The most common type of antenna feed in use today is RF coaxial cable. It is comfortable and provides a good level of performance. For this reason, a huge number of coaxial cables are produced for various purposes.
Uses
Cables of this type are used where it is necessary to transmit radio frequency electrical signals. Their most obvious area of use is broadcasting images to home televisions, but they are in demand in many other areas. Coaxial cables are also used for video surveillance and connection of receivers and antennas by commercial and industrial consumers. They are needed where it is required to transmit any high-frequency signals over a distance.
The design of coaxial cables minimizes loss and interference. Therefore, they are widely used in the organization of network connections. For example, digital coaxial cables were used in earlyforms of Ethernet local area networks, although today they have been replaced by optical fibers for faster data transfer rates and cheaper twisted pairs when the signal frequency is not as high.
History of Creation
RF coaxial cable is an important part of modern electronics. The first known implementation of it 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. It was not until 1929 that Bell Laboratories patented modern commercial coaxial cables, although their use was relatively small. For example, they were used in 1934 to transmit a television image from the Berlin Olympics to Leipzig. In 1936, a coaxial cable for 40 telephone connections was laid between London and Birmingham, and in the USA, between New York and Philadelphia, an experimental line was created to transmit television images.
Following the start of commercial use, other applications have been discovered that have proven themselves and are widely used in business and at home.
What is a coaxial cable?
It looks like a thick electric wire. Manufactured from parts that provide low-loss RF signal transmission. Its main elements are:
- center conductor;
- insulating dielectric;
- outer conductor;
- outer containment.
The center conductor is almost always made of copper. Sometimes a copper or aluminum alloy is used. May consist of one or more wires.
Insulating dielectric separates conductors and is one of the main causes of signal attenuation. May be solid or semi-air. It is carried out 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 provides the coaxial twisted cable with sufficient flexibility. Sometimes 2 or even 3 layers are used to improve shielding. This is usually achieved by placing one braid directly over the other, although copper foil or tape may be used in some cases. With the use of additional shielding layers, interference and radiation levels are significantly reduced.
The outer protective shell prevents the formation of parasitic feedback. It also provides protection against the ingress of dirt and moisture, and prevents damage to the cable by other mechanical means.
Working principle
Coaxial cable conducts current through both the inner and outer conductors. These currents are equal and have opposite polarity, as a result of which all fields remain in the cable, do not radiate beyond it and are not subject to interference. Therefore, no external objects affect the fields. Thus, the coaxial feeder is ideal for installation inside or near buildings or other objects. This is its main advantage compared to, for example, 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 its impedance, which is determined by its diameter and dielectric material. The parameter is measured in ohms. Its most common meanings are:
- 50 ohm coaxial cable. Provides minimal signal loss for a given conductor weight. Usually used for professional purposes.
- 75 ohm coaxial cable. It has a minimum weight for a given level of losses. Widely used in home TV and Hi-Fi equipment.
- The 93 ohm cable was previously used to link computers and connect monitors.
Other values of impedance are available, but they are much less in demand.
Fade out
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 dielectric and conductor materials. Signal attenuation is proportional to cable length. Specifications usually state loss in decibels per unit length. For example, RG-6A/U coaxial cable has an attenuation of 1.0 dB/10 m at 100 MHz. Unfortunately, not all manufacturers use the same length, so additional calculations may be needed for comparison.
Maximum allowable power
Although this specification is not very important for low signal applications, it can become a problem when transmitting high power. Typically, limitations arise due to heat losses in the cable. If high power transmission is expected, check the operating voltage.
Shortening factor
This is the ratio of the signal transmission speed in a coaxial cable to the speed of its propagation in a vacuum (the speed of light).
In some cases, the value of the velocity factor can be of great importance, for example, when the phase of the signal is important. It is always less than 1. In many respects, it is 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.
Capacity
Because the inner and outer conductors form capacitance, it is proportional to their diameters, cable length, and dielectric constant.
Max voltage
In some cases, the voltage reaches high values. Because of this, the cable may fail. The voltage rises as a result of the high level of standing waves and high power. Before choosing a specific type of coaxial cable, it is necessary to check how it withstands the expected voltage.
Physical dimensions
This characteristic is important for a number of reasons. conductorslarger diameters often have lower losses and higher power ratings. For example, the popular 75 ohm RG-6/U coaxial cable has an outer diameter of 6.86 mm.
In addition, the cable must match the available hole sizes and match the correct type of plug. Often the latter are made to fit popular cable types.
Installation Guide
RF coaxial cable is widely used to transmit signal from transmitter or receiver. Although it is very easy to install, in order for it to last for a long time, certain conditions must be met. This is especially important because it is often installed outdoors and must withstand harsh environmental conditions.
Things such as moisture can degrade its effectiveness over time. Such a decrease goes unnoticed until it reaches the point where it becomes unusable. A few simple precautions will keep your coax cable efficient and significantly slow down cable degradation.
Select order
There is a huge variety of coaxial cables on the market, and at first glance, the choice may seem difficult. The first decision to be made is to choose a suitable impedance. Home Hi-Fi and video antennas use 75 ohm coaxial cable. Professional users and radio amateurs typically select 50 ohm impedance.
The next decision to make is the level of signal attenuation that is acceptable. Howless loss, the larger the diameter of the cable, as well as its cost. Typically, you can find several brands with the same characteristics, and often the decision on a particular type will depend on the position of the suppliers.
Once a suitable cable has been found, it can be purchased and installed.
Protection from adverse weather conditions
When outdoor installation, for example, coaxial cable for video surveillance, it is very important to ensure it is properly protected. This is of great importance, because any moisture will lead to a significant increase in losses. If the dielectric separating the inner and outer conductors gets wet, this will degrade its performance and increase attenuation. Humidity also oxidizes the braid and reduces its conductivity.
That's why it's so important to seal the end of the cable if it's used outdoors and keep moisture out. It is necessary to ensure that the outer shell remains intact and is not damaged during installation and further operation.
An additional method to prevent large amounts of moisture from entering the cable is to form an up and down loop. This prevents the water that has penetrated inside from moving on. However, moisture will still spread by capillary action, so it's always best to make sure 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. OtherwiseIn this case, the internal structure may be damaged and 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 may increase the level of losses.
If the cable is physically damaged, make sure that its sheath remains intact. If its integrity is broken, then this can lead to the penetration of moisture, 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. Ordinary stamps cannot be laid in the ground, since their shell is not designed for these conditions. But they can be laid in a channel specially designed for this. This has the advantage that the cable is easy to replace. However, make sure that no water enters the channel. An alternative is to use a cable specially designed for laying in the ground, the sheath of which can withstand such conditions.
Ending
When installing a coaxial cable, it is important to properly terminate it. 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 residential connectors often have poor RF performance, there are few alternatives. Higher quality connections are needed for professional use, although in this case you should also make sure thatthat they are suitable for the frequencies used. Some cheap models do not meet the requirements and degrade the performance of the coaxial cable. Therefore, it is better to purchase connectors from trusted sellers.
Proper installation of a coaxial cable will provide many years of service. However, wear and tear and environmental influences will cause it to need to be replaced after a while. Because performance degradation occurs slowly, it may not be noticeable. It becomes apparent only after the complete replacement of the cable.
