Handiham World for 16 June 2010
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What coax should I use?
The repeaters I want to use are all just a bit too far away for me to work with an indoor antenna or a handheld radio. I want to install an outdoor antenna so that I can use several different VHF and UHF repeaters. I already have a dual-band 2m/70cm vertical antenna, but what kind of coax should I use? I am thinking about RG-58 or RG-8X, because they are cheaper and easy to work with than the thicker RG-8 or RG-213. My cable run will be about 100 feet.
Since the repeaters you plan to work are probably located in different compass directions, your choice of a vertical antenna is a good one, as long as the repeaters are not so far away that you would need a directional antenna with more gain. The directional antenna usually means an extra investment in a rotator system, a considerable expense and an additional accessory to maintain over the ensuing years.
One thing you will not want to skimp on is your feedline, especially if it is to be used for VHF and UHF work, and when the feedline is going to be run for a considerable distance. A short run of RG-8X, under 25 feet, is probably acceptable for VHF work. The problem with these thin, cheaper feedlines is that they lose quite a significant amount of signal – both on receive and on transmit – and the savings in initial cost for the coax are quickly offset by the poor performance they introduce to your otherwise well-designed system. RG-58 is such thin, fragile coax that it is a poor choice for anything but temporary use or short connecting cables used in test situations. It is very lossy and should not be used over long runs, even for HF operation. Its fragility means that it can easily break.
Let’s take a look at the loss for a typical VHF frequency, 146.52 MHz for three common types of coax, all assuming a 100 foot run.
RG-58: Power in = 100 Watts. Power out = 34 Watts. Total loss is 4.7 dB. Ouch!
RG-8X: Power in = 100 Watts. Power out = 39 Watts. Total loss is 4.1 dB. Ouch!
RG-213: Power in = 100 Watts. Power out = 55 Watts. Total loss is 2.6 dB.
As you can see, the unfortunate truth is that all of these cables have significant loss, but the cheaper cables will end up turning most of your signal into heat. Only the RG-213 comes close to being acceptable for VHF use with a 100 foot run.
Now for something really scary, let’s try a 70cm frequency, 446.0 MHz, with the same cable run.
RG-58: Power in = 100 Watts. Power out = 13 Watts. Total loss is 8.9 dB. Double Ouch!
RG-8X: Power in = 100 Watts. Power out = 15 Watts. Total loss is 8.2 dB. Double Ouch!
RG-213: Power in = 100 Watts. Power out = 32 Watts. Total loss is 5 dB. Ouch!
Even with the best of these three coax types, you are still getting less than one third of your signal to the antenna. Remember that it works the same way on receive. And why would you even bother with RG-58 or RG-8X for UHF work, when 100 Watts turns into only 15 Watts or less? Long runs of cheap, lossy cable might as well just be dummy loads!
As you can see from the results we have listed, the loss per distance unit of feedline goes up when the frequency goes up. Therefore, a cheaper grade of feedline might be acceptable for use on 3.9 MHz, but far too lossy for use at VHF or UHF. Another consideration is that if one intends to use even higher transmit power levels, cheaper coax must not be used because it may arc over and fail. It is generally acceptable only for lower power levels.
The results we listed are for SWR readings that are virtually perfect, 1:1. Since no antenna installation is perfect and minor mismatches occur in even a carefully-designed system, the actual loss will be even higher than what we listed. This makes using good feedline even more important.
To summarize, you will have several important choices to make when you plan your VHF/UHF antenna system. You will want to decide which repeaters you want to work, their compass directions from your station, and whether you will need to choose a directional antenna or a vertical antenna. The supporting structure will add height, which is generally a good thing for effective VHF/UHF work, but also add to the length of a feedline, and longer feedline runs mean more loss. If you want to try weak-signal work on VHF and UHF, you will need a rotator and a horizontally-polarized directional antenna. Unlike repeater operation, weak signal work on SSB and CW absolutely demands good quality feedline for the lowest loss possible. FM repeater operation is less demanding, and will require vertical polarization. You may be able make your horizontally-polarized system work for repeaters, but your vertically-polarized antenna will not be effective for weak signal work on SSB and CW.
Our recommendation is to use good quality feedline for every installation, avoiding higher loss coax except for short connectors and temporary use in sort runs.
Are you wondering how we calculated the loss for examples we used in this article? It was easy with the online calculator we found at the KC7HXC website!
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