The coaxial cable is an electric cable consisting of a central copper wire and a metal braid (screen) separated by a layer of dielectric (internal insulation) and placed in a common outer shell (Fig. 3).

Fig. 3. Coaxial cable

The coaxial cable until recently was very popular, which is associated with its high noise immunity (due to a metal braid), broader than in the case of twisted pair, bandwidth (over 1GHz), as well as large transmission transmission (to kilometer). It is more difficult to mechanically connect to unauthorized listening to the network, it also gives noticeably fewer electromagnetic emissions. However, the installation and repair of the coaxial cable is much more complicated than the twisted pair, and its value is higher (it is more expensive about 1.5 - 3 times). Comprehensible and installation of connectors at the ends of the cable. Now it is applied less often than twisted pair. The EIA / TIA-568 standard includes only one type of coaxial cable used in the Ethernet network.

The main use of the coaxial cable finds in networks with the Ti-Pa bus topology. At the same time, at the ends of the cable, it is necessary to install, terminators must be installed to prevent internal reflections of the signal, and one (and only one!) From the terminators must be grounded. Without grounding, the metal braid does not protect the network from external electromagnetic interference and does not reduce the emission transmitted over the network to the external environment. But when grounding braid in two or more locations, not only network equipment can be released, but also computers connected to the network.

Terminators must be necessarily coordinated with the cable, it is necessary that their resistance is equal to the wave resistance of the cable. For example, if a 50-ohm cable is used, only 50-ohm terminators are suitable for it.

Less often coaxial cables are used in networks with a star topology (for example, a passive star in the ARCNET network). In this case, the coordination problem is significantly simplified, since external terminators are not required at free ends.

The wave resistance of the cable is indicated in the accompanying documentation. Most often B. local networks The 50-ohm (RG-58, RG-11, RG-8) and 93-ohm cables are used (RG-62). Distributed 75-ohm cables in local networks are rarely used in the television technique. The brands of the coaxial cable are a bit. It is not considered particularly promising. It is not by chance that the use of coaxial cables is not provided on the FasteTherNet network. However, in many cases, the classic tire topology (and not a passive star) is very convenient. As already noted, it does not require application additional devices - concentrators.

There are two main types of coaxial cable:

Thin (Thin) cable having a diameter of about 0.5 cm, more flexible;

Thick (Thick) cable with a diameter of about 1 cm, significantly more hard. It is a classic alternativelyax cable that is already almost completely supplanted by a modern thin cable.

A thin cable is used to transmit for shorter distances than fat, since the signal in it is hurting stronger. But with a thin cable, it is much more convenient to work: it can be quickly paved to each computer, and thick requires tough fixation on the wall of the room.

Connecting to a thin cable (using BNC connectors Bayonet type) easier and does not require additional equipment. And to connect to a thick cable it is necessary to use special fairly expensive devices, piercing its shells and install contact both with central residential and screen. Thick cable-seeker is twice as thin, so the thin cable is much more used.

As in the case of twisted pairs, an important parameter of the coaxial cable is the type of its outer shell. In the same way, in this case, both Non-Plenum (PVC) and PLENUM cables are applied. Naturally, the Teflon cable is more precious than poly-vinyl chloride. Usually, the type of shell can be distinguished by color (for example, for PVC cable, BELDEN uses yellow color, and for Teflon - orange).

The typical values \u200b\u200bof the signal propagation delay in the coaxial cable are for a thin cable of about 5 ns / m, and for thick - about 4.5 ns / m.

There are variants of a coaxial cable with a double-screen (one screen is located inside the other and separated from it an additional layer of isolation). Such cables have better noise immunity and listening to listening, but they are a bit more expensive than usual.

Currently it is believed that the coaxial cable is outdated, in most cases it may well be replaced by twisted paired fiber optic cable. And new standards on cable systems No longer include it in the list of cable types.

Coaxial cable. What is it?

Probably, you have repeatedly heard such phrases like twisted para, shielded wire and high-frequency signal? So here coaxial cable - This variety twisted coupleBut with a much greater noise immunity, the most suitable conductor for the RF signal.

It consists of central veins (conductor), shielded layer (screen) and two insulating layers.

Internal insulator isolation central residential coaxial cable From the screen, external - to protect the cable from mechanical damage and electrical insulation.

Protection against interference coaxial cable. The cause of the occurrence of noise

What prevents interference in non-coaxial cable

It is necessary to immediately deal with the question of protection against interference. We will analyze the general principles of the nature of their occurrence and influence of interference to the transmission of information.

So, we all know that there are some interference in power lines. They represent bursts and, on the contrary, the disappearance of the nominal (of which should be) voltage in the cable (in the wire). On the graph (the dependences of the voltage in the cable from time) interference look like this:

The cause of interference is the electromagnetic fields from other signals and cables. As we know from the course of school physics, electricity has two components - electric and magnetic. The first is the current of the conductor current, and the second is an electromagnetic field that creates a current.

