Coaxial cable is used to transmit television signals. With the advent and development of video surveillance systems of various types and purposes, cable began to be used to transmit signals from video cameras to a centralized surveillance complex. For these purposes, both conventional and modern digital coaxial cable are used. This article will provide a small analysis of the types and features of the use of these cable products for video surveillance purposes.

Types and features of cable

This cable was invented in 1880 in Great Britain. The design feature of a coaxial cable is the combination on one axis of two conductors separated by a layer of dielectric material in a common outer shell. Initially, coaxial radio frequency cable was used to transmit television signals from public home network antennas and individual antenna devices to televisions, as well as in radio relay, radio transmission, satellite, and mobile radio communication systems. It is still used in these areas today. Basically, these are powerful cables with a large cross-section of the internal conductor and braid in a rigid sheath. In video surveillance systems, due to the peculiarities of installation in cramped conditions and a large number of cable bends, flexible cables of smaller cross-sections in a softer braid are used.

Types of cable products

There are two types of cable products that are used for installation in video surveillance systems:

• Regular coaxial;
• Combined (coaxial + 2 wires for connecting the power supply to video cameras and/or transmitting control signals) cable. A cable with a supporting steel cable is also manufactured for external aerial installation between buildings.

The use of a combined cable is preferable for many reasons:

• The cost of the cable is lower than the sum of the prices of a conventional coaxial cable and a two-core electrical wire with the same characteristics in terms of cross-section, core material, braiding and insulation;
• Easier installation, convenient installation with fewer fasteners and a correspondingly neat appearance, which is especially important when installing in office, retail and other public premises and buildings.

Marking

The most famous coaxial cable for video surveillance has the following markings:

• RK – cables produced in Russia;
• RG – imported.

There are significantly more brands of combined video cable - KVK:

• KVK-V – in a polyvinyl chloride outer shell for internal lining;
• KVK-P – in an outer shell of light-resistant polyethylene for external installation;
• KVK-Pt - similar in production to KVK-P, but has a steel cable for external installation between buildings and structures;
• KKSV and KKSP - for internal and external installation, with a single-core internal conductor;
• KKSVG and KKSPG are the same cables, but with a multi-core inner conductor;
• KVKng is a universal cable, often mistakenly called non-flammable by the designation “ng”, does not propagate fire when laid in groups.

Types of coaxial cable

Main parameters: the total thickness of the cable, the thickness and density of the braid, the cross-section of the internal conductor, the materials used in the production of the cable directly affect the characteristic impedance of the cable. Characteristic impedance is an electrical characteristic of a cable, measured in Ohms. It shows the possibility and quality of transmitting a television signal from a video camera to a receiving device (a video recorder with a monitor or a personal computer selected as the operator’s workplace). In a video surveillance system of a security facility, it is recommended to use a cable with the same characteristic impedance to ensure signal quality and the absence of additional interference and distortion.

Coaxial cable is also conventionally divided by total thickness:

• Thin – up to 50 mm, with a single braid and a thin outer shell, for installation in buildings at a distance of no more than 200 to the farthest video camera from the surveillance complex;
• Thick - up to 100 mm, with double braiding, a thick outer shell, allowing the transmission of video images from the camera without attenuation of the television signal over a distance of up to 650 m, which is very important for video surveillance systems in enterprises and warehouses.

Cable composition and design

The elements of a coaxial cable are:

• Internal electrical conductor or core;
• Shell made of dielectric material;
• Double-sided foil screen. Not applicable to all cable types;
• Copper braid of different densities;
• Outer shell.

The inner conductor is made:

• Made from single-core aluminum or copper wire;
• Copper-coated steel or aluminum wire;
• Stranded copper wire;
• Silver coated copper wire.

Copper and aluminum are used both in purified form and as their alloys. The inner conductor in a coaxial cable is the main element used to transmit the signal. The material of the inner conductor or central core is easily determined by external inspection of the cable cut: silver color - aluminum or copper-plated steel, completely golden - copper. The larger the cross-section, the better the signal can be transmitted. But we should not forget that the price per 1 linear meter of cable will increase in direct proportion and its rigidity will increase, which is not always acceptable.

A sheath of dielectric material insulates the inner conductor from the braid. Made from monolithic or foamed polyethylene or polyurethane. Monolithic material is more suitable for laying through rooms with high humidity, better protects against interference, mechanical damage to the central core when compressed, due to its rigidity it is limited when it is necessary to lay through corridors and rooms with many turns, where a flexible cable with porous foam insulation is more applicable material.

The cable braid serves as a second conductor and a ground shield to protect the center conductor. Sometimes it is supplemented with a metal foil screen. The denser the braid with more copper content in the wire, the better the video signal is provided.

The outer sheath of the cable is protection from external influences. Made from polyvinyl chloride plastic.

Standard cable products of the RK, RG brands are coaxial braided cables with insulation made of monolithic or porous polyethylene. A single or stranded inner conductor made of copper or copper-clad steel wire. The outer conductor is aluminum foil + copper braid or two silver-plated copper braids. The shell is made of flame retardant plastic.

Coaxial Cable Selection

A coaxial cable for video surveillance that is suitable in all respects is selected based on the tasks and conditions when organizing a surveillance system at each specific security facility. These tasks and conditions are set out in the design and estimate documentation, if available, or in the customer’s technical specifications. In the first case, the cable is selected. In the second option, which is much more common, a contractor or owner planning to install a video surveillance system himself should consider and evaluate several essential parameters:

• Distances to selected video camera installation locations;
• Availability of electrical distribution panels and lighting network boxes near the camera installation sites;
• Uniformity of the method of laying the cable to each chamber (internal, external, aerial on a cable);
• The presence of sources of electrical interference and electromagnetic interference along the selected cable laying line (power and lighting electrical routes, electric motors, powerful electrical appliances and other devices that create an electromagnetic field around themselves), which will lead to a loss of video image quality;
• Requirements for cable products in terms of color, thickness, possibility of installation behind suspended ceilings, in cable ducts, including those existing on site, to maintain the integrity of the interior of the premises;
• The need to record an audio signal.

The selected connectors for the coaxial cable are also important, allowing you to correctly connect the cable line to the video camera.

Having considered all the conditions and installation options, draw up a simple cable log with distance measurements, taking into account the geometry of the installation and the number of bends.

The determining factor is the length of the coaxial cable for video surveillance, because the cable laid to each camera must be made in one piece, without any connections, to ensure the quality of signal transmission.

It is necessary to take into account the protection of the cable from mechanical damage, indoor humidity, climatic conditions and air temperature during installation work during external installation, and the need for transitions between buildings. Taking this into account, we can draw conclusions about the possibility and necessity of using certain types of cable. Often different types of cable are used in one system: regular, combined, on a cable.

In conclusion, it is worth saying that the choice of cable is very important. But this is only one element of the video surveillance system and the objectives can only be achieved by correctly selecting the entire list of necessary equipment, which will serve as topics for other articles.

Coaxial cable is widely used to transmit packets of information signals in computer networks, television signals in cable television systems, video surveillance and many other radio engineering systems. With the advent of wireless data transmission technologies Bluetooth or more modern and high-speed Wi-Fi, the use of coaxial cable has decreased.

Coaxial cable for transmitting information packets

Disadvantages of new technologies:

• small range of action;
• low noise immunity;
• low speed of information transfer process.

Networks using coaxial cables are still in demand and are widely used, especially at military facilities, where noise immunity is of paramount importance.

Cable design

A coaxial cable has two conductors:

1. Central monolithic copper core. In some standards, it is found in a multi-core twisted version made of copper alloys and coated with a silver layer. This core is the core of the cable; it is in a dielectric sheath made of polyethylene.
2. The outer shielding conductor looks like a woven mesh made of copper, copper alloys or aluminum. Some cable designs allow two shielding layers with a dielectric layer between them. The first is used as a central core for transmitting signals, the other dissipates external electromagnetic interference.

General view of the coaxial cable design

In most cases, the external protection of the cable from mechanical influences is provided by a UV-resistant polyvinide chloride sheath. There are more expensive protection options with Teflon coating.

Types of coaxial cables

There are many varieties of coaxial cables, some of which are several centimeters in diameter. They are used to transmit high power radio and television signals to antennas in 1-5 kW transmitters.

