Coax Tutorial Includes:
Coax feeder Coax specifications overview Coax impedance Coax loss / attenuation Coax power rating Coax velocity factor Coax environmental Coax installation tips Coax cable types Tips for choosing right coax cable Buying TV coax: points to note
Installing coax cable can seem like a simple wiring task, but a few precautions, hints, and tips applied to the installation can make a huge difference after a few months.
While coax cable may perform perfectly well when it is first installed, factors such as the ingress of moisture may cause the performance to degrade steadily over time. As the performance reduction often occurs slowly, it may pass unnoticed until it 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. In this way, the investment in the coaxial cable can be extended as much as possible.
Coax cable areas to address
The hints and tips to help install coax cable can be grouped into a number of categories:
- Choice of correct 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.
- Coax cable weather-proofing When installing coax cable externally it is very important to ensure the cable is adequately weather-proofed. This is an obvious precaution, but one that is not implemented well enough on a large number of installations.
Putting in place sufficient weather-proofing 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 will also give rise to an increase in the level of passive intermodulation. The oxidised braid may form small non-linear interfaces that will act as small mixers giving rise to this form of intermodulation distortion when used with a transmitter.
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.
- Beware of coax bend radius 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.
- Don’t score or rupture coax cable sheath: 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.
- Don’t crush cable during installation 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.
- How to bury coax cable: 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 the outer sheath of this type of coax cable can withstand the constant moisture and other conditions.
- Coax terminations / connections It is important to terminate the cable correctly – making the terminations of the coax cable waterproof. The end of the cable provides a very easy path in for moisture and can even lead to the whole cable becoming waterlogged.
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. That said connectors are not always fully waterproof.
In view of the fact that RF connectors are not fully waterproof, it is wise to cover the whole connection with self amalgamating tape. In this way the connector will be protected from the elements.
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 taking the best care in preventing water ingress and reducing other forms of environmental degradation, the best life can be expected from the coaxial cable.
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