The most common type of antenna feeder used today is the coaxial RF cable. It is convenient and provides a good level of performance. For this reason, a huge number of coaxial cables are available for various purposes.
Areas of use
Cables of this type are used where it is necessary to transmit radio frequency electrical signals. The most obvious area of their use is broadcasting images to home TVs, but they are in demand in many other areas. Coaxial cables are also used for video surveillance and for connecting receivers and antennas to commercial and industrial consumers. They are needed where you want to transmit any high-frequency signals over a distance.
Coaxial cable design minimizes loss and interference. Therefore, they are widely used in networking. For example, digital coaxial cables were used in early forms of local Ethernet networks, although today they have been replaced by optical fibers that provide higher data transfer speeds and cheaper twisted pairs when the signal frequency is not so high.
History of creation
RF coaxial cable is an important part of modern electronics. Its first known implementation appeared in 1884, when Ernst von Siemens (one of the founders of Siemens) patented his idea, but at that time there were no applications for it. Only in 1929, Bell Laboratories patented modern commercial coaxial cables, although their use was relatively small. For example, they were used in 1934 to transmit television images from the Berlin Olympics to Leipzig. In 1936, a coaxial cable for 40 telephone connections was made between London and Birmingham, and an experimental line for transmitting television images was created in the USA, between New York and Philadelphia.
After the start of commercial use, other applications were discovered that have proven themselves well and are widely used in business and home.
What is coaxial cable?
Outwardly, it looks like a thick electric wire. It is made of parts that provide low-loss radio frequency signals. Its main elements are:
- central conductor;
- insulating dielectric;
- external conductor;
- outer protective shell.
The center conductor is almost always made of copper. Sometimes copper or aluminum alloy is used. It can consist of one or several wires.
An insulating dielectric separates the conductors and is one of the main causes of signal attenuation. It can be solid or semi-air. It is made in the form of long tubes made of polyethylene or fluoroplastic, or foam, most of which is air.
The outer conductor is usually a copper braid. This gives the coaxial twisted cable ample flexibility. To improve shielding, 2 or even 3 layers are sometimes used. This is usually achieved by placing one braid directly above another, although in some cases copper foil or tape may be used. When using additional shielding layers, the levels of interference and radiation are significantly reduced.
The outer protective sheath prevents the formation of spurious feedback. It also provides protection against penetration of dirt and moisture, prevents cable damage by other mechanical means.
Principle of operation
Coaxial cable conducts current through both internal and external conductors. These currents are equal and have opposite polarity, as a result of which all fields remain in the cable, are not radiated beyond its limits and are not subject to interference. Therefore, no external objects affect the fields. Thus, the coaxial feeder is ideal for installation inside buildings or near them or other objects. This is its main advantage compared, for example, with twisted pair.
When choosing the type of coaxial cable, its specifications and parameters must be taken into account.
Impedance
The most important characteristic of a coaxial cable is wave impedance, which is determined by its diameter and dielectric material. The parameter is measured in ohms. The following values are most common:
- 50 ohm coaxial cable. Provides minimal signal loss for a given conductor weight. Commonly used for professional purposes.
- 75 ohm coaxial cable. It has a minimum weight for a given level of losses. It is widely used in home television and Hi-Fi equipment.
- A 93-ohm cable was previously used to connect computers and connect monitors.
Other wave impedance values are available, but they are much less in demand.
Attenuation
Another important parameter of a coaxial twisted cable is its signal transmission loss. Their level depends on a number of factors, including the quality of the materials of the dielectric and the conductor. Signal attenuation is proportional to the cable length. Losses in decibels per unit length are usually indicated in specifications. For example, the RG-6A / U coaxial cable attenuation at 100 MHz is 1.0 dB / 10 m. Unfortunately, not all manufacturers use the same length, so additional calculations may be necessary for comparison.
Maximum power
Although this specification is not very important for low signal applications, it can be a problem when transmitting high power. Typically, restrictions arise due to heat loss in the cable. If high power transmission is expected, the operating voltage must be checked.
Shortening factor
It is the ratio of the signal transmission speed in a coaxial cable to its speed of propagation in vacuum (speed of light).
