Embarrassingly Simple NMO question

[sallen07]

OK I've looked on-line for the answer to this question (honest!) and have found nothing. And it's not like mobile antennas come with an instruction manual unless it's for tuning. What would the manual say? "Screw antenna onto base" ??

So all of the NMO mobile antennas I have purchased come with a little o-ring which goes into the a groove in the base of the antenna. Some (all?) also come with a larger rubber gasket that fits the bottom of the base. My question is, "Are there certain circumstances where you use one or the other, or do you always use both?"

See? I said it would be simple!

 

[mrweather]

I always use the larger rubber gasket. I don't think I've ever used the o-ring one.

 

[mmckenna]

The Larsen NMOQ and larger antenna coils seem to come with both. It might be an individual preference thing. I always used the larger gasket rather than the O-ring. It does make the coil sit very slightly higher on the mount, so maybe the O-ring is intended for applications where the NMO mount is sitting down a bit lower.

On the Larsen antennas, you don't want to use both as they would interfere.

 

[KevinC]

When I got my Larsen tri-band it mentioned when to use each one, but I can't find any reference to that in current documents. I find the O-ring can "catch" when attaching the antenna and either tear or bind up.

 

[N9JIG]

Since I live in a hot-dry desert environment I put some lithium grease on the o-ring before putting it in the little channel, lest it dry up and crack over the summer. I usually restore the grease each spring and have never had an issue.

As for the flat washer that goes around the outside of the NMO mount I use them on most antennas except my current EM-Wave. These are equipped with a softer rubber base that seals the NMO from the elements, negating the need for the washer.

One other thing to watch for is that some whips and mounts are engineered poorly and the center conductor might not engage well with the washer in place, the pin is too short or the mount itself runs shy. Sometimes it works fine stationary but at speed the flex is just enough to break the connection. Removing the washer solved the connection issue but then reduces the water-tightness of the mount.

 

[k6cpo]

If I remember correctly, the NMO I installed on the roof of my truck had an O-ring to seal the mount against the metal of the ruck and there's another one that seals the base of the antenna against the metal of the truck. The installation has been in place since 2013 and there's never been a hit of leakage, despite being parked outside in all kinds of weather.

 

[gh6406]

The instructions that came with my Larsen Tri-Band says to use the Flat Rubber washer for Permanent installs, and to use the O-Ring for installs that the antenna may/will be changed out often.