SWR on mobile 2M antenna

[KJ4YEV]

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

[K0BG]

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.

[KJ4YEV]

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

[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.

[K8ERV]

There is also the possibility that you get better capacitive coupling to the roof with the coil. ??

TOM K8ERV Montrose Colo

[K0BG]

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.

[KM3F]

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.