Posted by: GW7AAV | July 23, 2010

Don’t Starve Your Antenna

Coaxial cable is one of the things radio amateurs often take for granted and is also one of the things least understood by those installing professional systems as well. The higher we go in frequency the more important it is to use the lowest loss cable you can afford. I often see radio amateurs who have installed expensive collinears or beams for 144 MHz and above who are quite happy to use some cheap RG-58 that they picked up at a rally or on eBay. One problem is that they do not understand was the loss factors mean or just how far out for the claimed figures can be. Even good RG-58 should not really be used on two metres or above. Some of the cheap (so called RG-58) can be up to five times as lossy than the standard, which is around 6.1 dB per 100 feet at 144 MHz or 10.4 dB per 100 feet at 440 MHz. In a poor location with a less than perfect antenna that might mean you may never hear anything apart from a mobile station sat on your drive. Things are not usually that bad because usually we keep cables shorter, but it is still a dumb idea to not invest in good quality cables.

Simon M1AVV wrote on his blog recently: A few years ago I made up a cable of black RG8-Mini, about 11 metres long with PL259 plugs on and sometimes used it for portable operation. I thought it would probably be better than RG58 so wasn’t too worried about the loss. But this week when I was on Muncaster Fell I had a listen around 433 MHz and didn’t hear very much. I could hear more on my handheld’s own antenna than on the 2m/70cm mobile whip that I had set up on a pole.When I got home I measured the loss of this 11m of RG8-Mini on 2m and 70cm. With a very short piece of RG58 I was measuring 9 watts on 70cm. Replacing that with my coax cable reduced that to 2 watts! That’s about 6.5dB loss or 0.6dB per metre. RG58 is supposed to be better than that on 70cm. At 144 MHz, the loss was about 2dB which is more like RG58 should be.

Next day he wrote: Made a 5 metre pl259 cable this morning from some old rg58 which i’d had for years on 70cm, I was getting 6watts out for 9 watts in so the loss was under 2db or 0.4db per metre. That’s better than the 0.6db for that rg8-mini.

So Simon who was not using a low loss cable due to the weight factor when out portable back packing was loosing one third of the power from his rig with RG-58 and seven ninths of the power with the dodgy RG-8X (RG-8 mini), which should have actually been less lossy than the RG-58. The question is do you know how much power you are loosing? and how many dB of attenuation is down to using the wrong cable? Ask yourself as well can I shorten this cable? Do you really need that thirty feet of spare tucked behind the desk just in case you decide to rearrange the shack?

RG-58U should be 6.1 dB per 100 feet at 144 MHz or 10.4 dB per 100 feet at 440 MHz attenuation.

RG-8X should be 4.5 dB per 100 feet at 144 MHz or 8.1 dB per 100 feet at 440 MHz attenuation.

RG-213 should be 2.8 dB per 100 feet at 144 MHz or 5.1 dB per 100 feet at 440 MHz attenuation.

LMR-400 should be 1.5 dB per 100 feet at 144 MHz or 2.7 dB per 100 feet at 440 MHz attenuation.

Westflex 103 should be 1.5 dB per 100 feet at 144 MHz or 2.5 dB per 100 feet at 440 MHz attenuation.

Personally I like to use Westflex 103 and have two 100 metre drums standing by for my big antenna and mast rethink. The only thing to do for back pack portable is to minimise the cable lengths and test each one you make to be sure you are not handicaping yourself before you start. Have a think, why does that chap down the road get far better contacts than you when you both have similar set ups? Is it your choice of coaxial cable?

Some of you will be saying “I’m okay I just do HF and surely RG-58 is good enough there” but on 10 metres the loss is still 2.5 dB, so a weak signal might not be heard and even down on 80 metre it might just be enough to drop a signal bellow you noise floor. Feeder runs on HF tend to be much longer than on VHF and above due to the length of the antenna the feed point will be a long way from the shack and therefore the loss will be greater. On HF open wire feeders are the way to go but that is another story.

Coxail Cable Performance Charts – http://www.universal-radio.com/catalog/cable/coaxperf.html

UK Supplier of Westflex 103 – http://www.whwestlake.co.uk/

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