What the heck happened?

[OkieBoyKJ5JFG]

This morning, during a local net, the TYT TH-9800 in my car refused to transmit. I've been using a Comet SBB-2 on a permanent NMO mount on the roof and when I installed it, SWRs were in the range of 1.01 to as much as 1.4:1 all the way up and down from 151 to 467 and throughout the 2m and 70cm bands. When I got home, I stuck the SW-102 on the radio/antenna and got SWRs in the 4.7-4.8 area throughout the whole range of frequencies. I swapped it out for a Nagoya NMO-72 and SWRs were all 1.4:1 or better. Applying Koch's Postulates, I put the SBB-2 back on and SWRs were in the high 4s again. I put the Nagoya back on and SWRs were good again.

So far, so good, right? But wait... I took the SBB-2 into the house where I attached it to an identical radio and put it on a mag mount on a pizza pan and the SWRs are excellent -- 1.1:1 or lower in all the frequency ranges. I'm completely mystified. How could this happen?

 

[mmckenna]

May be poor contact with the NMO mount center button.

The level of that center button varies based on the installation, and it could be yours sits a bit low and the antenna is not making contact very well. Over time vibration/movement might have changed things. The NMO mount in the house may sit a bit higher.

I've seen that happen once or twice, but it was with the 'chrome nut' style 1/4 wave antennas from some manufacturers. Haven't seen it happen with better antennas.

Might be something else, but based on your description, that is where I'd start looking.

 

[nd5y]

I could also be the grounded base part of the antenna is smaller in diameter than the NMO center contact and shorts when when screwed down all the way. I had that happen with a Comet antenna and cheap no-name NMO mount.

 

[OkieBoyKJ5JFG]

May be poor contact with the NMO mount center button.

The level of that center button varies based on the installation, and it could be yours sits a bit low and the antenna is not making contact very well. Over time vibration/movement might have changed things. The NMO mount in the house may sit a bit higher.

I've seen that happen once or twice, but it was with the 'chrome nut' style 1/4 wave antennas from some manufacturers. Haven't seen it happen with better antennas.

Might be something else, but based on your description, that is where I'd start looking.

That makes sense. Things could shift around a bit with bumping, etc. When I'm a little more motivated, I'll take it back out and try it again, maybe see if i can tell whether it's making good contact.

 

[OkieBoyKJ5JFG]

I could also be the grounded base part of the antenna is smaller in diameter than the NMO center contact and shorts when when screwed down all the way. I had that happen with a Comet antenna and cheap no-name NMO mount.

The mount is a Larsen NMO-K, but I guess anything is possible. It is interesting that the Comet antenna has better SWRs on the mag mount than it ever had on the Larsen. They were good with the initial installation on the Larsen, but they're remarkable on the mag mount.