Types of coaxial cables

To build local computer networks, cables of two standards are used:

1. Category RG-58/U or 58A/U, 10BASE-2 standard. It is called a thin coaxial cable, with a diameter of up to 6 mm, with a characteristic impedance of 50 Ohms, model RG-58/U - with a solid copper wire in the center of the cable, in category 58A/U the core consists of twisted stranded copper wire. The cable is capable of transmitting information without significant signal loss, at speeds of up to 10 Mbit/s, up to 185 m, the cables are connected directly to the computer network adapter card.

1. Categories RG-11 or RG-8, 10BASE-5 standard:

• 10BASE-5 - in everyday life they call a thick coaxial cable, with a cross-section of 12 mm, the central core, the diameter of which is much thicker, allows you to transmit signals with less attenuation.
• In the RG-11 category, the resistance is 75 Ohms, in the RG-8 model - 50 Ohms. The signal transmission speed is 10 Mbit/s, as in cables of 10BASE-2 standards, but the distance is much greater, up to 500 m. Very often, this cable is used as a backbone connecting several separate local networks, connecting it through a transceiver. This is a device equipped with a “vampire tooth” connector, which, when connected, pierces the insulating sheath and provides electrical contact with the conductor.

One of the leading manufacturers of coaxial cables is the Belden company; its developments to improve production, increase noise immunity, and signal transmission range are used by many companies. Some technical solutions have been adopted as international standards in production.

Belden cable

Cable networks have become widespread due to companies providing certain services for consumers in receiving and processing information. With the transition to digital broadcasting, the quality of cable TV has improved, high-speed Internet, satellite television packages and IP telephony have appeared. The range up to 2100 MHz and frequencies from 5-65 MHz have become more actively used for transmitting reverse channel signals.

Belden cable has proven itself in the development of these technologies, the developers realized that the costs of creating noise-resistant wireless communication equipment are very high. It is cheaper and more effective to eliminate electromagnetic interference by creating reliable shielding of coaxial cables.

General view of Belden cable

Belden's DUOBOND® PLUS shielding technology meets the industry's most advanced customer demands.

Features of technical solutions of the DUOBOND® PLUS cable:

1. Shielding is provided by sheet aluminum foil and copper woven mesh, the structure of the outer shielding shell is three-layer, aluminum, polyester, and aluminum again. All this is securely glued around a dielectric layer of foamed, nitrogen-charged polyethylene, which separates the central wire and the multilayer shield. This cable manufacturing technology prevents displacement of the shielding layer when cutting for fastening connectors and when bending during installation. This technical solution, developed by Belden specialists, is already an industry standard for all manufacturers.
2. To increase the mechanical strength of the shielding layer and ensure anti-interference efficiency of 85 dB, galvanic copper 99.99% coated with a layer of tin is used.
3. The outer three-layer shielding shell has a unique connection method with the closing overlap, which provides electrical contact to the aluminum coating. This creates a one-piece shielding tube made of laminated foil.
4. Cables, DUOBOND® PLUS, are created with shielding that fully comply with the RG 6, RG 11 and RG 59 standards. This greatly simplifies the work of operators during installation; the same techniques, tools and connectors are used as when working with other cables of these standards.

Main parameters of DUOBOND® PLUS cables

Options	RG 59	RG 6	(PRG 11)
Wave R, in Ohm	75	75	75
Capacitance, pF/m	55	56	55
Shielding effect, in dB	>85	>85	>90
Ø central wire, mm	0.8	1	1.55
Ø dielectric, in mm	3.66	4.75	7.25
Foil type	Al/Pe/Al	Cu	Cu
Braid density, %	50	50	50
Ø outer conductor, in mm	4.2	5.25	7.9
Shell Ø, mm	6	6.9	10.1
Minimum bend radius, mm	35	35	100
Clock frequency of transmitted signals in MHz		signal attenuation in dB/100m
5	1.8	1.7	0.9
50	4	4.5	2.7
100	5.6	6.4	3.9
200	7.4	9.1	5.7
300	12.6	11	6.9
400	16.2	13.2	8.2
800	23.2	19.2	12
1350	30.7	25.6	16.1
1750	35.3	29.6	18.7
2400	41.9	35.5	22.5

Domestic manufacturers produce coaxial cables with individual markings, but in compliance with international standards.

Cable RK-75 and RK-50

The shielding mesh of these cables is made of thin soft copper wire, the density of which is 90%. There are combined options, a lavsan aluminum coating is wrapped in a mesh of tinned copper wire with a density of 50%. The combined screen provides greater noise immunity.

The insulation of the central wire consists of nitrogen-foamed polyethylene, which ensures a low attenuation coefficient of the transmitted signal. The central rod is made of solid annealed copper wire or of a thin stranded structure. This gives greater flexibility to the wire and better conductivity for electrical signals. The structure of the RK-75 coaxial cable is the same as the RK-50, the difference is in the diameter of the individual elements and wave impedance. It determines their purpose and scope:

• RK-75 is more often used in backbone computer networks between local branches of a large network;
• RK-50 has a wide range of applications; it is used on radio transmitting and receiving devices, for connecting boards for various purposes in separate units, transmitting video signals, and in local computer networks.

For outdoor use of cables, grades with an outer sheath made of light-stabilizing polyethylene are used; for installation inside buildings, cables with a sheath made of PVC plastic are used.

Technical characteristics of RK cables

Cable brand	Inner core	Characteristic impedance W, Ohm	Attenuation, dB/m at frequency		Diameter D, mm
			10 MHz	100 MHz
RK-50-2-11 (RK-119)	Single wire	50±2	0.05	0.18	4.0±0.3
RK-50-2-13 (RK-19)	"	50±2	0.05	0.18	4.0±0.3
RK-50-3-11 (RK-159)	"	50±2	0.04	0.13	5.3±0.3
RK-50-3-13 (RK-55)	"	50±2	0.03	0.13	5.0±0.3
RK-50-4-11 (RK-129)	"	50±2	0.03	0.1	9.6±0.6
RK-50-4-13 (RK-29)	"	50±2	0.03	0.1	9.6±0.6
RK-50-7-11 (RK-147)	Stranded	50±2	0.02	0.08	10.3±0.6
RK-50-7-15 (RK-47)	"	50±2	0.02	0.08	10.3±0.6
RK-50-7-12 (RK-128)	"	50±2	0.02	0.09	11.2±0.7
RK-50-7-16 (RK-28)	"	50±2	0.02	0.09	11.2±0.7
RK-50-11-11 (RK-148)	"	50±2	0.018	0.06	14.0±0.8
RK-50-11-13 (RK-48)	"	50±2	0.018	0.06	14.0±0.8
RK-75-4-11 (RK-101)	Single wire	50±2	0.032	0.1	7.3±0.4
RK-75-4-15 (RK-1)	"	50±2	0.032	0.1	7.3±0.4
Note: The table contains data provided by manufacturers. Linear interpolation can be used to estimate the attenuation at 27 MHz.

Combined cable KVK-V(P)-2

The peculiarity of this brand is that two insulated copper wires are added to the usual coaxial cable between the screen and the outer insulating sheath. Additional stranded wires with a cross-section of 0.5 for controlling system options and 0.75 mm/sq. for power supply. The TV signal is transmitted via the coaxial component of the cable, and the camera is controlled or powered via additional wires.

General view of the combined cable KVK-V(P)-2

This cable provides high-quality transmission of color and black-and-white video signals over a distance of up to 450 m.

Designs and technical parameters of combined cables

Marking of domestic cables of the Republic of Kazakhstan

The basis of the marking is the sequence of letters and numbers:

• the first number 50 or 75 is the value of the wave impedance Ohm;
• the second number is the Ø of the central core insulation;
• in the third number, the first digit is the insulation material with the heat resistance category, the second and third digits determine the development number.

Example of decoding RK 75-4,9-322A:

• R – radio frequency;
• K – coaxial cable;
• number 75 – wave impedance in Ohm/m;
• 4 or 9 – Ø for external insulation;
• 3 - semi-air, foam insulation with heat resistance 1250 C;
• number 22 – production development number;
• The last letter in the group of symbols, A, denotes the main purpose category “Antenna”.

Depending on the application and demand, manufacturers make many different brands of coaxial cables. Combined cable models are widely used for video surveillance systems and intercoms.

Installation features

Each coaxial cable described has good flexibility.