The electromagnetic field is distributed in an environment in the form of a sphere in infinity. Passing through unprotected (not coaxial) Cable, the electromagnetic signal affects the magnetic component of the electrical signal in the cable and causes interference in it, rejecting the signal voltage from the nominal.

Imagine that we process (read) voltage 10 V with a certain clock frequencyFor example, in 1Hz. This means that we instantly write off the stress testimony in line every second. What happens if it is at the time of reading the interference will strongly dismiss the voltage, for example from 10 volts to 7.4 volts? Right, error, we consider false information! We illustrate this moment:

But we must remember that the voltage is measured from the hull (or from the minus). And the chip is that in electronics (in electronics of high-frequency signals) a large negative role is played precisely high-frequency interferenceAnd here it, in fact, truth: at the moment when the interference acts on central residential coaxial cable, the same interference acts on screen coaxial cableand the voltage is measured from the housing (which is connected to the screen), so the potential difference between on-screen part of the coaxial cable And its central residential remains unchanged.

Therefore, the main task in protection against interference when transmitting the signal is to keep the screen layer or the wire as close as possible to the central and always at the same distance.

What better protects from electromagnetic interference - twisted pair or coaxial cable?

Immediately answer the question. Coaxial cable protects against interference better than twisted para.

IN twisted pare Two wires of suits among themselves and are isolated from each other. The positive wire with bends can be given to a hundred millimeter from a minus, which removes, actually, plus from the case. In addition, the residentials of the positive and minus wire themselves, at the expense of isolation, have a certain gap among themselves. An interference can slip, but the probability is quite small.

IN Coaxial Cable Open Layer In a circle, fully envelops the central core. No interference can not go through the central custody, bypassing the coaxial screen. In addition, the quality of the material from which the coaxial cable is manufactured, according to the requirements of the State Standard, exceeds the quality of materials for twisted par. Point.

Wave resistance of coaxial cables.

Wave resistance

Basic characteristics of the coaxial cable - wave resistance. This is a magnitude, in general, characterizing attenuation signal amplitudes in coaxial cable 1 mongo meter.

It turns out from the expression of private from the voltage of the signal, transmitted by coaxial cabledivided by current wherein voltage in coaxial cable, Merils in Omah.

But most importantly, remember that it characterizes - the attenuation of the transmitted signal. This is the essence of the wave resistance of coaxial cables. Reducing the amplitude of voltage and current - there is a signal attenuation.

In order to plunge into wave resistance of coaxial cables Deeper, you need to know many different concepts about the theory of electromagnetic waves, such as amplitude excluding attenuation, active routine resistance, attenuation coefficient electromagnetic waves in a coaxial waveguide, several permanent electrical values, then build a pair of integral wave graphs and understand that after all, 77 ohms are ideal for Soviet television, 30 ohms is ideal for everything except Soviet television, but the 50 ohm is the Golden Middle between Soviet Television , coax cable and everything else!

But it is better - remember the essence, and the rest - Believe the word)

Standards of wave resistances of coaxial cables:

50 ohms. The most common coaxial Cable Standard. Optimal characteristics According to the transmitted signal power, electrical insulation (plus from minus), the minimum loss of the signal when the radio signal is transmitted.

75 ohm. It was widespread in the USSR in terms of television and video transmission and, it is noteworthy, it is optimally suitable for these purposes.

100 ohms, 150 ohms, 200 ohm. Used extremely rare, in highly specialized tasks.

Also, important characteristics are:

elasticity;
rigidity;
diameter of internal isolation;
screen type;
metal conductor;
degree of shielding.

Have questions? Write in the comments) We will reply!

Coaxial cable is used to transmit a television signal. With the emergence and development of video surveillance systems of different types And the destination cable began to be used to transmit a signal from video cameras to a centralized surveillance complex. For these purposes, both ordinary and modern digital coaxial cable is used. This article will present a small analysis of the species and features of the use of this cable products for video surveillance purposes.

Cable species and features

This cable was invented in 1880 in the UK. Constructive feature Coaxial cable is a combination on one axis of two conductors separated by a layer of dielectric material in a common outer sheath. Initially, the coaxial radio frequency cable was used to transmit a television signal from the antennas of the general use of the public network and individual antenna devices to televisions, as well as in systems of a radio relay, radio transmission, satellite, mobile radio communications. In these areas, it is applied now. Basically, these are powerful cables with a large cross section of the inner conductor and braid in a rigid shell. In video surveillance systems due to the features of the gasket in cramped conditions, flexible cables of smaller cables in a softer braid are used in a large number of cable bends.

