Tram 1/4 wave UHF

[dmg1969]

Hello all. A quick question regarding NMO mount antennas. I have an NMO mount in the center of the roof of my vehicle. Until this weekend, I used it for a Laird tri-band scanner antenna. I recently got my GMRS license and bought an inexpensive Radioddity HT. I bought a Tram 1126-B 1/4 wave UHF antenna to use with the GMRS radio. I put that on the roof and moved the scanner antenna to a lip mounted Comet NMO mount.

Should the new Tram antenna have come with a gasket? When I installed the Tram antenna on the existing roof NMO, there was still a gap between the antenna base and the car. So, I took the washer off of the Laird scanner antenna NMO mount as water isn't nearly as much an issue since it is a lip mount. I guess I need to pick up a pack of the NMO gaskets.

Dave

 

[nd5y]

Those "chrome nut" type antennas are not designed to use an extra gasket below the antenna.
The bottom of the NMO external threaded ring (not the antenna) should have an O-ring that seals the roof. The antenna should have a large rubber gasket that seals the whip and the top of the NMO mount.

If the antenna nut doesn't fit flush against the roof surface then either:
You installed it wrong.
You used the wrong NMO mount for the roof thickness.
The NMO mount is some inferior brand that is too high.
The antenna is some inferior brand with a threaded nut that is too short or the internal threads don't go up high enough or something is preventing it from being screwed down all the way.

 

[mmckenna]

Should the new Tram antenna have come with a gasket? When I installed the Tram antenna on the existing roof NMO, there was still a gap between the antenna base and the car. So, I took the washer off of the Laird scanner antenna NMO mount as water isn't nearly as much an issue since it is a lip mount. I guess I need to pick up a pack of the NMO gaskets.

As said, those types of 1/4 wave antennas don't come with gaskets. You can add one, but you need to be sure that the antenna center pad is still making good contact with the NMO mount center button.

However, like I said on your original post, those antenna types will eventually leak, even with the gasket under it. The mount doesn't seal around the whip very well and moisture will eventually get in.

Instead of buying gaskets trying to make it work, just purchase one of the antenna types that does a proper job sealing. Won't cost much more and it'll be a better solution in the long run:

NMOQC : Quarter Wave Omnidirectinal Whip Antenna

Base / Whip; Unity; 136 - 512 MHz; .070 Dia Stainless Whip*This Antenna Requires a NMO Mount. Please click here to find all Thru-Hole NMO mounts or Magnetic NMO Mounts we carry.*This product is subjected to a minimum order quantity of 21*

www.arcantenna.com

EM-MX0450: X-NUT, MOBILE 1/4 WAVE ANTENNA 450-470 MHz

X-NUT, MOBILE 1/4 WAVE ANTENNA 450-470 MHz

www.arcantenna.com

 

[dmg1969]

The NMO mount itself was installed after I bought the car new. It was done by a reputable upfitter company that does public safety installs. It has worked well for the Laid tri-band antenna for 9+ years. I was not aware that this particular type of antenna (the chrome nut) was not meant to be used with a gasket. I figured that all NMO mount antenna bases were similar in the way they keep water out. Now I know that they are not.

The reason I used the gasket was because of the gap and I figured that one had to be used since there was one used with the Laird scanner antenna when they installed it. Other than having the scanner antenna professionally installed, I have no experience with NMO mounts, which is why I came here to ask. I wasn't attempting to "just make it work". It was simply a lack of knowledge on my part.

 

[mmckenna]

"Unity Gain" is 0dB, or the same as the 1/4 wave you have right now. If that antenna is working for you, then that's good enough. You could go with an antenna design that has some gain, but gain comes with tradeoffs. A 5/8th's wave UHF antenna will give you 3dB of gain, which may be helpful. However, it does that by compressing the radiation pattern down, which can have drawbacks in the mountains.


I'd still steer you towards the Larsen or EM Wave I linked to above. They have excellent sealing designs and will work with your NMO mount. I've never had any issues with either of those, and I've been running Larsen antennas for almost 35 years now, both on my personal vehicles and at work.

 

[dmg1969]

The NMO mount itself was installed after I bought the car new. It was done by a reputable upfitter company that does public safety installs. It has worked well for the Laid tri-band antenna for 9+ years. I was not aware that this particular type of antenna (the chrome nut) was not meant to be used with a gasket. I figured that all NMO mount antenna bases were similar in the way they keep water out. Now I know that they are not.