The radius of rotation during installation should not exceed 12 times the radius of the cable sheath. Over time, kinks can lead to pushing through the central core of the dielectric layer and shorting to the screen. It is not advisable to hang the cable for a long time, under its own weight, at a distance of more than 15 m, as this leads to stretching and breakage of the central core. Proper cutting of the cable ends for connecting connectors is of great importance.

Sequence and description of cable cutting for attaching connectors

The cable sheath is designed to protect it from moisture and external damage when used indoors and on the surface. Do not lay the cable underwater or underground. Capillary seepage of water will destroy the shielding sheath and the central rod.

Operation on the surface in rainy weather is allowed, but at the joints it is recommended to use silicone sealants, or, in extreme cases, electrical tape and plasticine. There are special moisture-resistant connectors, models SO-239 or PL-259, PL-258.

Sequence of arrangement of connecting connectors PL-259 and PL-258

The connections are made by soldering, change the value of the wave impedance, and are a source of reflected waves, which leads to signal distortion. Use industrial connectors.

Characteristic impedance

When working with electronic equipment, the signals of which are transmitted via a coaxial cable, it is necessary to understand that the characteristic impedance cannot be measured along the central core with a conventional ohmmeter. It is calculated based on the diameter of the central wire and the diameter of the shielding mesh:

• Rw = 91lg (dD);
• Rw – characteristic impedance in Ohm;
• D – Ø of the internal dielectric layer in mm;
• d – Ø of the inner central rod in mm.

There are calculated graphs of the dependence of wave impedance on the diameters of the dielectric layer and the central core.

Graph of the dependence of wave impedance on the ratio of the diameters of the dielectric layer and the central wire

Along the vertical axis we mark the value of the ratio Dd, along the horizontal axis the value Rw (characteristic impedance) is measured. The graph shows a directly proportional relationship; as the ratio increases, the resistance increases. Thus, by making diameter measurements and calculations, you can independently determine Rw from the graph.

Cable selection. Video

This video explains how to choose the right cable for TV.

When using coaxial cable when installing electronic equipment, it is recommended to follow the installation instructions, where the markings are usually indicated. Using the specified brands of cables, modern tools, connectors and other components, it is easy to carry out installation on your own.

The purpose of any radio transmission line is to transmit a signal from the source to the load with minimal losses and minimal distortion. And intra-unit installation and wires and cables connecting various radio-electronic devices, for example, a transmitting television camera with a video recorder, all of these are communication lines.

The design and principle of operation of communication lines depends on the frequency range of the signals that are planned to be transmitted over them.

Signals in the frequency range from 1 Hz to 30 kHz are audio signals and are usually transmitted through wires.

A wire contains one or more stranded wires or insulated strands protected by a lightweight non-metallic sheath or braid of fibrous materials. If the braid must withstand high mechanical loads and protect the wire from rodents, it is made of wire.

ADVICE
Use wires rather than coaxial cables to transmit audio signals

Signals in the frequency range from 30 kHz to 300 GHz are radio frequency signals. To transmit such signals, shielded wires and coaxial cables are used, and in the microwave range, starting from 3 GHz, waveguides are used.

Waveguides are conductive tubes of rectangular, circular or elliptical cross-section that allow the wave to propagate along the length of the tube, reflecting from its walls. The advantages of a waveguide compared to a coaxial cable are low power losses, low standing wave ratio and high operating frequency, but they are expensive, bulky, difficult to install, and, despite the emergence of the so-called. flexible waveguides, not designed for repeated bends and kinks.

Coaxial cable(from the Latin co - together and axis - axis) is a communication cable made up of one or more (up to 20 or more) coaxial pairs, in which both conductors - internal and external, are coaxial cylinders separated by a layer of insulation (polyethylene, air polyethylene, fluoroplastic or other).

The video signal passes through the central core, while the screen is used to equalize the zero potential of the end devices - a video camera and a video monitor, for example. The shield also protects the central core from external electromagnetic interference (EMI). To improve the performance of the electrical shield, good coaxial cables include a return wire.

Coaxial cable is the most common means of transmitting video signals.

The idea of ​​a coaxial cable structure is that all interference is induced only in the screen. If it is reliably grounded, then the interference is “discharged” through the grounding circuit.

The coaxial cable completes the circuit between the source and the receiver, where the central core of the cable is the signal wire and the shield is the ground wire. Therefore, transmission over a coaxial cable is called asymmetric transmission.

In electronic equipment, a simple coaxial cable is most often used, containing one central core surrounded by a screen (Fig. 1), or a triaxial cable having two central cores.

Rice. 1 Coaxial cable

ADVICE
Use triax cables to transmit chrominance (C) and luminance (Y) signals from satellite receivers, DVD players, and other devices with an S-Video interface.

Rice. 2 Triaxial (bicoaxial) cable

Coaxial cable is the most common means of transmitting video signals. Depending on the type of video signal, it can be transmitted from sources to receivers using a coaxial cable with a characteristic impedance of 75 Ohms over the distances shown in Table 1.

Signal type	Signal type	Bandwidth, MHz	Distance, m
Composite CV without amplifier with amplifier	analog	6	50-100 200-300
S-Video without amplifier with amplifier	analog	6	50-100 200-300
Component UXGA HDTV/1080i	analog	300 30	5-30 5-30
SDI standard without amplifier standard with amplifier	digital	270 Mbit/s 270 Mbit/s	50-200 200-300

Main characteristics of coaxial cables

The main characteristics of coaxial cables are:

• Characteristic impedance;
• Return loss;
• Attenuation.

Linear wave impedance

The short wires and cables used in conventional electronic equipment have negligible ohmic resistance, inductance and capacitance and do not affect the signal. However, if a signal must be transmitted over a fairly long distance, many different factors are included in the complex picture of information transfer. High-frequency signals are especially susceptible to influence. Then resistance, inductance and capacitance begin to play a significant role and significantly affect signal transmission.

From the point of view of electrodynamics, a coaxial cable can be represented as a circuit consisting of resistances (R), inductances (L), capacitors (C) and conductors (G) per unit length (Fig. 3). If the cable is of considerable length, then the combination of elements R, L and C acts as a coarse low-pass filter, which in turn affects the amplitude and phase of various components of the video signal. The higher the signal frequencies, the more they are affected by non-ideal cable properties.

Rice. 3 Introduction to coaxial cable

Each cable has a uniform structure and its own characteristic impedance (impedance), which is determined by the elements R, L, C and G per unit length.

The main advantage of single-ended video transmission is based on the fact that the characteristic impedance of the transmission medium does not depend on frequency (this applies mainly to medium and high frequencies), while the phase shift is proportional to frequency.

The amplitude and phase characteristics of a coaxial cable at low frequencies depend to a large extent on the frequency itself, but since in such cases the cable length is quite short compared to the wavelength of the signal, the effect on signal transmission is negligible.

When the characteristic impedance of the coaxial cable matches the output impedance of the video source and the input impedance of the receiving device, maximum energy transfer occurs between the source and the receiver, such a transmission line is called matched.

For high frequency signals such as video, impedance matching is of utmost importance.

For high frequency signals such as video, impedance matching is of utmost importance. When the impedance is mismatched, all or part of the video signal is reflected back to the source, affecting not only the output stage but also the image quality. Reflection of 100% of the signal occurs when the end of the cable is either short-circuited or left open (unshorted). All (100%) signal energy (voltage times current) is transmitted only when there is a match between the source, transmission media, and receiver. This is why the last element in the video signal chain always ends with a 75 ohm load, which is called a terminator (see Fig. 4).

ADVICE
To ensure matching between the source, transmission media and receiver, the last element in the coaxial line includes a 75-Ohm terminator.

Rice. 4. Design elements of a coaxial line

In television, all equipment transmitting or receiving video signals has a characteristic impedance of 75 ohms. Therefore, you need to use a coaxial cable with an impedance of 75 ohms. But manufacturers also produce other equipment, for example, with an impedance of 50 ohms (which in some cases is used for broadcast or RF equipment), but then impedance converters (passive or active) must be used between such sources and 75 ohm receivers.

75 ohms of coaxial cable is the complex impedance determined by the voltage/current ratio at each point in the cable. This is not active resistance and therefore cannot be measured with a regular multimeter.

The impedance of a coaxial cable is determined by the formula:

Characteristic impedance is independent of cable length and frequency, but depends on capacitance and inductance per unit length.

