Ham Radio Q and A Episode 2: Selecting coax feed line cable

What’s the best type of coax cable to purchase for my antenna system? Watch this episode of Ham Radio Q and A to find out.

Thank you to everyone that gave me the positive feedback on the first episode of Ham Radio Q and A. I will admit that the first episode probably wasn’t the most polished, but doing a show like this is a learning experience, so please keep watching. I guarantee that future episodes will continue to improve.

You too can get your ham radio question answered, just send me an email to kb9vbr@jpole-antenna.com or leave your question in the comments below. Do you have a question about antennas, equipment, or operating practices? Or is there some ‘How do I. . . “ or ‘Why is it . . .’ question that you’d like answered. Drop me a line and I’ll put it on the show.

In this week’s episode, Kevin from Kentucky asks:

What kind of coax do you think I need to buy ? I’m guessing 50 ft and is it a male or female end that connects to the antenna?

All right, great question. You can watch the entire episode here, or continue reading below:

Quite often I’ve found that our choice of feed line for our antenna systems is determined more by budget and not need, or what’s the right cable for the situation. You know, I get that. Money is tight for everyone, myself included, and I’ll try to find a good deal whenever I can. But saving money on cable really can have a detrimental effect to your enjoyment of the ham radio hobby.

For example: You spend money on a good antenna and a fancy rig, and now you get ready to install the two and take a measurement of how much cable you need. You take and look at the prices. Holy smokes! Maybe instead of that super low-loss stuff I can get something a bit cheaper. You end up with 150 feet of RG-58 because you want your shack in the basement and the antenna needs to be located on the edge of the property. Your cable is so long and thin that no matter how much power you run through it, you’re running in a deficit.

Ham-Radio-Q-and-A-Episode-2-keep-feedline-short

So, let’s step back and look at the rules of feed line cable:

  1. Keep you feed line runs short as possible
  2. Use cable with loss characteristics appropriate to your operating frequencies

First, a little physics. All cables will exhibit loss. As the electrons flow through wire, they meet resistance and slow down. One way to lessen the loss is to use a thicker, or heavier cable.

Also RF energy does not travel through the entire thickness of the wire, but floats along the outer surface through a phenomenon known as the skin effect. The higher the frequency of the radio wave, the closer to the surface of the wire the rf energy will travel. so a cable with a thicker center conductor will have more surface area for the RF energy, lessening the losses.

So, on the HF bands, a the type of cable is not as critical as when you move up the frequency spectrum. RG-8U or RG-213 for that matter will be sufficient below 30 Mhz, but have increasing losses in the VHF and UHF bands.

With this in mind, try to use the best quality cable appropriate to your operating frequency that you can afford. Oftentimes the cost of the feed line will me more than the price of the antenna you’re using. But that’s ok. It’s easier to upgrade your antenna than to run another piece of coax.

So, when you look through the catalog of coax feed lines, you’ll see a myriad of options and sizes. You’ll also see charts and graphs showing how much loss a cable will have at a certain frequency. Cables are produced for different frequencies, applications, and uses. So what cable do you pick.

First look at the length of cable you need, and then the operating frequency. Now, looking at the chart of losses for popular cables, see how much loss is acceptable to you. Rule of thumb, you don’t loss to me more than the gain increase your antenna provides. Pick a cable that meets that requirement. Now look at the price, is the cable in your budget? If its not, try to shorten the cable run. Can the antenna be located somewhere else so you use less cable? Do this balancing act until you come up with a cable that has the length, loss, the price you an afford.

So back to the question, For 50 feet on a VHF antenna, I’d recommend RG-8U. A 50 foot run on RG-8U is going to have 1.4 db of loss, which isn’t bad. Next step up would be LMR-400, but at that length and frequency, I don’t think the benefit is that great compared to the cost. A step down would be RG-8X, or mini-8. I like to use Mini 8 for temporary use because its light weight, but its loss on VHF is high enough that I wouldn’t permanently install that cable for a VHF antenna.

Finally, the connectors. Antennas and mobile/base radios have SO-239 connectors. These are also known as the UHF female connector. Feed line has PL-259 connectors, the UHF male connector. The PL-259 mates to the SO-239. So order the cable with Pl-259 connectors.

There’s another type of connection called an N connector. N connectors are designed for low loss VHF and UHF applications, and you’ll often find them on UHF and higher radio equipment. But for general amateur radio use, the UHF connectors are commonplace.

Links and Resources:

Feed Line Attenuation Chart
http://www.jpole-antenna.com/faq/coax-feedline-attenuation-chart/

MPD Digital

Cable Experts
http://www.cablexperts.com/cfdocs/index.cfm?BSKT=0&USA_ship=1

The Wire Man
https://www.thewireman.com/