There is hardly a person who has never seen a coaxial cable. How it works, what are its advantages, what are its areas of application - many have yet to figure this out.
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How coaxial cable works
The coaxial cable consists of:
- inner conductor (central core);
- dielectric;
- outer conductor (braid);
- outer cover.
If we consider the cable in cross section, we can see that both of its conductors are located on the same axis. Hence the name of the cable: in English coaxial - coaxial.
The inner conductor in a good cable is made of copper. Now cheap products use aluminum or even copper-plated steel. The dielectric in a high-quality cable is polyethylene, and in high-frequency cables it is fluoroplastic.In inexpensive options, various foamed plastics are used.
The classic material for braiding is copper, and the braiding of quality products is carried out with a dense weave, without gaps. In lower quality cables, copper alloys, sometimes steel alloys, are used to make the outer conductor, rare weaving is used to reduce the cost, and in some cases foil.
Scope of coaxial cable, its pros and cons
The most common use of a coaxial cable is to transmit high-frequency currents (RF, microwave and higher). In many cases this is done communication between antenna and transmitter or between the antenna and the receiver, as well as in cable television systems. Such a signal can also be transmitted using a two-wire line - it is cheaper.
In some cases, this is done, but such a line has a serious drawback - the electric field in it passes through open space, and if a third-party conductive object gets into it, this will cause signal distortion - attenuation, reflection, etc. And for a coaxial cable, the electric field is completely inside, so when laying, you don’t have to worry that the line will pass by metal objects (or they may subsequently be in close proximity to the cable) - they will not affect the operation of the transmission line.
The disadvantages of coaxial cable include its high cost. Also a disadvantage is the high complexity of repairing a damaged line.
Previously, coaxial cables were widely used for organizing data transmission lines in computer networks. Today, transmission rates have risen to levels that RF cable cannot provide, so this application is rapidly phasing out.
The difference between coaxial cable and armored cable and shielded wire
Often coaxial cable is confused with shielded wire and even armored power cable. If there is a certain external similarity of the design (“core-insulation-metal flexible casing”), their purpose and principle of operation are different.
In a coaxial cable, the braid acts as a second conductor that completes the circuit. A load current necessarily flows through it (sometimes even on the inner and outer sides is different). The braid may have contact with ground for safety purposes, it may not have it - this does not affect its operation. It is also incorrect to call it a screen - it does not carry a global screening function.
For an armored cable, the outer metal braid protects the insulating layer and the core from mechanical stress. It has high strength, and it is always grounded according to safety requirements. In normal mode, no current flows through it.
In a shielded wire, the outer conductive sheath is designed to protect the conductor from external interference. If it is necessary to protect against low-frequency interference (up to 1 MHz), then the screen is grounded only on one side of the wire. For interference above 1 MHz, the screen serves as a good antenna, so it is grounded all the way at several points (as often as possible). In normal mode, no current should flow across the screen either.
Technical parameters of coaxial cable
One of the main parameters that you need to pay attention to when choosing a cable is its characteristic impedance. Although this parameter is measured in ohms, it cannot be measured with a conventional tester in ohmmeter mode, and it does not depend on the length of the cable segment.
The wave impedance of a line is determined by the ratio of its linear inductance to the linear capacitance, which, in turn, depends on the ratio of the diameters of the central core and the braid, as well as on the properties of the dielectric. Therefore, in the absence of devices, you can “measure” the wave resistance using a caliper - you need to find the diameter of the core d and braid D, and substitute the values into the formula.
Here also:
- Z is the desired wave resistance;
- Er - dielectric permittivity of the dielectric (for polyethylene, you can take 2.5, and for foamed material - 1.5).
The resistance of the cable can be anything with reasonable dimensions, but products are standardly produced with the following values:
- 50 ohm;
- 75 ohm;
- 120 Ohm (quite a rare option).
It is impossible to say that a 75 ohm cable is better than a 50 ohm cable (or vice versa). Each must be applied in its place - the characteristic impedance of the transmitter output Zand, communication lines (cables) Z and load should be the same Zn, only in this case the transfer of energy from the source to the load will occur without losses and reflections.
There are certain practical limitations on the manufacture of cables with high impedance. Cables of 200 ohms and above must be very thin stranded or have a large outer conductor diameter (to maintain a large D/d ratio).Such a product is more difficult to use, therefore, for paths with high resistance, either two-wire lines or matching devices are used.
Another important coax parameter is damping. Measured in dB/m. In general, the thicker the cable (more precisely, the larger the diameter of the central core), the less the signal attenuates in it with each meter of length. But this parameter is also affected by the materials from which the communication line is made. Ohmic losses are determined by the material of the central core and braid. Dielectric losses contribute. These losses increase with increasing signal frequency; special insulating materials (PTFE, etc.) are used to reduce them. Foamed dielectrics used in inexpensive cables contribute to increased attenuation.
Another important characteristic of coaxial cable is velocity factor. This parameter is needed where it is necessary to know the length of the cable in the wavelengths of the transmitted signal (for example, in resistance transformers). The electrical length and the physical length of the cable do not match because the speed of light in vacuum is greater than the speed of light in the cable's dielectric. For cable with polyethylene dielectric Kreproach=0.66, for fluoroplastic - 0.86. For cheap products with foam insulator - unpredictable, but closer to 0.9. In foreign technical literature, the value of the deceleration coefficient is used - Kslowed down=1/Kreproach.
Also, the coaxial cable has other characteristics - the minimum bending radius (depends mainly on the outer diameter), the dielectric strength of the insulator, etc. They are also sometimes needed to select a coax.
Coaxial cable marking
Domestic products had alphanumeric marking (it can be found even now). The cable was designated by the letters RK (radio frequency cable), followed by numbers indicating:
- wave resistance;
- cable thickness in mm;
- Catalogue number.
Thus, the RK-75-4 cable denoted products with a wave impedance of 75 ohms and an insulation diameter of 4 mm.
The international designation also begins with two letters:
- RG RF cable;
- DG - cable for digital networks;
- SAT, DJ - for satellite broadcasting networks (high-frequency cable).
Next comes the figure, which obviously does not carry technical information (to decrypt it, you will have to look into the cable passport). Further there may be more letters denoting additional properties. An example of designation - RG8U - a 50 Ohm RF cable with a reduced diameter of the central core and a reduced braid density.
Having understood the differences between coaxial cable and other cable products and having learned the influence of its parameters on performance, you can successfully use this product in the areas for which it is intended.
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