This formula means that the characteristic impedance does not depend on cable length and frequency, but depends on the capacitance and inductance per unit length. However, this is not the case if the cable length exceeds 200 meters. In this case, resistance and capacitance matter and affect the video signal.

Losses in a coaxial cable consist of two components: dielectric losses and losses in conductors. Losses in insulation depend only on its dielectric properties and do not depend on the size of the cable. Losses in conductors are strictly related to their dimensions, and to a greater extent to the cross-section of the central conductor, because the main part of the electromagnetic field propagates along the cable, greatly decreasing towards the screen. Obviously, as the cable size increases, the field concentration around the central conductor decreases, therefore, the losses also decrease.

Losses in a coaxial cable consist of two components: dielectric losses and losses in conductors.

Deviations of linear wave impedance of a cable line are expressed using return losses.

The assessment of the line operating mode is characterized by Traveling Wave Ratio (TWR), which characterizes the degree of coordination of the line with the load. If the BPV is equal to one, the line is fully matched to the load. In practice, such lines do not exist due to the impossibility of perfectly matching the load with the line.

The reciprocal of the traveling wave coefficient is called standing wave ratio.

It is clear that cable length uniformity is essential to meeting characteristic impedance requirements. The quality of the cable depends on the accuracy and uniformity of the central core, dielectric and screen. These factors determine the values ​​of C and L per unit length of cable. That is why special attention must be paid to cable routing and termination.

Rules for laying coaxial cable

• Loops and bends disrupt the uniformity of the cable. This results in high-frequency loss, which is the loss of fine image detail, as well as image doubling due to signal reflections. The picture quality will be better if the loop bend is 10 times the diameter of the coaxial cable. This is equivalent to saying: “the loop radius must be at least 5 diameters or 10 radii of the cable.
• When laying coaxial cable, follow the manufacturer's instructions regarding permissible bend radii and recommended distances between attachment points.
• When laying, do not scatter the cable on the floor. If you accidentally step on it or place a heavy object, signal transmission will deteriorate sharply.
• When pulling the cable, do not apply great mechanical force to it, do not try to pull it through a small hole in the wall or a narrow box. This can lead to deformation or internal breakage of the central core and shielding braid.
• Do not run coaxial cable near power wires or other sources of electromagnetic interference.
• Breaking the cable in the middle and sealing the resulting ends will result in some signal loss, especially if the ends are poorly sealed or low-quality BNC connectors are used. A good termination gives a signal loss of only 0.3 - 0.5 dB. If there are not too many terminations on one cable, the signal will suffer only slightly.
• To move from connector to connector, use special adapters (Fig. 5).

Rice. 5 Adapters for video signal

1 – BNC plug to RCA socket; 2 – BNC socket to RCA plug; 3 – BNC female-female; 4 – RCA-female-female; 5 – BNC plug to T-splitter with two BNC sockets; 6 – BNC plug to Y-splitter with two BNC sockets; 7 – BNC socket with 75 Ohm terminator; 8 – 3.5mm stereo plug to splitter with two RCA sockets.

The degree of distortion of sinusoidal signals by communication lines is assessed by such characteristics as attenuation And bandwidth.

Attenuation shows how much the power of the reference sinusoidal signal at the output of a communication line decreases in relation to the signal power at the input of this line.

The signal attenuation per 100 feet of some popular foreign cables is shown in Table 1.

Table 1. Signal attenuation in coaxial cables

Cable type	Characteristic impedance (Ohm)	Signal attenuation per 100 feet of length, dB
Frequencies, MHz		1	10	100	1000
RG-59/U	72	0,6	1,1	3,4	12
RG-6/U	72	0,4	0,8	2,7	9,8
RG-11/U	72	0,2	0,4	1,3	5,2
RG-58/U	50	0,4	1,3	4,5	18,1
RG-8/U	50	0,2	0,5	1,5	4,8

ADVICE
When choosing a brand of coaxial cable for installation, always ensure that its bandwidth exceeds the spectrum width of the transmitted signal.

Noise and electromagnetic interference

How well a coaxial cable's shield protects the core from noise and EMI depends on the percentage of shielding. As a rule, manufacturers indicate figures from 90 to 99% in specifications. But keep in mind that even if 100% shielding is promised, it is impossible to achieve 100% protection from external interference. The penetration of EMF into a coaxial cable depends on the frequency used.

How well a coaxial cable's shield protects the core from noise and EMI depends on the percentage of shielding.

Theoretically, only frequencies above 50 kHz are successfully suppressed - mainly due to the weakening of the skin effect. All frequencies below this induce an electric current in the screen to a lesser or greater extent. How strong the electric current is depends on the strength of the magnetic field. It is clear that we are primarily interested in the radiation of industrial frequency current (50 or 60 Hz), which surrounds almost all technical devices.

This is why problems arise when coaxial cable is run parallel to electrical wires and cables. The magnitude of the induced voltage in the central core of a coaxial cable depends, firstly, on the current strength on a given power line. Secondly, it depends on how far the coaxial cable runs from the power cable. And finally, it depends on the length of these cables running together. Sometimes proximity within 100 m has no effect, but if a large current flows through the power cable, then even 50 m can affect the quality of the signal. When installing, try (whenever possible) to ensure that the power and coaxial cables do not run too close to each other. To significantly reduce EMF, it is necessary that the distance between them be at least 30 cm.

On the monitor screen, interference from the electrical network appears as several thick horizontal stripes, slowly sliding up or down. The creep frequency is determined by the difference between the frequency of the video signal fields and the industrial frequency and can range from 0 to 1 Hz. As a result, stationary or very slowly moving stripes appear on the screen.

Coaxial cable design

Everyone who is more or less connected with radio engineering knows how a coaxial cable works. However, some aspects of their design often cause annoying errors. For example, many people confuse the insulation of a coaxial cable with its sheath.

In RF coaxial cables, insulation is usually called the structure that isolates the inner conductor from the outer one, but the material that covers the outside of the cable is called the sheath.

In RF coaxial cables, insulation is the structure that isolates the inner conductor from the outer conductor.

Typically, in catalogs and price lists, in the “Diameter” column, the insulation diameter of the coaxial cable is indicated without taking into account the thickness of the braid and sheath. Therefore, if the outer diameter of the cable is important to you (for example, for laying it through pre-assembled boxes of a certain size), you should clarify it in advance.

Copper is one of the best conductors for coaxial cable. Only gold and silver have higher performance indicators (resistance, corrosion), but they are too expensive for cable production. Many people believe that the best cables are made from copper-plated steel, but this is not true. Copper-plated steel is simply cheaper and possibly stiffer, but for long cables it is better to use copper. Copper-clad steel coaxial cables are acceptable for a community antenna where the transmitted signals are HF modulated (VHF or UHF, MB or UHF). Namely, at higher frequencies the so-called skin effect (surface effect) is more pronounced: the actual signal flows onto the copper surface of the conductor (not the screen, but the central conductor).

ADVICE
When choosing a brand of coaxial cable for installation, give preference to cables with copper cores.

By degree of hardness Coaxial cables can be divided into 4 groups:

• flexible;
• semi-flexible;
• semi-rigid;
• hard.

Flexible cables are those that can withstand up to 50,000 bends or more. For such cables, the shield is a braid of thin wires. Since the braid is not a continuous conductor and has a significant distance between the wires, the electromagnetic field “leaks out” through the holes. In addition, for electric current, the braid represents a huge number of contacts between the wires, which leads to an increase in its resistance and, ultimately, increases the signal attenuation in cables of this type.

Flexible cables are not suitable for transmitting signals over distances exceeding 50 m.

IN semi-flexible In coaxial cables, to increase the degree of shielding and reduce electrical resistance and, therefore, attenuation, metal foil is first applied to the insulation, and braided on top of it. These cables have much lower attenuation than flexible cables, but they are much less flexible. Such cables are widely used in cable television networks, but are not widely used in radio systems.

Semi-rigid Coaxial cables have a solid welded outer conductor. In 95% of designs, this conductor has a spiral or annular corrugation. Cables of this type have a low attenuation coefficient and excellent shielding. Depending on the size and insulation material, they can provide transmission of quite high power (up to 5 kW at a frequency of 100 MHz for the domestic cable RK50-17-51).

Hard Coaxial cables, more like water pipes than radio frequency cables, are designed primarily to carry high power signals.

