Moderated Discussion Areas
ContinuousWave: Small Boat Electrical
Antenna Cable SWR
|Author||Topic: Antenna Cable SWR|
posted 03-04-2008 09:04 PM ET (US)
When I launch this spring, I'll be starting out with the old Shakespeare fiberglass antenna that came with my Revenge 21 hull when I bought it. It has a long cable that was originally wrapped up and tied under the deck.
I would like to cut the cable to a more manageable length but I've always heard about the risks of developing a standing wave on the cable (the measurement is referred to as standing wave ratio or SWR) that could damage the transmitter.
Is there a risk if I shorten the cable? Is there a way to calculate the proper cable length (or rule of thumb)?
I know there are better antennas out there and I may upgrade if I'm not satisfied with how this one performs. Any guidance on SWR risk or lack thereof would be greatly appreciated.
posted 03-04-2008 09:13 PM ET (US)
Your question is filled with misconceptions, but don't worry, just about everyone has those same misconceptions.
There are only two things which determine the VSWR on a transmission line:
--the characteristic impedance of the transmission line
--the characteristic impedance of the antenna.
Neither of those characteristics change with length of the transmission line. Changing the length of the transmission line between the transmitter and the antenna cannot change the VSWR on the transmission line, except to reduce it due to attentuation in the transmission line.
posted 03-04-2008 09:26 PM ET (US)
Byron Goodman, W1DX, was the Technical Editor of QST Magazine, the radio amateur's journal, and wrote an article about SWR and transmission lines in 1960. It dispelled all these myths, but they die hard. Here we are 48 years later and the same misinformation gets repeated over and over.
As for cutting the transmission line, I would not worry about it. Just don't cut it too short--you don't want to have to splice it. If you have a couple of feet extra, just wind it into a coil with a diameter of about four inches. The coil will form an inductance which will help choke off any antenna currents flowing back from the antenna on the outside of the outer conductor of the coaxial transmission line.
posted 03-05-2008 12:42 AM ET (US)
Your greater risk is inducing more loss due to a poorly installed connector. I believe that some out there propagate( sorry,bad radio joke )these fears to keep thier products from comming back and having to confront the paying customer and not so handy Do-It-Yourselfer about the botched connector. The weeny coax that Shakespeare installs on thier antennas is a case of less is more.
Good luck and as Jim said, leave a little extra so you can do it over if you have to.
posted 03-05-2008 08:10 PM ET (US)
Thanks Jim. Thanks roloaddict.
I knew the expertise was here. I also had thought the SWR thing might be a misconception, but it's been better than 20 years since I've messed with such things and I've forgotton a few details. Oh yea, and I've seen a more than a few boats with a big ol' wad of coax stuffed up under the deck.
I'll do as you suggest, cut it the right length, make a good connection with the, uh.. connector, and leave a couple of feet to change my radio location if I want to.
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