Types of cable products

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

Normal coaxial;
Combined (coaxial + 2 wires for connecting the power supply of video cameras and / or transmission of control signals) Cable. A cable with a carrier steel cable for an outer air gasket between buildings is also manufactured.

The use of a combined cable is preferable to many parameters:

The cost of the cable is lower than the price sum of the conventional coaxial cable and the two-housing electrical wire with the same characteristics by section, the material lived, braid and isolation;
Lighter gasket, convenient installation with a smaller number of fasteners and, accordingly, tidy appearanceThat is especially important when laying in office, shopping and other public spaces and buildings.

Marking

The most famous coaxial cable for video surveillance such labels:

RK - Cables produced in Russia;
RG - imported production.

Brands of a combined video cable - KVK is much larger:

KVK-B - in a polyvinyl chloride outer sheath for the inner laying;
KVK-P - in the outer sheath of light-resistant polyethylene for outdoor gasket;
KVK-PT - similar to the manufacture of KVK-P, but has a steel cable for outdoor gasket between buildings and structures;
KKSP and CSSP - for internal and outdoor gaskets, with a single-core internal conductor;
KKSVG and KKSPG are the same cables, but with a stranded internal conductor;
The SVGG is a universal cable, often erroneous for the designation "NG" non-combustible, does not spread combustion with a group laying.

Types of coaxial cable

The main parameters: the total thickness of the cable, the thickness and density of the braid, the cross section of the inner conductor, the materials used in the production of the cable directly affect the wave resistance of the cable. Wave resistance - this electrical characteristic of the cable measured in OM. It shows the possibility and quality of television signal transmission from the camcorder to the receiving device (video recorder with a monitor or personal computerselected as the operator's workplace). In the security video surveillance system, it is recommended to use a cable with the same wave resistance to ensure the quality of the signal, the lack of additional interference and distortion.

Also conditionally divide coaxial cable for general thickness:

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

Composition and cable design

The elements of the coax cable are:

Internal electric conductor or central lived;
A shell of dielectric material;
Screen from double-sided foil. It is not used in all types of cable;
Braid of different density of copper;
Outdoor shell.

Internal conductor make:

From single-core aluminum or copper wire;
Coated wire or aluminum wire copper;
Copper strainer;
Copper wire covered with a layer of silver.

Copper and aluminum are used both in purified form and as their alloys. The internal conductor in the coaxial cable is the main element that serves to transmit the signal. The material of the internal conductor or the central core is easily determined by the external inspection of the cable cutoff: silver color - aluminum or steel with copper coating, completely golden - copper. The more section, the better signal can be transmitted. But we should not forget that the price is directly in proportion to the price of 1 cable meter and increase its rigidity, which is not always acceptable.

The shell from the dielectric material isolates the inner conductor from the braid. It is performed from monolithic or foamed polyethylene or polyurethane. The monolithic material is more suitable when laying through rooms with high humidity, it is better protecting from the pressing, mechanical damage to the central core during squeezing, due to the stiffness, it is limited due to the need for gaskets on corridors and rooms with a plurality of turns, where a flexible cable with insulation from foamed porous material.

The cable braid serves as the second conductor and the grounded screen to protect the central conductor. Sometimes complemented by a metal foil screen. The denser of the braid with a high content of copper wire, the better video signal is provided.

The outer cable shell is protection against external influences. Perched from polyvinyl chloride plastic.

Standard cable products of stamps RK, Rg - coaxial braid cables with insulation from monolithic or porous polyethylene. Single-core or stranded internal copper conductor or copper coated steel wire. External conductor - aluminum foil + copper braid or two silver plated copper braids. The shell is made of plastic that does not spread the burning.

Choosing a coaxial cable

Suitable in all parameters a coaxial cable for video surveillance is selected based on the tasks and conditions when organizing a surveillance system at each specific object of protection. These tasks and conditions are set out in design and estimate documentation, if it is either in the technical task of the customer. In the first case, the cable is selected. In the second variant, which meets much more often, the contractor or the owner, going to mount a video surveillance system on his own, should consider and evaluate several significant parameters:

Distances to selected locations of the camcorder;
The presence of distribution panels, lighting boxes not far from camera installation locations;
Uniformity of the cable laying method to each chamber (internal, outer, air on the cable);
The presence of sources of electrical flooring and electromagnetic interference on the selected cable laying line (power and lighting electrotrassia, electric motors, powerful electrical appliances and other devices that create an electromagnetic field around them), which will lead to loss of video quality;
Requirements for cable products in color, thickness, the possibilities of laying over suspended ceilings, in cable channels, including existing on the object, to maintain the integrity of the interior of the premises;
Need to record a beep.

The selected connectors for the coaxial cable are also important, which allow you to correctly connect the cable line to the camcorder.

Having considered all the conditions and options for laying, make the simplest cable magazine with distinction distances, taking into account the geometry of the gasket, the number of bends.