ADVICE
When choosing a coaxial cable for installation, use soft cables only for jumpers, and make the main line from semi-flexible cables.

It should be noted that radio frequency cables, located most of the time in open spaces (radio masts, roofs, etc.), must be resistant to high and low temperatures and their changes, to moisture and solar radiation. To increase mechanical strength, some coaxial cables are equipped with a metal cable that takes on the main loads.

Radio frequency cables, located most of the time in open spaces (radio masts, roofs, etc.), must be resistant to high and low temperatures and temperature changes, moisture and solar radiation.

As already mentioned, a typical coaxial cable consists of a center conductor, an internal dielectric, a shield and an outer sheath (Fig. 1).

The center conductor of the cable is designed to transmit a signal from one point to another. It is made from materials that conduct electrical signals well. Copper is usually used, which is suitable for these purposes in terms of its electrical, mechanical and cost parameters. The central conductor can be either single-core or multi-core.

Single-core– this is a central conductor made in the form of one straight conductor. Solid conductor is well formed, but not very flexible. Therefore, cables with a single conductor are usually used in fixed installations.

Twisted stranded- is a conductor consisting of many thin conductors twisted together. These cables are flexible, lighter and are mainly used in mobile installations. However, the characteristics of such a cable will be slightly lower than a cable with a single-core conductor of the same size.

Internal dielectric, also called internal cable insulation, plays an important role in coaxial cables. First of all, it is a material that insulates the center conductor from the shield. But, in addition, it determines the impedance and capacitance of the cable. Typically, polyethylene is used in general purpose cables, and fluorine-containing polymers are used to produce non-flammable cables.

Cheap cables have a dielectric made of solid polyethylene. More serious manufacturers use foamed polyethylene, which provides lower linear signal attenuation in the cable at high frequencies.

It is worth noting that some manufacturers foam the dielectric chemically. The result is a low-density polyethylene compound that is susceptible to mechanical damage and unstable to environmental influences in the form of temperature and humidity.

The highest quality cable is obtained with a physically foamed dielectric (gas injected foam polyethylene). It contains up to 60% air bubbles, thereby reducing the attenuation of high signal frequencies. In terms of strength, physically foamed polyethylene does not differ from ordinary solid non-foamed polyethylene, providing the necessary flexibility and resistance to mechanical stress. And finally, being highly resistant to temperature fluctuations and humidity, the physically foamed dielectric will ensure stable parameters and long-term operation of the cable.

ADVICE
When choosing a brand of coaxial cable for installation, give preference to cables with physically foamed dielectric.

Screen fulfills two important roles. It acts as a second conductor connected to the equipment's common ground wire. At the same time, it shields the signal conductor from extraneous radiation. There are different shielding methods for cables that perform different tasks. These include foil screen, braided screen, and foil/braid combinations.

Braid- a screen that is made of many thin conductors woven into a mesh that encloses a central conductor with an internal dielectric. Braid usually has less resistance than foil and has better resistance to extraneous electromagnetic fields and electromagnetic interference. Braid can be combined with other types of shields, such as aluminum or copper foil, to provide the required shielding percentage.

Foil can provide up to 100% shielding when combined with braid. Considering that the braid can provide shielding efficiency of up to 90%, to get 100% you need two braids, which significantly increases the cost of the cable, its weight and impairs flexibility. It is much easier to achieve 100% shielding efficiency using a combination of braid and foil.

Provides the necessary protection for the cable's internal components outer shell. The sheath protects the cable from climatic, chemical, and exposure to sunlight. Based on the type of sheath, cables can be divided into standard and special cables.

A standard cable has a regular, most often polyvinyl chloride sheath, which protects the cable (or multicore) from mechanical stress and moisture, and also plays the role of electrical insulation.

To transmit RGBHV, S-Video and component signals, several coaxial cables can be combined into a multicore (Fig. 6) with a common sheath. The number of coaxial cables in a multicore can be from two to six; in addition, balanced audio pairs and power conductors can be added to a multicore, which makes them even more versatile.

Filled(Plenum) - standard installation involves laying cables through walls and ceilings. A possible fire inside a building places special demands on the cable sheath. Cables of the Plenum type have a fire-resistant sheath, which uses special compounds. This ensures low flammability and smoke emission if the cable is exposed to fire. Such a cable can be laid without a pipeline, which reduces installation costs.

Halogen-free– low emission of smoke and vapors, absence of halogens in the cable sheath material are required by European safety regulations (IEC33203 flammability test, IEC61034 smoke emission test, IEC754-1 corrosion resistance).

To transmit RGBHV, S-Video and component signals, several coaxial cables can be combined into a multicore (Fig. 6) with a common sheath. The number of coaxial cables in a multicore can be from two to six; in addition, balanced audio pairs and power conductors can be added to a multicore, which makes them even more versatile.

Rice. 6 Multicore cross-section

ADVICE
To transmit a large number of signals of different types over one cable, use multicores.

During installation, special attention must be paid to preventing moisture from entering the cable. This problem is especially acute when using cables with cord insulation. First of all, it is necessary to seal (moisture-proof) the cable when installing connectors.

A separate class of coaxial cables consists of cables for underground installation.

When constructing an antenna-feeder path (AFT), the following scheme is usually followed. A semi-rigid cable with good characteristics is selected as the main transmission system. They are connected directly to the radio equipment at one end and the antenna at the other using short sections of flexible cable, the so-called. jumpers (Fig. 7). This scheme is convenient and economically beneficial, because If you connect a semi-rigid cable directly to devices, then due to the large bending radius, you would have to use at least 6 m more cable, and this is more expensive than two short jumpers, and it is simply inconvenient to service equipment without jumpers. However, when operating at sufficiently high frequencies (800-900 MHz), even short jumpers on flexible cables can significantly weaken and distort the signal. Therefore, it is more expedient to use a semi-rigid thin cable as jumpers in this part of the AFL, because The difference in price between them relative to the entire AFT is insignificant.

Rice. 7 Coaxial jumper

There are three types of BNC connectors: threaded, soldered, and crimped.

Another important element when connecting a coaxial cable to equipment is the connector (connector). When selecting this seemingly simple device, you must be guided by two criteria: good electrical characteristics and ease of cable termination.

Connectors

In television, a coaxial cable termination is widely used, which is called a BNC connector (after the first letters of the last names of the creators of Bayonet-Neil-Concelman). There are three types of BNC connectors: threaded, soldered, and crimped.

Rice. 8 BNC type connector (cable)

Structurally, the connector looks like this: inside a metal sleeve with a slip-on locking coupling (when it is turned, the detachable connection is securely fixed) there is a thin central signal contact. On the other side of the sleeve there is a contact tube for the screen braid. The signal conductor passes through this tube and is inserted into a pin that fits into the central contact. Another tube is put on the contact tube, which, in fact, is crimped with a special tool. The central contact is nickel, silver-plated and gold-plated. The sleeve itself is most often nickel-plated.

ADVICE
Experience has proven that crimped BNC connectors are the most reliable. They require special and expensive crimping tools, but they are well worth the expense. More than 50% of problems encountered during system installation are the result of poor or incorrect cable termination.

The most common BNC connectors are plug-in connectors (male contact connections). There are also female connectors, right angle adapters, BNC-to-BNC adapters (often called "barrels"), 75 ohm terminations (or "dummy loads"), BNC to other connection types, etc. d.

For consumer equipment, the coaxial cable can be cut into an RCA connector (also known as a “tulip” connector due to the flower-like shape of older connectors). This is a very simple and cheap connector, but it is designed exclusively for indoor use and is not suitable for professional equipment.

Rice. 9

RCA connectors are used for unbalanced transmission of line-level analog signals, mainly from various recording devices. In addition, this connector is used in the SPDIF digital interface. The well-known company Canare produces crimp-type RCA connectors for installation on coaxial wires.

RCA is an inherently “wrong” connector, since the connection of the signal contact of the plug to the signal contact of the socket occurs before the connection of the ground contacts. Some companies, such as Neutrik, make RCA style plugs with an extended spring-loaded ground pin that connects to the jack's ground pin before the signal pin.

ADVICE
If possible, avoid using RCA type connectors.

All RCA connectors can be divided into two groups. Some are designed to transmit an analog signal, and the second are designed to transmit a digital SPDIF signal, as a result of which they have a characteristic impedance of 75 Ohms. The wiring (or crimping) of both connectors is completely unambiguous: the central contact is signal, and the cylinder around the central contact is common.

