SWR on mobile 2M antenna
My mobile 2M rig consists of an Icom IC-V80 HT and magnetic mount single-band antennas on top of two vehicles.
I decided to check the SWR on both vehicles. On the first, I was able to set the SWR to a very low value, with almost no reflected power.
On the other, I was not able to set it reliably. It started out at over 3.5:1, and would change by moving the coax ever so slightly. Even moving the coax 6" would drop it to less than 1:1.
I assumed it was a bad piece of coax, and replaced it. I even soldered the connections at the antenna (it was a pressure fit connection), still same problem.
I found that making a small loop with three turns near the antenna gets rid of the problem, and now I can set it for less than 1:1 and keep it there.
What's going on here? Thanks in advance! Al
The issue is common mode. Remember, RF must return to its source, and when using a mag mount, that's back down the coax cable. Even a slight change in antenna positioning is enough to upset the SWR reading, particularly one built into the radio.
I understand and agree with that. I still don't understand why looping three turns of coax together (which I assume adds a little inductance) fixes the problem. Al
Originally Posted by K0BG
At VHF it doesn't take much of an inductor to choke off common mode. Sometimes, even a single turn, mix 31 bead (or even a few turns of coax) is all it takes.
There is also the possibility that you get better capacitive coupling to the roof with the coil. ??
TOM K8ERV Montrose Colo
I doubt that Tom.
The truth is, mag mounts are meant to be temporary, but far too many make one permanent (It is one of my peeves, by the way). As a result, you put up with whatever common mode you get.
There are some metalic paints that will not allow the antenna to couple to the metal roof very well.
Iv'e seen it a few times.
Take the same antenna on another car and it matches down ok, with no changes.
An antenna analyzer look at it might be enlightening.