What type of antenna cable to use

Years ago when I wanted to put up my first real antenna I looked far and wide to find out what type of antenna cable to use. Is there a difference for distance? How about just receiving instead of transmitting? Different types of cable for different frequencies? How about for the power I would be using? Are there different types of cable conductors that perform differently?

The answer is yes, to all of those questions. What follows is a very basic introduction to antenna cable and why it is important. It is intended for the beginner.

The basics

A little further down we will get a little more into it, but here is the general rule: larger cable means less loss.

Loss refers to both receiving a signal and transmitting one. To help you understand I will use a very rough generalization; the larger the cable, the further away you can hear someone else and the further away someone else can hear your transmission.

The longer the cable run, the more loss, so you need a larger cable for longer runs.

The more power you transmit, the larger cable needed to carry that much current.

The higher the frequency, the larger the cable you need to reduce reflectivity (loss).

Pure copper core cable is much better than aluminum or coated aluminum.

Why this is important

So lets say you have a nice big amateur radio, say a 2m all mode rig that can output 100w (and you are licensed). You then get this really cool yagi antenna almost the size of your roof and put it up on a 100′ tower in your back yard. You spend all your money on the radio, antenna, and tower so you look at some cheap RG8x cable. You think to yourself, what difference does it really make? I need 250′ of cable and I have lots of power on a great antenna, surely I can deal with a little loss in the cable, right?

If you do the math in that scenario you will find that out of that 100w of power you are getting about 9w to the antenna, the rest is lost in the line. If on the other hand you used LMR400 cable you would get around 42w to the antenna, almost five times the power!

You can also shorten the length to 100′ and get around 39w with RG8x or 71w with LMR400.

This also changes with frequency so while the numbers I have provided are good for 2m (145mhz), they change for what is considered HF radio. For example 250′ of RG8x when used at 40m (7mhz) gets you about 65 watts out instead of the 9w you would get at 2m (145mhz).

There are of course smaller cables than RG8x and larger cables than LMR400 but those two represent the smallest and largest you are likely to see premade at most amateur radio supply shops so I tend to stick with them. If you are looking more at places that sell and service CB radios, you might run into RG58 cables which is even worse than RG8x so stay away from them.

So far we have only talked about transmitting, but how about receiving? The same rules apply except the loss is on incoming signals. This means that a signal that is loud and clear on 250′ of LMR400 might not be heard at all on 250′ of RG8x.

To sum this up in something easy to remember, you need to use the largest cable you can afford and that is practical given the application.

What I mean by it being practical is that if you are doing a mobile install, that 8′ piece of RG8x will get you 46watts usable to the antenna from a 50watt radio and is a small flexible cable. Using the heavy and stiff LMR400 cable in that case is just overkill and is so inflexible you might not even get it where it needs to be in a car.

Conductor types

One problem I see when purchasing cable is that they will use an aluminum core, or an aluminum core coated with copper, and try to pass that off as the same thing as pure copper. People are drawn to it because it appears to be the same size and style, but far cheaper. Unfortunately the aluminum core does not conduct as well as copper which causes more loss.

This little “trick” has been common in wire used for power and speakers for many years with people purchasing 16ga wire and wondering why it won’t handle the current it is supposed to. While not as common in serious radio cable I have seen many examples in aftermarket premade cables for CB radios such as those sold at truck stops.

Where to get more information

The calculations provided here came from a wonderful Coax Loss Calculator on QSL.net and absolutely should be bookmarked by anyone even remotely into radios.

 

Please follow and like us:

Leave a Comment