Rules for cutting connectors

• Never use improvised tools to cut connectors - you can easily damage the central core and cable shield. Use the special cable stripping and crimping tool shown in Fig. 7 and 8.
• Select cable connectors that match the selected cable type. If the cable is thicker than the diameter in the connector shank, it will not be possible to assemble it, and if it is thinner, the first accidental tug will pull the cable out of the connector.
• When cutting, do not use much physical effort. If the connector does not assemble, then you are doing something wrong.

The main purpose of a coaxial cable is signal transmission in various fields of technology:

• communication systems;
• broadcast networks;
• computer networks;
• antenna-feeder systems;
• Automated control systems and other production and research technical systems;
• remote control, measurement and control systems;
• alarm and automation systems;
• objective control and video surveillance systems;
• communication channels of various radio-electronic devices of mobile objects (ships, aircraft, etc.);
• intra-unit and inter-unit communications as part of radio-electronic equipment;
• communication channels in household and amateur equipment;
• military equipment and other special applications.

Device

The coaxial cable (see figure) consists of:

• A - shells (used for insulation and protection from external influences) made of light-stabilized (that is, resistant to ultraviolet radiation from the sun) polyethylene, polyvinyl chloride, fluoroplastic tape or other insulating material;
• B - external conductor (screen) in the form of braid, foil, film coated with a layer of aluminum and their combinations, as well as a corrugated tube, twisted metal tapes, etc. made of copper, copper or aluminum alloy;
• C - insulation made in the form of solid (polyethylene, foamed polyethylene, solid fluoroplastic, fluoroplastic tape, etc.) or semi-air (cordial-tubular layer, washers, etc.) dielectric filling, ensuring constancy of the relative position (alignment) of the internal and external conductors;
• D - internal conductor in the form of a single straight (as in the figure) or twisted into a spiral wire, stranded wire, tube made of copper, copper alloy, aluminum alloy, copper-plated steel, copper-plated aluminum, silver-plated copper, etc.

Due to the coincidence of the centers of both conductors, as well as a certain ratio between the diameter of the central core and the screen, a standing wave regime is formed inside the cable in the radial direction, which makes it possible to reduce the loss of electromagnetic energy through radiation to almost zero. At the same time, the screen provides protection from external electromagnetic interference.

There are several common misconceptions about coaxial cable.

A common misconception is that all white cables are good.

Not all white cables are high quality, and not all high quality cables are white! The basis of this misconception is the external similarity of cheap cables with the products of the world's leading manufacturers. The main differences between quality cables and counterfeits are physically foamed dielectric with gas injection and double foil (foil - polyester - foil) as a continuous shield. Physically, a foamed dielectric is a structure of insulated cells filled with gas. It does not absorb water and is more resistant to mechanical stress. The dielectric constant of such a material is close to ideal and remains for 15 years or more, and therefore, losses in the cable as a result of aging are close to the original ones.

Since manufacturers of cheap cables cannot afford expensive technologies, they use chemically foamed dielectric. It absorbs moisture like a sponge when the outer shell is damaged and is sensitive to external mechanical influences. In addition, as a result of aging, losses in it increase (Fig. 1). Also, cheap cables do not use double foil (but only single foil) as the main shield, which reduces the shielding effect and makes the cable sensitive to external interference (radio extenders, SENAO, etc.). Therefore, such a cable cannot be used in interactive networks with a return channel. While questionable cables use copper braid (soldered cable), quality cables use tinned copper braid. The combination "tin - aluminum" is more preferable compared to "copper - aluminum". That is, if the outer sheath of the cable is damaged or the connector is leaking, moisture enters the outer conductor, and as a result of an electrochemical reaction, the aluminum foil is destroyed. This leads to a significant reduction in the shielding properties of the cable.

• the performance characteristics of cheap cables deteriorate over time;
• the shielding properties of such cables are lower than those of high-quality cables from global manufacturers;
• Although cheap cables have better characteristics than the domestic cable RK75-4-11, they should not be used in networks where a return channel is supposed to be used. The scope of application of these cables is non-critical cabling with a high signal level, if there are no special shielding requirements.

Unreasonable exaggeration of the importance of secondary braiding

There is an opinion that the thicker the braid, the better the cable. This is not entirely true! As for low losses in the cable... Like, the thicker the braid, the less losses! Indeed, attenuation in a coaxial cable consists of conductor losses, dielectric losses and radiation losses. The last parameter is considered separately and characterizes the effectiveness of shielding.

So let's start in order:

1. Losses in conductors depend on the frequency of the signal, due to a decrease in the thickness of the skin layer and a corresponding decrease in conductivity. The use of high-quality copper in the cables, either in the cladding layer of the center conductor or for the entire center conductor, reduces the overall attenuation in the cable.
2. Losses in the dielectric also depend on the signal frequency. The power loss in the dielectric is spent on the reorientation of dielectric molecules in the RF field. As the dielectric constant of the material increases, the power loss also increases. The use of physically foamed (rather than solid) polyethylene as a dielectric allows one to reduce the amount of losses in the dielectric. By physically foamed dielectric we mean foaming with gas injection. In this case, isolated micropores filled with an inert gas (nitrogen) are created in the dielectric. It is this structure that ensures low losses in the dielectric and guarantees its stability over many years of operation. The use of such a dielectric in CAVEL cables ensures a reduction in parameters due to aging by only 5%, and in BELDEN cables - by 1%. In cables where, for reasons of economy, this technology is not used, the parameters are reduced by 50...70%. Hence the rule: we are not so rich as to buy cheap things!
3. The effectiveness of shielding determines the relative level of power emitted by the cable into the air and, at the same time, the degree of protection of the cable from external interference. Shielding factor (expressed in decibels) is defined as the ratio of the signal power of external interference to the power generated by that interference in the cable.

A high degree of shielding in cables is achieved through the use of a two-layer combined shield - aluminum foil and a braid of twisted conductors. As the first screen, a polystyrene tape is used, laminated on both sides with aluminum, and as the second layer, braids made of tinned copper - CuSn or aluminum AL (this applies to high-quality cables) are used. So it is this first layer that performs the main shielding functions. In addition, the shielding properties of copper are higher than those of aluminum, therefore, where 40% copper is enough, 80% aluminum is needed! In other words, identical cables but with different braid densities, such as 40% and 80%, will have the same attenuation.

For cheap cables, a three-layer (AL-film-AL) first screen is an unaffordable luxury. In the best case, foil with a polyester backing is used, and usually aluminum is sprayed onto the backing. This is where thick braiding is essential! But, alas, “the economy must be economical.” Hence the rule: free cheese is only in a mousetrap.

As for increased strength... If the cables are subject to tension during installation or there are long sags (stretching under the influence of their own weight), then in such cases a central core made of copper-clad steel is used. And in such cables, it is the steel central core that serves as the reinforcing element, and not the braid, even the thickest one. By the way, the quality of the clad layer is also a very important issue, because we remember the skin effect!

And directly about shielding: the main shielding functions are performed by a layer of foil (in high-quality cables), and the braid plays a secondary shielding function and is more intended to transmit current, as well as give flexibility to the cable. That is, the greater the braiding density, the greater the current that can be transmitted (for example, when powering amplifiers remotely). The effect of braid density on shielding efficiency is shown in the table.

The table shows that when the braiding density increases from 40% to 70%, the shielding coefficient increases by only 5 dB, while the cost of the cable increases. Hence the rule: if there is no difference, why pay more? Perhaps this is the only place where you can save on cable.

The coaxial cable produced by these companies is designed in accordance with the international standard IEC 1196, adopted for radio frequency cable, and is certified by ISO 9001 and 9002, which confirms the quality of the products.

Coaxial cables are the most important passive element in cable television networks. Their quality and reliability significantly affect the service life of cable installations.

• when purchasing a “white cable”, it is a good idea to check the name of the manufacturer (indicated on the cable), and if it is not one of those listed, you need to make sure whether the manufacturer has the appropriate quality certificates;
• It’s hardly worth saving on buying 30 m of cable and buying a fake if you can buy a high-quality cable once and for life;
• You shouldn’t overpay for thick braiding, and if you need increased shielding, then there are special cables for this, but that’s another story...

Next, I would like to delve deeper into a number of problems and issues faced by consumers of coaxial cable. Among many questions, questions about the sheath of coaxial cables arise quite often.