In some cases, the value of the shortening factor can be of great importance, for example, when the phase of the signal is important. It is always less than 1. It is largely determined by the material of the dielectric. For digital coaxial cables with a solid polyethylene dielectric, the shortening factor is 0.66, and with polyethylene foam - in the range of 0.80-0.88.
Capacitance
Since the inner and outer conductors form a capacitance, it is proportional to their diameters, cable length, and dielectric constant.
Maximum voltage
In some cases, the voltage reaches high values. Because of this, the cable may fail. The voltage rises as a result of a high level of standing waves and high power. Before choosing a specific type of coaxial cable, you need to check how it can withstand the expected voltage.
Physical dimensions
This characteristic is important for a number of reasons. Larger diameter conductors often have lower losses and higher rated power. For example, the popular 75 ohm coaxial cable RG-6 / U has an outer diameter of 6.86 mm.
In addition, the cable must match the size of the holes and fit the plugs of the desired type. Often the latter are made to fit the sizes of popular types of cables.
Installation guide
A radio frequency coaxial cable is widely used to transmit a signal from a transmitter or receiver. Although it is very easy to install so that it lasts a long time, certain conditions must be observed. This is especially important because it is often installed outside and must withstand the harsh environmental conditions.
Factors such as moisture penetration over time can degrade its effectiveness. Such a reduction goes unnoticed until it reaches a point where it will be impossible to use. A few simple precautions will keep the coaxial cable efficient and significantly slow down its degradation.
Selection order
There is a huge variety of coaxial cables on the market, and at first glance the choice may seem difficult. The first decision that needs to be made is to choose the appropriate impedance. Home Hi-Fi and video antennas use 75 ohm coaxial cable. Professional users and hams select a 50 ohm impedance as standard.
The next decision to make is an acceptable level of signal attenuation. The smaller the loss, the greater the diameter of the cable, as well as its cost. As a rule, you can find several brands with the same characteristics, and often the decision on a particular type will depend on the position of suppliers.
Once a suitable cable is found, it can be purchased and installed.
Weather protection
For outdoor installation, for example, a coaxial cable for video surveillance, it is very important to ensure its proper protection. This is of great importance, because any moisture will lead to a significant increase in losses. If the dielectric separating the internal and external conductors gets wet, this will degrade its characteristics and increase the attenuation. Humidity also leads to oxidation of the braid and a decrease in its conductivity.
Therefore, it is so important to seal the end of the cable, if it is used outdoors, and to prevent moisture from entering. It is necessary to ensure that the outer shell remains intact and is not damaged during installation and further operation.
An additional method of preventing large amounts of moisture from entering the cable is to loop up and down. This prevents the water that has penetrated inside from moving on. However, moisture will still spread in a capillary manner, so it is always best to make sure that the ends are properly sealed and protected.
General installation recommendations
All cables have a bend radius. To prevent damage, they should not bend more than this value. Otherwise, the internal structure may be damaged and the level of losses will increase significantly.
You should also take care that the cable is not deformed. This will lead to a change in its size and wave resistance. In addition, damage to the dielectric can increase the level of losses.
If the cable is physically damaged, make sure that its sheath remains intact. If its integrity is violated, this can lead to moisture penetration, oxidation of the wire and wetting of the dielectric, which will increase the level of losses.
In some cases, the coaxial cable needs to be buried. Conventional stamps cannot be laid in the ground, since their shell is not designed for these conditions. But they can be laid in the channel, specially designed for this. This has the advantage that the cable is easy to replace. However, make sure that no water enters the canal. An alternative is the use of a cable specially designed for laying in the ground, the sheath of which can withstand such conditions.
Termination
When installing a coaxial cable, it is important to terminate it correctly. In most cases, the physical termination is a plug, and the end device is either an antenna or a receiver. Connections must be made correctly and efficiently.
Although home connectors often have poor RF characteristics, there are few alternatives. For professional needs, higher quality connections are needed, although in this case you should also make sure that they are suitable for the frequencies used. Some cheap models do not meet the requirements and impair the operation of the coaxial cable. Therefore, it is better to purchase connectors from trusted sellers.
Correct installation of coaxial cable will ensure many years of its service. However, wear and tear and environmental influences will lead to the need for replacement after some time. Since performance degradation is slow, you may not notice it. It becomes apparent only after a complete cable replacement.