The determining factor is the length of the coaxial cable for video surveillance, because the cable deployed to each chamber must be performed by a solid 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, humidity in the premises, climatic conditions and air temperature during installation work in the outdoor gasket, the need for transitions between buildings. With this in mind, it is possible to draw conclusions about the possibility and need to apply certain types of cable. Often, various types of cable are used in one system: the usual, combined, on the cable.

In conclusion, it is worth saying that the selection of the cable is very important. But this is just one element of the video surveillance system and achieve the tasks you can only select the entire list of the necessary equipment, which will serve as other articles.

The main parameters of the coaxial cable

Impedance - The main indicator that determines the ability to transmit the signal energy over the cable between the source and the receiver. All items on the signal path, connectors and cable itself must have one impedance. Failure to follow this rule leads to internal reflections in the cable, which can lead to the appearance of double contours in the image. The most common reason for the appearance of reflections is poor-quality connectors or their improper installation, as well as the use of connectors and cables of different impedance.
Standard impedance of video cables is 75 ohms.

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

Resistance - Indicator of the quality of the conductor, literally showing what part of the signal energy will turn into heat. The result of such losses is to reduce the signal level, and, accordingly, the dynamic brightness of the image.
Resistance is measured in Omah (?), And is called otherwise as resistance dC or active resistance. For cables, resistance is indicated as OM 100 meters (? / 100m) or OM per 1000 feet (? / 1,000 FEET) and can also be referred to as the row resistance.
Resistance depends on the material of the conductor, its size and temperature.
The best cables have signaling conductors from chemically pure copper or covered with a thin layer of silver.

Capacity. By design, any coaxial cable is an elongated capacitor. The capacity is measured in the Farades (F), and the cable capacity in the picofarades per meter (PF / M) or in the Picoparades per foot (PF / FT).
The cable capacity affects the high-frequency components of the video signal, that is, to clarity and image detail. Capacity is determined by the quality of the dielectric and the cable design. This parameter is especially important when transmitting digital signals.

Applied to video surveillance systems coaxial cables of all types (reduction cables, trunk cable, switchgear, subscriber cable) should have a 75 ohm wave resistance.
The conditional designations of domestic coaxial cables according to GOST 11326.0.78 has the following form: RK.W-D-MN-Q.
The first two letters (RK) indicate the type of radio frequency cable, coaxial.
The first number W is the value of the nominal wave resistance (50, 75, 100, 150, 200 Ohms).
The second number D corresponds to the nominal diameter of the insulation with a rounded to a smaller nearest integer for diameters more than 2 mm (with the exception of a diameter of 2.95 mm, which is rounded up to 3 mm and a diameter of 3.7 mm, which is not rounded).
Depending on the diameter of the insulation of the cable, subdivided into subminiature (up to 1 mm), miniature (1.5-2.95 mm), the medium-fastest (3.7-11.5 mm) and large-sized (more than 11.5 mm). The nominal diameter of the coaxial cable isolation should be equal to one of the following ranges:
0.15; 0.3; 0.6; 0.87; one; 1.5; 2.2; 2.95; 3.7; 4.6; 4.8; 5.6; 7.25; nine; 11.5; 13; 17.3; 24; 33; 44; 60; 75 mm.
For connections between the equipment, cables from 5.6 to 7.5 mm are used, 9-13 mm cables are used for trunk connections. Usually the best 11.5 mm.
The number "M" denotes a group of insulation and a category of cable heat resistance:

1 cables with solid insulation of conventional heat resistance;
2-cables with solid insulation of high heat resistance;
3 cables with semi-insulation of conventional heat resistance;
4-cables with half-resistant semilation;
5-cables with air insulation of conventional heat resistance;
6-cables with air insulation of increased heat resistance;
7-cables of high heat resistance.

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

C - cable of increased homogeneity and phase stability;
G - hermetic;
B - has armored cars;
OP - has over the shell sash steel galvanized wires.

For example: RK-75-4-11-C is a radio frequency coaxial with a nominal wave resistance of 75 ohms, a nominal insulation diameter of 4.6 mm, with a solid insulation of a conventional heat resistance, the sequence number of development 1, an increased uniform cable.

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

When installing coaxial cables, it is necessary to observe the minimum bend radii (stipulated in the standard or that on cables of different brands).
So, for the cable of RK-75-4-11, the minimum bend radius at T\u003e + 5 ° C is 40 mm, and with T< +5°C - 70 мм.
Figing the cable under a smaller radius is not recommended. It should also be borne in mind that under the action of its own weight, the cable is pulled out.
This must be taken into account when laying a cable (vertically) and between buildings. It should be fixed to the wall (mast) or auxiliary cable every 1-2 m.

When storing cables with air and semi-use insulation, their ends should be protected from moisture penetration inside the cable, and during operation it is necessary to use hermetic connectors.