Which shell is better: polyethylene or polyvinyl chloride?

Very often this issue is considered without taking into account the specific operating conditions of the coaxial cable.

These conditions include the following:

• Climatic operating conditions
  This group includes parameters of coaxial cable resistance to non-electrical and non-mechanical influences of the external environment. This is resistance to the effects of high and low temperatures, humidity, solar radiation, and aggressive environments.
• Mechanical operating conditions
  This group includes parameters of coaxial cable resistance to mechanical stress. This is resistance to vibration, linear loads, bends, and dynamic effects of dust.

Polyvinyl chloride plastic compound is most widely used for the sheaths of imported coaxial radio frequency cables. At normal and elevated temperatures, polyvinyl chloride plastic provides greater cable flexibility and ease of installation of connectors than polyethylene.

It is non-flammable and can be white, which improves the appearance of the cable.

However, at elevated temperatures, the plasticizer contained in the shell can migrate into the polyethylene dielectric, significantly increasing its dielectric losses. Global manufacturers of cable products eliminate this drawback by using a special plastic compound with non-migrating plasticizers.

The special plastic compound is based on the use of high-quality primary polyvinyl chloride, which makes it possible to realize all the advantages of this type of shell.

Manufacturers of cheap cables cannot afford to use expensive materials.

The plastic compound used by these manufacturers from recycled materials is significantly inferior to special polyvinyl chloride in a number of parameters. These are high moisture absorption, low resistance to ultraviolet irradiation, low strength and elasticity. All these shortcomings lead to rapid aging of the shell and loss of its protective functions.

As a consequence of these processes, instability of the electrical parameters of the coaxial cable occurs, which often begins to accurately monitor weather conditions by changing its electrical characteristics. Fatigue and a decrease in the mechanical strength of the coaxial cable sheath is most clearly manifested in its transverse breakage during long vertical sags without intermediate fastenings, which is often practiced in our country.

The shell, made of high-quality polyvinyl chloride plastic, has no such disadvantages. Operational parameters are indicated in catalogs, but you cannot demand more from the shell than what is included in it by the manufacturer.

Creating extreme operating conditions for a coaxial cable usually leads to the accumulation of sad experience rather than to stable operation.

Sub-main and distribution coaxial cables with a sheath made of polyvinyl chloride plastic from foreign cable manufacturers are used mainly for installation in rooms and climatic conditions corresponding to the temperature range of this sheath.

In coaxial radio frequency cables intended for primary operation when exposed to low temperatures or sudden changes in temperature, the use of polyvinyl chloride plastic is undesirable.

Polyethylenes of various grades were most widely used for the sheaths of domestic coaxial radio frequency cable.

In fact, in the manufacture of shells, it is not pure polyethylene that is used, but polyethylene compositions, which are a mixture of several modifications of the original polyethylene with the addition of stabilizers. Stabilizers increase the resistance of polyethylene to thermal aging.

In the sheath of a coaxial radio frequency cable, high-density polyethylene (low pressure) is usually used for external installation, and low-density polyethylene (high pressure) for underground installation.

High-density polyethylene is resistant to abrasive wear and provides more reliable protection against mechanical stress.

Since pure polyethylene ages quickly enough in the light and microcracks appear in it, compositions of light-stabilized polyethylene containing at least 2.5% fine soot are used to protect shells from ultraviolet irradiation. Light-stabilized polyethylene is black in color. The percentage of fine soot content in the polyethylene sheaths of the coaxial radio frequency cable from global cable manufacturers is much higher than the generally accepted standard, which allows this coaxial cable to operate stably in the African climate.

The polyethylene shell, in comparison with polyvinyl chloride plastic, has a wider range of operating temperatures and is less critical to sudden temperature changes.

The moisture absorption of a polyethylene shell, compared to a polyvinyl chloride shell, is 20 times less.

The mechanical, operational and technological properties of polyethylene and polyvinyl chloride plastic are presented in a small table:

With the massive arrival of imported coaxial cables with a PVC sheath on our market, the polyethylene sheath was undeservedly forgotten and relegated to the background. The decisive role in this was played by the low electrical characteristics of the domestic coaxial radio frequency cable. Indirectly, these shortcomings also affected the reputation of the polyethylene casing, which, in spite of everything, passed the most important test with honor - the test of time.

The stability of the parameters of the domestic cable, produced 10-15 years ago, is ensured by the quality of the materials used in it and, first of all, the polyethylene sheath, which provided and continues to provide protection for these materials from environmental influences, despite the past years.

In light of the above, the polyethylene sheath of the coaxial radio frequency cable seems to be most preferable for use in the climatic conditions of Russia.

Statements that coaxial RF cable with a polyethylene sheath is difficult to install and that it is impossible to install connectors on it are based on certain gaps in the knowledge of technological techniques and tools used in installation work with coaxial cable.

These gaps are easily removable, and the results obtained from the use of a polyethylene shell pay for the costs of eliminating these gaps.

At low ambient temperatures, the polyethylene-sheathed coaxial cable is kept in a room at room temperature. The installation itself requires certain preparation and installation location in order to minimize the time of exposure to low temperatures on the coaxial cable and the installer. When installing connectors on a polyethylene shell, a tool is used that reduces labor costs and significantly reduces installation time.

The world's leading companies producing cable products carefully monitor trends in the Russian market. Now in the product line supplied, each of them contains a coaxial radio frequency cable of various standards with a polyethylene sheath.

Time has shown that the polyethylene sheath of coaxial radio frequency cable has proven to be in demand in our professional market.

A well-known manufacturer producing cables with these characteristics is Helukabel.
Halogen-free coaxial cables are used to transmit high-frequency signals in various electronic equipment, especially transmitters and receivers, computers, industrial and consumer electronics, where it is necessary to prevent the spread of fire due to fire. The various mechanical, thermal and electrical characteristics of coaxial cables allow them to be used for transmitting signals up to the gigahertz range.

The technical characteristics of the cable are presented below using the links.

Basic parameters of coaxial cable

Impedance - the main indicator that determines the possibility of transmitting signal energy through a cable between the source and the receiver. All elements in the signal path, connectors and the cable itself must have the same impedance. Failure to do so will result in internal reflections in the cable, which may result in double contours appearing in the image. The most common cause of reflections is poor-quality connectors or their incorrect installation, as well as the use of connectors and cables of different impedances.
The standard impedance of video cables is 75 ohms.

Attenuation - indicator of signal energy loss inside the cable. Each cable has its own frequency properties, so the attenuation at different frequencies is also different and the higher the frequency, the greater the attenuation.

Resistance - an indicator of the quality of the conductor, literally showing how much of the signal energy will turn into heat. The result of such losses is a decrease in the signal level, and, accordingly, the dynamic brightness of the image.
Resistance is measured in ohms (?), and is otherwise referred to as direct current resistance or active resistance. For cables, resistance is specified as Ohms per 100 meters (?/100m) or Ohms per 1000 feet (?/1,000 feet) and may also be referred to as linear resistance.
Resistance depends on the conductor material, its size and temperature.
The best cables have chemically pure copper signal conductors or are coated with a thin layer of silver.

Capacity. By design, any coaxial cable is an elongated capacitor. Capacitance is measured in farads (F), and cable capacitance is measured in picofarads per meter (pF/m) or picofarads per foot (pF/ft).
The cable capacitance affects the high-frequency components of the video signal, that is, the clarity and detail of the image. Capacitance is determined by the quality of the dielectric and the design of the cable. This parameter is especially important when transmitting digital signals.

