RF Coax Cable Installation Guide

- essential points about how to install RF coax cable and the key points to note to ensure the best performance is obtained from the coaxial cable installation.

RF coax cable is widely used for a variety of professional RF applications where RF power either from a transmitter or to a receiver needs to be transferred from one point to another. While RF coaxial cable is very easy to install, it is necessary to ensure that a number of points are observed to ensure the coax cable installation is satisfactory initially, and then lasts. This is particularly important because many coax cable installations are external and need to withstand the rigours of the environment.

While coax cable may perform perfectly well when first installed, factors such as the ingress of moisture may cause the performance to degrade over time. Accordingly this performance reduction may pass un-noticed until the performance has reduced to a point where it may not be usable. By adopting a few simple precautions, the performance of the RF coaxial cable installation can be preserved and a much slower rate of degradation seen.

Coax cable areas to address

The hints and tips to help install coax cable can be grouped into a number of categories:

  1. Choosing the right coaxial cable
  2. Weatherproofing the coax
  3. General installation
  4. Terminations / connections for the RF cable

Choosing the right coax cable

There is an enormous variety of coax cables on the market, and at first sight the choice may not appear easy. The first decision to make, prior to any installation is to choose the required impedance. Domestic hi-fi and video antenna feeds use 75 ohm coax cable. Professional, CB, and amateur radio standardise on 50 ohm cable. Once this choice has been made the next decision will probably be made on the level of signal loss that is acceptable. Typically the lower the loss, the greater the diameter of the cable, and also its cost. Typically there are several cables with similar performance figures and often the decision of the exact type number will depend on the stock position of suppliers. Once a suitable cable has been found then it can be purchased and installed.

Weatherproofing the coaxial cable

When installing coax cable externally it is very important to ensure the cable is adequately weatherproofed. This is critical because any moisture entering the RF coax cable will produce a considerable increase in the level of loss. If any moisture passes into the dielectric material spacing the inner and outer conductors, this will impair the performance of the dielectric, and increase the level of loss. Moisture will also cause the outer braid to oxidise, and reduce the conductivity between the small conductors making up the braid.

It is therefore very important to seal the end of the cable if it is to be used externally, and ensure that no moisture enters. It is also necessary to ensure that the outer sheath of the cable remains intact and is not damaged during installation or further use.

An additional method of preventing large amounts of moisture entering the cable is to loop it up and down. In this way it is more difficult for water to enter the cable and then move along it. However if some moister enters the cable it will move into it by capillary action, so it is always best to ensure that the ends are properly sealed and protected.

General installation tips for RF coax cable

All cables have a bend radius. In order to prevent damage they should not be bent into curves tighter than this. If RF coax cable is bent beyond its limit then damage to the inner construction of the cable may result. In turn this can lead to much higher levels of loss.

In a similar line, care should be taken to ensure that the cable is not crushed, or likely to be crushed. If the RF cable does suffer damage in this way, the dimensions of the cable will be changed and it will not maintain its characteristic impedance. Additionally if the dielectric between the two concentric conductors in the coax cable is damaged, then there is the likelihood of an increase in the level of loss.

While on the subject of physical damage to the cable, it is necessary to ensure that the sheath of the cable remains intact. If it is broken in any place, then this may allow moisture to enter if it is used externally, and this will cause oxidation and moisture retention in the dielectric that will increase the level of loss.

On some occasions it is necessary to bury coaxial cable. Ideally, normal cable should not be buried directly as this relies purely on the outer sheath for protection and it is not designed for these conditions. Instead it can be run through buried conduit manufactured for carrying buried cables. This has the advantage that it is easy to replace. However ensure that the conduit does not become water logged. Alternatively solution to using some form of conduit is to use a form of coax cable known as "bury direct". This is designed for being buried, and its outer sheath can withstand these conditions.

Coax terminations / connections

When installing RF coax cable, it is important to terminate the cable correctly. In most instances the coax cable will be physically terminated using an RF connector, the electrical termination being either at the antenna or in the receiver. Accordingly the connections to the connectors must be made correctly and the right quality RF connectors should be used.

Although connectors for domestic installations are often poor in terms of their electrical radio frequency performance, there is little alternative to using them in view of the fact that they have to mate with the RF connectors on the equipment. For professional applications, RF connectors can be far better, although it is necessary to ensure that the connectors are suitable for the frequencies used. Some cheap versions of RF connectors may not meet the full specification and can thereby impair the performance of the RF coax cable. It is therefore wide to always buy connectors from reputable sources.

By correctly installing a RF coax cable it can provide many years of satisfactory service. However wear, and exposure to the elements will mean that after some time it may be prudent to replace the RF coax cable. As the degradation in performance will be slow, it may mean that this is not noticed. Only when it is ultimately replaced will a major difference be seen.

By Ian Poole

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