Combat two segments of the coaxial cable can be different ways including soldering. The easiest way to connect soldering with a wire bandage is shown in Fig. 3-1. In this case, part of the insulation of the cable is not restored, which leads to a violation of the wave resistance at the site of the soldering, in addition, the loss of signal increases. Therefore, such a way to splicing cables is suitable only on radio frequencies of meter waves (up to 200 ... 300 MHz). However, it sometimes has to be used when connecting the syphase antennas, assembling filters of addition and other devices.

Fig. 3-1 Splicing of coaxial cables with a wire bandage:
1, 2 - naked mounting wire;
3 - soldering central conductors.

The most common way to splicing cable segments soldering is to the joint (Fig. 3-2).

Fig. 3-2.Connection of cables in the method of jack:
1 - cutting braids and soldering central conductors;
2 - restoration of isolation;
3 - Lining a wire bandage on a braid.

The cutting of the ends of the cables is to remove the protective shell, shielding braids, isolation and sweeping.
To remove the protective polyethylene and polyvinyl chloride shell on the cable, they make a longitudinal and ring incision to a special mounting knife.

On each of the ends compiled, the outer shell is cut into two parts with a length of 80 mm, which are rejected into the side opposite from the end and is temporarily fixed. The copper braid at the ends of the cable is 100 mm. Braid strands are rejected in the opposite side of the side. The unpleasted part of the braid shifted into the same direction. From each end of the cable from the central wire, an insulation is removed by 30 mm. Before stripping, the multi-breed central custody is broken and each wire of the current-carrying lives are cleaned with sandpaper, folded widow.

If the central wire is multi-way, the internal conductors of the cable ends are connected to hollow. If it is single-wire and quite thick (for example, at the RK-75-9-12 cable, the diameter of the internal conductor is 1.37 mm), then both ends of the central wire should be cut up to half with the help of the appliance at about 10 mm, and soldering overlay one to another so that there are no protruding parts.

If the central wires are thin, they can be folded with 10 mm (enter each other), and then produce a soldering. Pre-site soldering is covered with flux from rosin in alcohol. Place of soldering of central wires is best placed in a bath with a molten solder Pos-60 on 10 ... 15 s. The solder with the help of the acid should not be used.

In order not to change the wave resistance, it is necessary to restore the inner insulation on the site of the spliced \u200b\u200bportion of the cable (pre-manufactured from the internal polyethylene insulation cable). In the tube make a longitudinal cut and put on the place of soldering. Suts of the tube and place of compound with insulation are heated to the spreading of polyethylene.

At the next stage, braid cables are spliced. To do this, they are shifted again to the ends of the cables. The ends of the braids for greater strength can be wrapped with multiple turns tinned naked mounting wire, and then after processing the connection location with a flux, produce a soldering, as shown in the figure.

In certain cases, it is better to impose on top of the connected section with reduced insulation, a part of a tin or copper foil with a thickness of 0.1 ... 0.2 mm, as shown in Fig.3-3.

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

To protect against the penetration of moisture and giving strength to the compound along its entire length, it is advisable to clean the power of the PCV tightly.

Fig. 3-3.The variant of splicing coaxial cables.

The manual for RD 78.145-93 indicates the following way to spoffing coax cable:

Remove from the ends of the cable intended for the compound, the upper polyethylene sheath at a length of at least 30 mm from the ends;
dissolve a metal braid consisting of thin copper wires at one end of the cable by 20 mm, on the other end to trim the same length and from the loose copper wire braid twist 4 harness and fill;
- to end the braid of the second end of the cable around the circumference at a length of at least 5 mm (in order to avoid the melting of the polyethylene insulation of the central core, under the braid, it is necessary to put a protective insulation of cable paper in 2 layers);
- release the central core core from insulation for a length of at least 15 mm;
- twist the central veins of two cables between themselves and solder.
The length of the bare layer must be 15 mm;
- cut the removal insulation of the central core, impose it on the junction of the central lived and, straightening the soldering iron, close the spike;
- soldering irradiated four harnesses to the irradiated second cable braid symmetrically from all sides;
- Wear on the finished connection of two cables removed cut along the outer insulation and melted it with a soldering iron with the main insulation of the cable.

When soldering the central core, it is impossible to allow its overheating, since the displacement occurs and the uniformity of the wave resistance is disturbed.
When installing cables and cutting braids, the latter cannot be cut: the braid must be broken, twisted into one or two pigtails and lead.
Separating the cable, it is necessary to ensure that the central lived accidentally was cut and so as not to close the wire braid on it.

With such an embezzlement of the cable, its homogeneity is practically not violated. Otherwise, repeats may appear on the video monitoring device screen, vertical stripes and deterioration of the cable.