Coaxial cables of all types used for video surveillance systems (drop-in cables, trunk cable, distribution cable, subscriber cable) must have a characteristic impedance of 75 Ohms.
The symbols of domestic coaxial cables according to GOST 11326.0.78 are as follows: RK.W-d-mn-q.
The first two letters (RK) indicate the type of cable - radio frequency, coaxial.
The first number W means the value of the nominal impedance (50, 75, 100, 150, 200 Ohms).
The second number d corresponds to the nominal insulation diameter rounded to the nearest whole number for diameters greater than 2 mm (except for 2.95 mm diameter, which is rounded to 3 mm and 3.7 mm diameter, which is not rounded).
Depending on the insulation diameter, cables are divided into subminiature (up to 1 mm), miniature (1.5-2.95 mm), medium-sized (3.7-11.5 mm) and large-sized (more than 11.5 mm). The nominal diameter of the insulation of the coaxial cable must be equal to one of the following values:
0.15; 0.3; 0.6; 0.87; 1; 1.5; 2.2; 2.95; 3.7; 4.6; 4.8; 5.6; 7.25; 9; 11.5; 13; 17.3; 24; 33; 44; 60; 75 mm.
For connections between equipment, cables from 5.6 to 7.5 mm are mainly used; for trunk connections, cables of 9-13 mm are used. Usually the best is 11.5mm.
The number “m” indicates the insulation group and heat resistance category of the cable:

1-cables with continuous insulation of normal heat resistance;
2-cables with continuous insulation of increased heat resistance;
3-cables with semi-air insulation of normal heat resistance;
4-cables with semi-air insulation of increased heat resistance;
5-cables with air insulation of normal heat resistance;
6-cables with air insulation of increased heat resistance;
7-high heat resistance cables.

The number "n" indicates the serial number of the development.
In some cases, an additional letter (q) is introduced into the symbol:

C - cable of increased homogeneity and phase stability;
G - sealed;
B - has armored cover;
OP - has galvanized steel wires extending over the shell.

For example: RK-75-4-11-S - this means radio frequency, coaxial with a nominal impedance of 75 Ohms, a nominal insulation diameter of 4.6 mm, with continuous insulation of normal heat resistance, development serial number 1, cable of increased homogeneity.

The markings and designations of imported cables are established by international and national standards, as well as the manufacturers’ own standards (the most common series of brands are RG, DG, etc.)

When installing coaxial cables, it is necessary to observe the minimum bending radii (specified in the standard or specifications for cables of different brands).
Thus, for cable RK-75-4-11 the minimum bending radius at t> +5°C is 40 mm, and at t< +5°C - 70 мм.
It is not recommended to bend the cable to a smaller radius. It should also be taken into account that the cable is stretched under the influence of its own weight.
This must be taken into account when laying cables (vertically) and between buildings. It should be secured to the wall (mast) or auxiliary cable every 1-2 m.

When storing air- and semi-air-insulated cables, their ends must be protected from moisture penetration into the cable, and during operation, sealed connectors must be used.

You can splice two pieces of coaxial cable in different ways, including soldering. The simplest method of soldering connection using a wire band is shown in Fig. 3-1. In this case, part of the cable insulation is not restored, which leads to a violation of the wave impedance at the soldering site, in addition, signal losses increase. Therefore, this method of splicing cables is suitable only at radio frequencies of meter waves (up to 200...300 MHz). However, it sometimes has to be used when connecting common-mode antennas, assembling addition filters and other devices.

Rice. 3-1 Splicing coaxial cables using wire bands:
1, 2 - bare installation wire;
3 - soldering of central conductors.

The most common method of splicing cable sections by soldering is at the butt (Fig. 3-2).

Rice. 3-2.Connecting cables using the butt method:
1 - cutting the braid and soldering the central conductors;
2 - restoration of insulation;
3 - overlay of a wire bandage on the braid.

Cutting the ends of cables involves removing their protective sheath, shielding braid, insulation and stripping the cores.
To remove the protective polyethylene and polyvinyl chloride sheath on the cable, a longitudinal and circular cut is made with a special mounting knife.

At each of the assembled ends, the outer sheath is cut into two parts 80 mm long, which are bent in the direction opposite to the end of the cable and temporarily secured. The copper braiding at the ends of the cable is unraveled by 15 mm. The braid strands are bent in the direction opposite to the connection. The unbraided part of the braid is shifted in the same direction. At each end of the cable, 30 mm of insulation is removed from the central wire. Before stripping, the multi-wire central core is unraveled and each wire of the current-carrying cores is cleaned with sandpaper folded into widows.

If the central wire is multi-wire, the inner conductors of the cable ends are connected in a winding. If it is single-wire and thick enough (for example, for a cable brand RK-75-9-12, the diameter of the inner conductor is 1.37 mm), then both ends of the central wire should be cut down to half using a needle file of approximately 10 mm, tinned, and solder, place one on top of the other so that there are no protruding parts.

If the central wires are thin, they can be overlapped by 10 mm (overlapping each other), and then soldered. The soldering area is first coated with a flux made from a solution of rosin in alcohol. It is best to place the soldering area of ​​the central wires in a bath of molten POS-60 solder for 10...15 s. Acid soldering should not be used.

In order not to change the characteristic impedance, it is necessary to restore the internal insulation at the site of the spliced ​​section of the cable (preliminarily made from the internal polyethylene insulation removed from the cable). A longitudinal cut is made in the tube and placed over the soldering area. The seams of the tube and the joints with the insulation are heated until the polyethylene spreads.

At the next stage, the cable braids are spliced. To do this, they are moved again to the ends of the cables. For greater strength, the ends of the braids can be wrapped with several turns of tinned bare mounting wire, and then, after treating the joint with flux, soldering can be done, as shown in the figure.

In certain cases, it is better to place a piece of tin or copper foil with a thickness of 0.1...0.2 mm over the connected section with restored insulation, as shown in Fig. 3-3.

At the last stage, the bent ends of the protective sheath are applied to the braid. If necessary, they are shortened.

To protect against moisture penetration and give strength to the connection along its entire length, it is advisable to tightly wrap it with PVC electrical tape.

Rice. 3-3.Option for splicing coaxial cables.

The manual to RD 78.145-93 indicates the following method for splicing a coaxial cable:

Remove the upper polyethylene sheath from the ends of the cable intended for connection to a length of at least 30 mm from the ends;
unravel the metal braid consisting of thin copper wires at one end of the cable by 20 mm, cut it to the same length at the other end and twist 4 braided copper wires from the loose copper wires and tin them;
- tin the braid of the second end of the cable along the circumference for a length of at least 5 mm (to avoid melting of the polyethylene insulation of the central core, under the braid, it is necessary to put protective insulation made of cable paper in 2 layers);
- remove the central core of the cable from insulation to a length of at least 15 mm;
- twist the central cores of the two cables together and solder.
The length of the exposed layer should be 15 mm;
- cut the removed insulation of the central core, apply it to the junction of the central cores and, straightening it with a soldering iron, seal the junction;
- solder the four tinned bundles to the tinned braid of the second cable symmetrically on all sides;
- put the removed outer insulation, cut lengthwise, onto the finished connection of the two cables and melt it with the main cable insulation using a soldering iron.

When soldering the central core, it must not be allowed to overheat, since this causes displacement and the uniformity of wave resistance is disrupted.
When installing cables and cutting braids, the latter cannot be cut: the braid must be unbraided, twisted into one or two braids and tinned.
When cutting the cable, you must ensure that the central core is not accidentally cut and that the wire braid is not shorted to it.

With such termination of the cable, its homogeneity is practically not disturbed. Otherwise, repetitions and vertical stripes may appear on the screen of the video monitoring device and the noise immunity of the cable may deteriorate.

If the coaxial cable is run parallel to the electrical network, problems arise. The magnitude of the EMF induced in the central core depends, firstly, on the current flowing through the network cable, which, in turn, depends on the load current consumption along this line. Secondly, it depends on how far the coaxial cable runs from the power cable. And finally, it depends on the length of these cables running together. Sometimes proximity within 100 m has no effect, but if a large current flows through the power cable, then even 50 m can affect the quality of the video signal. When installing, try (whenever possible) to ensure that the power and coaxial cables do not run very close to each other. To significantly reduce electromagnetic interference, it is necessary that the distance between them be at least 30 cm.
On the video monitor screen, electrical interference appears in the form of several thick horizontal stripes, slowly sliding up or down. The speed of their movement is determined by the difference between the frequency of the video signal fields and the industrial frequency, and can range from 0 to 1 Hz. As a result, stationary or very slowly moving stripes appear on the screen. Other frequencies appear in the form of different noise patterns - depending on the source of interference. The main rule is that the higher the frequency of the induced unwanted signal, the finer the details of the noise pattern. Periodic disturbances, such as lightning or a passing car, will produce an irregular noise pattern.

Breaking the cable in the middle and sealing the resulting ends will result in some signal loss, especially if the ends are poorly sealed or low-quality BNC connectors are used. A good seal gives a signal loss of no more than 0.3:0.5 dB. If there are not too many of these splices in the cable, then signal loss is negligible.