Gasket on NMO connection

[VETCOMMS]

I recently installed a 6" tram uhf antenna on a mag mount on my vehicle. 

I noticed with the rubber gasket/O-ring installed around the threads of the NMO mount the SWR is 8.  Without it,  it's 1.01.  I like the idea of that o-ring being there from a water/ weather intrusion point but I'm wondering if the o- ring is preventing a good ground?  

[SteveShannon]

1 hour ago, VETCOMMS said:

I recently installed a 6" tram uhf antenna on a mag mount on my vehicle. 

I noticed with the rubber gasket/O-ring installed around the threads of the NMO mount the SWR is 8.  Without it,  it's 1.01.  I like the idea of that o-ring being there from a water/ weather intrusion point but I'm wondering if the o- ring is preventing a good ground?  

Sounds like it, but it shouldn’t matter.  Have you tried a different mag mount?  

[VETCOMMS]

14 minutes ago, Sshannon said:

Sounds like it, but it shouldn’t matter.  Have you tried a different mag mount?  

No.   I only have the one mount though.   Are certain models more consistent in performance?  I tried the same thing on my fender mount with and without the gasket with similar results.  I wonder if I need to tighten the antenna more to flatten the gasket more and that the contacts aren't quite touching between the antenna and the mag mount. 

[WSAM454]

Yes, that is a good possibility, especially if the antenna shows the same problem with a different NMO mount.  Could the ring be too large in diameter?  Do you have another  antenna that you use on the fender mount, and can you compare the O rings on the two of them?

[VETCOMMS]

30 minutes ago, WSAM454 said:

Yes, that is a good possibility, especially if the antenna shows the same problem with a different NMO mount.  Could the ring be too large in diameter?  Do you have another  antenna that you use on the fender mount, and can you compare the O rings on the two of them?

The ring is snug around the threads.   I only have the one o-ring as the other antenna is a Laird with an integrated o-ring.  Maybe I can put the o-ring under under the other antenna and see what the swr does on the different antenna.  Thx. 

[tcp2525]

On 5/11/2024 at 7:30 AM, VETCOMMS said:

I recently installed a 6" tram uhf antenna on a mag mount on my vehicle. 

I noticed with the rubber gasket/O-ring installed around the threads of the NMO mount the SWR is 8.  Without it,  it's 1.01.  I like the idea of that o-ring being there from a water/ weather intrusion point but I'm wondering if the o- ring is preventing a good ground?  

No! Absolutely no. The NMO is designed to get its ground from underneath the sheet metal it's mounted to. The top side of the sheet metal is usually painted and will insulate. If you want to assure a good ground, rough up the bottom of the sheet metal with sand paper or wire brush. I've installed dozens of NMO mounts through the roof of vehicles and none of the above mentioned is really necessary. I never had grounding issues.