If the coaxial cable is laid parallel to the power grid, problems occur. The magnitude of the EMF induced in the central dwell depends, first, from the current flowing through the network cable, which, in turn, depends on the current consumption current on this line. Secondly, it depends on how far the coaxial cable runs from the power cable. And finally, it depends on what length these cables are running together. Sometimes the neighborhood for 100 m does not have any influence, but if a high current flows on a power cable, then even 50 m can affect the quality of the video signal. When installing, try (always, when it is possible) to make power and coaxial cables not very close to each other. For a tangible decrease in electromagnetic interference, it is necessary that the distance between them amounted to at least 30 cm.
On the screen of the video monitor, the tipping of the power grid is of a few fatty horizontal stripes, slowly sliding up or down. The speed of their movement is determined by the difference between the frequency of the video fields and the industrial frequency, and can be from 0 to 1 Hz. As a result, motionless or very slowly moving strips appear on the screen. Other frequencies are manifested in the form of various noise paintings - depending on the source of the press. The main rule is that, the higher the frequency of the induced unwanted signal, the thinner the details of the noise picture. Periodic tips, like a zipper or a passing car, will give an irregular picture of noise.

The cable break in the middle and sealing the resulting ends will lead to some signal lossEspecially if the ends are chosen badly or used low-quality BNC connectors. Good seal gives a signal loss not more than 0.3: 0.5 dB. If there are not too many similar splices in the cable, then the signal loss is insignificant.

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

communication systems;
broadcast networks;
computer networks;
antenna feeder systems;
ACS and other manufacturing and research technical systems;
systems remote control, measurement and control;
alarm and automation systems;
objective control systems and video surveillance;
communication channels of various radio electronic devices of mobile objects (ships, aircraft, etc.);
intublock and inter-block connections in the composition of radio-electronic equipment;
communication channels in household and amateur technology;
military equipment and other areas of special application.

Device

Coaxial cable (see Figure) consists of:

A - shells (serves to isolation and protection against external influences) from light stabilized (that is, resistant to ultraviolet radiation of the Sun) of polyethylene, polyvinyl chloride, oxiva fluoroplastic tape or other insulating material;
B is an external conductor (screen) in the form of braid, foil coated with a layer of film aluminum and their combinations, as well as a corrugated tube, observing metal ribbons, and other copper, copper or aluminum alloy;
C - insulation made in the form of solid (polyethylene, foamed polyethylene, solid fluoroplastic, fluoroplastic tape, etc.) or semi-separated (Caddle-tubular hanging, washers, etc.) of dielectric filling, ensuring the constancy of the mutual position (alignment) of internal and external conductors;
D is an internal conductor in the form of a single straight line (as in the figure) or a pig in the spiral of the wire, a multi-moist wire, tubes performed from copper, copper alloy, aluminum alloy, coched steel, copper-mounted aluminum, silver plated copper, etc.

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

There are several common misconceptions about the coaxial cable.

Widespread misconception that all white cables are good

Not all white cables are high-quality, and not all high-quality cables - white! This delusion is based on the external similarity of cheap cables with products of leading global manufacturers. The main differences in high-quality cables from fakes are physically foamed dielectric with gas injection and double foil (foil - polyester - foil) as a solid screen. The physically foamed dielectric is the structure of isolated cells filled with gas. It does not absorb water and more resistant to mechanical exposure. The dielectric constant of such material is close to the ideal and preserved for 15 years and more, and therefore, the losses in the cable as a result of aging are close to the initial one.

Since cheap cable manufacturers cannot afford expensive technologies, they use a chemically foamed dielectric. It as a sponge absorbs moisture with a damaged outer sheath and is sensitive to external mechanical effects. In addition, as a result of aging, losses increase in it (Fig. 1). Also in cheap cables do not apply a double foil (and only single) as the main screen, which reduces the shielding effect and makes the cable sensitive to external interference (radio suites, SENAO, etc.). Therefore, such a cable cannot be used in interactive networks with reverse channel. If a copper braid is used in dubious cables (a soldering cable), then in high-quality cables, a braid of copper tinned is used. The combination of "tin - aluminum" is more preferable in comparison with "copper - aluminum". That is, if the outer cable shell is damaged or a leakage, the moisture enters the outer conductor, and as a result of the electrochemical reaction, the aluminum foil is destroyed. This leads to a significant reduction in the shielding properties of the cable.

the operational characteristics of cheap cables are worsen over time;
the shielding properties of such cables are lower than that of high-quality cables of world manufacturers;
although cheap cables have characteristics better than the domestic cable RK75-4-11, they should not be used in networks where it is planned to use the reverse channel. The scope of these cables - non-silent cable wiring with a high signal level, if there are no special scrapping requirements.