The reason I used the gasket was because of the gap and I figured that one had to be used since there was one used with the Laird scanner antenna when they installed it. Other than having the scanner antenna professionally installed, I have no experience with NMO mounts, which is why I came here to ask. I wasn't attempting to "just make it work". It was simply a lack of knowledge on my part.

I had another post asking some questions about this. I just deleted it and bought the emWave X-Nut antenna. In the meantime, I am just going to pull that Tram antenna off and put a rain cap on it so no moisture gets in.

 

[mmckenna]

I am just going to pull that Tram antenna off and put a rain cap on it so no moisture gets in.

I wouldn't go that far. The NMO mount isn't going to rust, it's just corrosion you need to watch out for. Just remove it periodically and clean it. That'll be fine until you get the EM Wave antenna in.

 

[prcguy]

All the NMO mounts I have installed over the years (more than a hundred) had an O ring between the hockey puck and the roof. If the O ring is greased during installation they should be waterproof for the life of the install. I have never seen one properly installed that leaked that either had an antenna or protective cover installed. If they are left in the rain with no antenna or cover they will eventually have problems.

If the NMO mount is installed well and there is a slight gap between the Tram antenna and the roof, its ok to put a gasket between them if it doesn't prevent the antenna from screwing down tight and making good electrical contact.

 

[mmckenna]

All the NMO mounts I have installed over the years (more than a hundred) had an O ring between the hockey puck and the roof. If the O ring is greased during installation they should be waterproof for the life of the install. I have never seen one properly installed that leaked that either had an antenna or protective cover installed. If they are left in the rain with no antenna or cover they will eventually have problems.

If that was in response to mine, I probably didn't word it well.

Yeah, I've never had a properly installed NMO mount leak. Never. Not once.

What I have had issues with are the 'chrome nut' style antennas. Moisture somehow got down under the whip base where it meets the center button of the NMO mount. Corrosion ensued and damaged the NMO mount. I replaced the NMO mount and installed a new Larsen NMOQ 1/4 wave antenna.

I think with the chrome nut antennas and the whip flexing a lot, moisture eventually makes its way down the rubber cone where the whip passes through. Granted, these antennas had been installed for a while. The particular one I remember was on a late 1990's Jeep Cherokee used by our animal control officer. Had another one on a late 1990's Chevy Suburban used by our PD. It was always the chrome nut 1/4 waves.

Whenever I would get a chance, I'd pull the antennas off their vehicles and check/clean underneath. These two often escaped me. All their newer vehicles (Ford police interceptor utilities or F150's) got the Larsen NMOQ's and haven't had any issues.

 

[prcguy]

If that was in response to mine, I probably didn't word it well.

Yeah, I've never had a properly installed NMO mount leak. Never. Not once.

What I have had issues with are the 'chrome nut' style antennas. Moisture somehow got down under the whip base where it meets the center button of the NMO mount. Corrosion ensued and damaged the NMO mount. I replaced the NMO mount and installed a new Larsen NMOQ 1/4 wave antenna.

I think with the chrome nut antennas and the whip flexing a lot, moisture eventually makes its way down the rubber cone where the whip passes through. Granted, these antennas had been installed for a while. The particular one I remember was on a late 1990's Jeep Cherokee used by our animal control officer. Had another one on a late 1990's Chevy Suburban used by our PD. It was always the chrome nut 1/4 waves.

Whenever I would get a chance, I'd pull the antennas off their vehicles and check/clean underneath. These two often escaped me. All their newer vehicles (Ford police interceptor utilities or F150's) got the Larsen NMOQ's and haven't had any issues.

If I install an NMO mount and antenna where the antenna will never come off I will completely coat the top of the NMO mount with the contact and the threads with silicone grease and the same up inside the antenna. This will have no effect on performance through UHF even with the center contacts buried in the grease.

For my personal antennas I only run the grease on the O ring under the puck when the antenna is first installed and all my personal antennas get taken off and swapped around weekly. The grease would get messy in that case and since the antennas are off some of the time I get to inspect the mount quite often.

 

[dmg1969]

I received the EM Wave antenna and have it installed. I will have to see how it works out over the next week.