coax length optimization

my pole is 16' tall. how long should my coax be to get the best results? i have only had mobile units up until now, this is my first base station.

More info needed.
What frequencies?
What power?
What budget?

Bruce

redbullet said:

my pole is 16' tall. how long should my coax be to get the best results? i have only had mobile units up until now, this is my first base station.

Click to expand...

Not to sound flippant, but the cable should be exactly as long as it needs to be to get from the antenna to your radio, and maybe a bit of slack.
There is no magic length for coax in normal cases. Ideally you want to use the least amount possible to reduce feed line losses.

Which cable you use is a more important question. Length affects losses, so if you have a long run, you may need to use a higher grade cable than if the run was shorter.
Higher frequencies suffer from higher attenuation in the cable, so if you are using 700/800MHz, you may need to use a higher grade cable than if you were using it with lower frequencies.

In normal cases where you tune the antenna to match feed line, length is less important. Feed line length comes into play when diplexing without a diplexer and filtering. Most cases you'll be fine just fine tuning the antenna.

coax

well im trying to put base up the antenna dont have a set point on it. but the antenna is 16 feet up and where i will be putting the radio. it would be about 28 feet total from antenna to where it will be setting. thats why i ask how long should i make the coax for best SWR match.

As they are trying to tell you above, if your antenna is properly tuned for the frequencies you intend to use then the coax length will have no affect on the SWR. Use an appropriate length.

That's a pretty short run (less than 50 feet) so most coax feedlines at that length would be fine. The loss factor you may be thinking about happens when the feedline goes up and over 50 feet as some of the coax loss charts show. Just my suggestion use LMR-400.

how long should i make the coax for best SWR match.

Click to expand...

This is an old story about making your coax cable multiples of a quarter-wavelength so to achieve the best power transfer to the antenna. Another old wives tale that I though had been laid to rest a long time ago.
Hopefully you have an external SWR meter - install it as near to the antenna as you can on a short jumper lead, tune the antenna to get best SWR. Now disconnect the jumper and meter, install your full length coax cable, check the SWR back at the transmitter, should be the same or even slightly better than before.
Note that best SWR may not be the resonant frequency of the antenna - but that's another story....