Unreasonable exaggeration of the importance of secondary braid

There is an opinion that the thick braid, the better the cable. This is not quite so! That to low losses in the cable ... they say, the thick of the braid - the less losses! Indeed, the attenuation in the coaxial cable is made up of losses in the conductors, losses in dielectric and emission losses. The latter parameter is selected separately and characterizes the screening efficiency.

Therefore, let's start in order:

Losses in conductors depend on the frequency of the signal, due to the reduction of the skin-layer thickness and the corresponding decrease in conductivity. Use in high-quality copper cables in the central conductor coating layer or for the entire central conductor allows you to reduce the overall attenuation in the cable.
Dielectric losses also depend on the frequency of the signal. Dielectric loss power is spent on the reorientation of the dielectric molecules in the RF field. With an increase in the dielectric permeability of the material, the loss power also grows. Application as a dielectric physically foamed (and not solid) polyethylene reduces the amount of losses in the dielectric. Under the physically foamed dielectric, we understand foaming with gas injection. In this case, the micropores filled with inert gas (nitrogen) micropores are created in the dielectric. This is the structure and provides low dielectric losses and ensures its stability over the years of operation. The use of such a dielectric in Cavel cables provides a decrease in parameters as a result of aging by only 5%, and in BELDEN cables - by 1%. In cables, where for reasons for saving such technology does not apply, the parameters are reduced by 50 ... 70%. Hence the rule: we are not so rich to buy cheap things!
The screening efficiency determines the relative power level emitted by the ether cable and, at the same time, the degree of cable protections from external interference. The shielding coefficient (expressed in decibels) is defined as the ratio of the power of the external interference signal to the power generated by this interference in the cable.

The high degree of shielding in cables is achieved through the use of a two-layer combined screen - aluminum foil and braids from twisted conductors. As the first screen, a polystyrene tape, laminated on two sides by aluminum, is used as a second layer, and braids are used from the mug of copper - CUSN or aluminum Al (which concerns high-quality cables). So this is this first layer and performs basic shielding functions. To all other things, the shielding properties of copper are higher than that of aluminum, therefore, where 40% of copper is enough, you need 80% aluminum! In other words, the same cables, but with different braid density, for example 40% and 80%, will have the same attenuation.

For cheap three-layer cables (Al-film-AL), the first screen is a non-disabilities. At best, a foil with a polyester substrate is used, and usually aluminum sprayed onto the substrate. That's where the thick braid is simply necessary! But, alas, "the economy should be economical." Hence the rule: free cheese only in a mousetrap.

As for increased strength ... If the cables are stretched in the gasket process or there are long checks (stretching under the action of their own weight), then in such cases the central lived from steel plated with copper is used. And in such cables, it is the steel central core that serves as a hardening element, and not a braid, even the most thick. By the way, the quality of the plated layer is also a very important question, because we remember about the skin effect!

And directly on the shielding: the main shielding functions performs a foil layer (in high-quality cables), and the braid plays a secondary shielding function and is more intended for transmitting current, as well as the flexibility of the cable. That is, the more the density of the braid, the greater the current can be conveyed (for example, with remote power of amplifiers). The effect of braid denotation on shielding efficiency is shown in the table.

From the table, it can be seen that with an increase in the flue density with 40% to 70%, the shielding coefficient increases only 5 dB, and the cost of the cable increases. Hence the rule: if there is no difference, why pay more? Perhaps this is the only one where you can save on the cable.

The coaxial cable released by the listed firms was developed in accordance with the international standard IEC 1196 adopted for the radio frequency cable, and has an ISO 9001 and 9002 certificate, which is confirmed by product quality.

Coaxial cables are the most important passive element in networks. cable television. Their quality and reliability significantly affect cable layout.

when buying a "white cable", it is not bad to clarify the name of the manufacturer (specified on the cable), and if it is not one of those that are listed, you need to make sure whether the manufacturer has the appropriate quality certificates;
it is hardly worth saving on the purchase of 30 m cables and buy a fake, if you can buy a quality cable once and for life;
it is not necessary to overpay for a thick braid, and if you need increased shielding, then there are special cables for this, but this is another story ...

Next, I would like to continue to touch a number of problems and issues faced by consumers of a coaxial cable. Among the many questions, quite often questions arise on the coaxial cable sheath.

What is the shell better: polyethylene or polyvinyl chloride?

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

These conditions include the following points:

Climatic operating conditions
This group includes the resistance parameters of the coaxial cable to non-electrical and non-mechanical effects of the external environment. It is resistance to the effects of increased and reduced temperature, humidity, solar radiation, aggressive media.
Mechanical operating conditions
This group includes the parameters of the coaxial cable resistance to mechanical effects. This is resistance to vibration, linear loads, flexibations, dynamic dust effects.

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

He 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 to a polyethylene dielectric, significantly increasing the dielectric losses in it. This disadvantage of global cable manufacturers eliminate the use of special plastic with non-making plasticizers.

