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Amateur Radio

Antennas -- Coax Cables

Why use Coaxial cables?
Coaxial feeder cables can provide convenient and sensibly efficient means of carrying RF signals. Signals remain contained within the outer braid/screen of the cables and remain unaffected by outside influences such as the presence of other conducting bodies.

Balanced twin wire feeder cables can carry RF signals even more efficiently and in open spaces might be the preferred type of feeder. The use of balanced twin feeders can eliminate the need for choke baluns. etc. However, twin wire feeders can be adversely affected by adjacent conducting bodies or other wires.

The use of a single unscreened wire to carry RF signals (fed against an RF earth connection) is not recommended. This can result in VERY high RF fields in and around the shack, a reduction in the amount of energy delivered to the antenna, and possibly to local electromagnetic interference. Not good practice!

Losses within feeder cables:
Losses along the length of a correctly terminated feeder cable are mainly due to resistive losses in the conductors, and also to dielectric losses in insulating materials. At very high frequencies, losses can occur in coax cables via poor quality braiding.

Resistive losses are due mainly to "skin effect" resistance which increases with frequency.

Additional losses can occur as a result of incorrectly terminated feeders. ie due to high SWR levels.

Twin line feeder should cause lower losses than will a coaxial feeder. For example, at 30MHz losses along a correctly terminated length of 30m is likely to be approximately:

                                                               Coax
                                                     RG58/U    RG213/U    Twin wire feeder
                                                       2.5dB       1.2dB             0.05dB

The benefit of using twin wire feeder over long distances is clear. Often feeder lengths are quite short so that at HF frequencies, coax can be a viable and practical choice. At VHF, losses can become quite large and special very low loss coaxial feeders become necessary. The following table gives losses that are to be expected for various types of coaxial feeders and maximum power ratings.

http://www.hamuniverse.com/coaxdata.html provides information about losses and power ratings of a wide range of feeder cables. Note that in this table, type names have been truncated, so that for example RG58 is listed as 58.

My own practice is to use RG58 for the short runs to my HF antennas that are situated in my loft space. I am presently using a run of 10m of CLS200 with 7m of RG58/U to feed my 5 element Yagi on 2m -- losses estimated to be at least 2.3 dB, but this is a temporary installation. I have a 2m J pole and a 70cm eight element Yagi. These are fed via a single 10m run of RG213/U into a diplexer and then via separate 3m runs of RG58/U. I estimate cable losses to be 2dB at 2m and 5.6dB at 70cm.

The very short runs of patch cabling around the shack are either of RG58 or CLF200. These are very much easier to manage than the larger diameter and much stiffer RG213. Feeder losses must be balanced against practicalities.

The need for baluns:
No matter what type of coax is used, an appropriate bulun should be included if feeding into or out from a balanced antenna.

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03/06/13