The basis of special plastic is the use of high-quality primary polyvinyl chloride, which allows you to realize all the advantages of the shell of this type.

Manufacturers of a cheap cable cannot afford the use of expensive materials.

As used by these manufacturers of plastic from secondary raw materials for a number of parameters is significantly inferior to special polyvinyl chloride. This is high moisture absorption, low resistance to ultraviolet radiation, low strength and elasticity. All these shortcomings lead to a rapid aging of the shell and losing her protective functions.

As a result of these processes, the instability of the electrical parameters of the coaxial cable occurs, which often begins to precisely track weather conditions by changing its electrical characteristics. The fatigue and reduction of the mechanical strength of the coaxial cable shell is most pronounced in its transverse cliff with long vertical vertices without intermediate fasteners, which is often practiced with us.

In the shell made of high-quality polyvinyl chloride plastic, there are no such disadvantages. Operational parameters are indicated in the catalogs, but it is impossible to demand from the shell more that it is laid by the manufacturer.

The creation of extreme conditions of operation of the coaxial cable leads, as a rule, to the accumulation of sad experience, and not to stable operation.

Submalt and distribution coaxial cables with a shell from a polyvinyl chloride plastic of foreign manufacturers of cable products are mainly used for laying indoors and climatic conditions corresponding to the temperature range of this shell.

In coaxial radio frequency cables intended for preferential operation when exposed to low temperatures or with a sharp change of temperature, the use of polyvinyl chloride plastic is undesirable.

Polyethylene various brands are most widely used for the shells of a domestic coaxial radio frequency cable.

In fact, the manufacture of shells uses non-pure polyethylene, but polyethylene compositions representing a mixture of several modifications of the initial polyethylene with the addition of stabilizers. Stabilizers increase the resistance of polyethylene to thermal aging.

In the coaxial radio frequency cable for external gasket, as a rule, high density polyethylene (low pressure) is used, for underground gasket - low-density polyethylene (high pressure).

Highly cute polyethylene racks to abrasive wear and provides more reliable protection against mechanical effects.

Since pure polyethylene quickly raises the light and microcracks appear in it, the compositions of light-stabilized polyethylene containing at least 2.5% small soot are used to protect the shells against ultraviolet irradiation. Light-stabilized polyethylene has a black color. The percentage of fine-dispersed soot in the polyethylene shells of the coaxial radio frequency cable of world cable manufacturers is much higher than the generally accepted standard, which allows this coaxial cable to work consistently in the climate of Africa.

The polyethylene shell, compared with the polyvinyl chloride plastic, has a wider range of operating temperatures, less critical to the sharp temperature drop.

Moisture absorption of the shell of polyethylene, compared with a polyvinyl chloride sheath, less than 20 times.

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

With a massive arrival on our market of imported coaxial cable with a polyvinyl chloride plastic shell, a polyethylene shell turned out to be undeservedly forgotten and moved to the background. A crucial role in this played low electrical characteristics of a domestic coaxial radio frequency cable. Indirectly, these disadvantages affected the reputation of a polyethylene shell, which, in spite of everything, with honor withstood the most important exam - the time check.

The stability of the parameters of the domestic cable released 10-15 years ago is ensured by the quality of the materials used in it and, first of all, the polyethylene shell, which provided and ensures the protection of these materials from the effects of the external environment, despite the past years.

In the light of the above, the polyethylene coaxial radio frequency cable is preferable to use in the climatic conditions of Russia.

Applications that a coaxial radio frequency cable with a plastic shell is difficult to lay that it is impossible to establish connectors, they are based on their specific gaps in the knowledge of technological methods and tools used in assembly work with a coaxial cable.

These spaces are easily eliminated, and the results obtained from the use of a polyethylene shell payback to eliminate the data of spaces.

With low ambient temperature, the coaxial cable in the polyethylene shell is maintained indoors with room temperature. Mounting itself requires a certain preparation, the installation site, to a minimum to reduce the exposure time for a low temperature on a coaxial cable and installer. When installing connectors on a polyethylene sheath, a tool is used to reduce labor costs and significantly reduce the installation time.

Leading world companies producing cable products carefully monitor trends russian market. Now in the line of products supplied, each of them is a coaxial radio frequency cable of various standards with a polyethylene sheath.

The time has shown that the polyethylene shell of the coaxial radio frequency cable turned out to be in demand by our professional market.

A well-known manufacturer producing a cable with data characteristics is Helukabel.
Non-containing halogen coaxial cables are used to transmit high-frequency signals in various electronic equipment, especially in transmitters and receiver, computers, in production and consumer electronics, where it is necessary to avoid the spread of fire as a result of ignition. Various mechanical, temperature and electrical characteristics of coaxial cables allow you to use them to transmit signals up to a gigahertovoy band.

The technical characteristics of the cable are presented below by reference.