NMO Mount Gasket

Does anyone ever add a gasket between the antenna itself and the sheetmetal of the vehicle? I'm curious how this would work when it comes to grounding the antenna, the picture below is kind of gasket I'm thinking of.


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That gasket is standard with many antennas and the mount will ground just fine through the NMO threads. I would never put the gasket you show between the mount and vehicle.
prcguy

Where would someone use this gasket?

prcguy said:

That gasket is standard with many antennas and the mount will ground just fine through the NMO threads. I would never put the gasket you show between the mount and vehicle.
prcguy

Click to expand...

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zane018 said:

Where would someone use this gasket?

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Click to expand...

I believe that is the gasket that goes between the bottom of the coil or base of the antenna that screws onto the NMO mount and the sheet metal of the vehicle to help prevent the paint from being damaged.

Those gaskets are not for the NMO mount. Those are designed to be installed AFTER the NMO mount is properly installed.

Those gaskets should fit around the NMO mount after it's installed on the vehicle. They work well for some of the "chrome nut" style antennas. I use them on mine to keep crud from building up in the gap that exists between the bottom of the NMO antenna nut and the roof of the vehicle. Not necessary, but they do help seal things up a bit better.

Different mounting surface thicknesses, NMO mount designs and different NMO antenna base designs all need to be considered. Sometimes these gaskets can keep the center pin of the antenna from properly contacting the NMO mount.

Installed, they'd look like this:



and with the antenna installed:

Is that a popsicle?

mass-man said:

Is that a popsicle?

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Yeah, 800MHz Popsicle.

That's why I took the photo. I'm close to the ocean, and freezing weather is pretty rare here.
Actually, being close to the ocean gives me some salt air exposure, so keeping the brass part of the NMO connection clean, dry and not corroded helps. The gaskets work well for that.

I've always used them. When I worked as a part time installer, at a two way radio shop, I had to replace damaged antenna's at various times. Many times a previous installer didn't install the rubber gasket in conjunction with the antenna. I would often find where moisture had made it's way under the antenna and made it's way under the base of the antenna and wicked it's way up the threads of the large nut. I've had to replace whole coax assemblies/NMO mounts, after customer complaints of noise, reduced reception, etc., due to corrosion developing where the antenna made contact with the mount. In 100% of the installs, on my personal vehicles, I've never had an issue with moisture wicking it's way up the threads of the nut when I used one of these gaskets.

If an NMO mount is installed properly there should be no moisture leaking past the rubber "O" ring under the large nut. I use a silicone(?) grease, made by Motorola, on the rubber "O" ring of the nut and the rubber gasket anytime I do an install.

every NMO install I do gets those. The older thicker ones are better the new ones that come with the Comtelco are paper thin and stretch too easy

Always, except ...

Water and radio do not play well together. Any way you do it, moisture and gunk will work tirelessly to get past the threads and wack out the antenna. I use a dab of silicon grease on the gasket/o-ring just to be sure. The jury is still out about grease on the threads. I've done it both ways and can't tell any difference over time.

Some base coil antennas come with an o-ring to seal between the base coil and the roof. That works assuming the roof is smooth. (The AS Mosaic antennas come with a clear plastic 'decal' that sticks down around the mount threads and seals to bad paint; it provides a nice smooth surface for the o-ring. I have a couple dozen floating around in my parts box for installing other antennas on rough paint. Silicon grease on top fills the tiny gaps.)

Installing a NMO on treadplate (truck toolbox or the roofs of some fire apparatus) is always a challenge. I normally use a die grinder to remove the offending humps, use the thick gasket and stick it down with moisture-cure urethane sealer just to make sure i'm watertight. Grease between the antenna and gasket.

(Side note: I am always amused when I find a 'professionally installed' antenna with three fingers of clear silicon sealer smeared around the base. Silicon sealer will not make a watertight seal to metal. It may sorta stick for a while, and will always fail after a short time. All of the boat engine builders specify moisture-cure urethane for that reason. I've seen my share of rotted transoms and corroded metal 'sealed' with box store goo. Been there, done that.)

The previous posts about the center pin not making good contact due to gasket thickness are all valid.

A $500 antenna install and a $50 radio will outperform a $5,000 radio and a $50 antenna every time.

stripping about 1-2mm of paint to bare metal around the hole + dielectric grease really helps the ground. Using a gasket (i really like the ones that come with some laird antennas that not only are a round gasket but have a lip that matches up with the antenna base) helps the antenna's o-ring (which tends to get lost if it even has one) seal out the mount.

always always always make sure that the o-ring on the NMO mount is in place, not damaged, and not out of the groove and squished, That right there is the number 1 reason ive seen an NMO leak.

Dont use silicone, like the above post said. Not only does it not work on metal, the fumes will break down some adhesives and make the headliner sag.

You should not need to strip any paint away on the outside of the vehicle body. The NMO mounts have teeth on the underside that will bite into the surface and provide the necessary ground connection.

Dielectric grease is an insulator, and it won't help the ground connection at all.

you do know that "branded" hole saws from motorola and laird and other antenna companies literally are designed to score or strip the paint on the top surface for grounding.

If not dielectric, what am i thinking of?
Waterproof but conductive.
Used on bulbs and battery terminals, sold in little packets at auto parts stores?

That is Dielectric grease. Motorola includes it with their antenna kits and yes NMO hole saws do strip away a small amount of paint.

Yeah, I know what you mean, same hole saw I use, but I've never seen a 1 - 2 mm stripped area around the outside of the hole. Maybe I'm missing something.

Dielectric is non conductive. The dielectric grease is non-conductive, but useful for sealing connections. I don't see how it would help with the grounding at all. I would help with sealing water out, and is what usually comes with the NMO mount kits. Good stuff. I have a big tube I use for some battery connections.

In the antediluvian days when vehicles had dome lights in the center of the headliner I was taught to drop the dome light and use the hole saw from the inside out while running a shop vac to catch the chips. The shop owner had us do it this way for two reasons. 1, we could not screw up and drill through the headliner. 2, you did not scratch the paint on the roof. The saw left a bright ring on the inside for the fingers of the mount to ground to which was covered with a coat of primer just before the dome light was reinstalled.

(Day 8 after purchasing my new to me 2013 GMC Sierra)


It didn't get a NMO mount for that one. If I'm using an antenna that doesn't have the O-ring in the base I use those little gaskets.

Depending on the original spec, there should always be two seals/gaskets between the exterior environment and the interior of the roof. The primary O-ring on the brass locking collar is one. In the case of "chrome nut" antennas there is the rubber base (under the nut) which seals to the locking collar and then the gasket originally pictured at the top of the thread. In the case of the outer O-ring style (common with loaded bases) you have the two O-rings to get through as the rest of the base is hermetically sealed). Some manufacturers go a step further and provide a seal in the top of the loaded base (Larsen has a seal present on some of their antennas).

Most Chinese antenna vendors only supply a seal in the top of the base...sometimes you can get away with adding the OPs gasket and sometimes you can't.

Why did you wait 8 days?

Did a couple of trucks the same day they were picked up from the dealer. My dad's new 2014 Chevy came to my shop direct from the dealer.

Honestly, though, always glad to see people doing the proper installs. I shudder at some of the antenna mounting jobs I've seen here at rr.

firefighter479 said:

I've always used them. When I worked as a part time installer, at a two way radio shop, I had to replace damaged antenna's at various times. Many times a previous installer didn't install the rubber gasket in conjunction with the antenna. I would often find where moisture had made it's way under the antenna and made it's way under the base of the antenna and wicked it's way up the threads of the large nut. I've had to replace whole coax assemblies/NMO mounts, after customer complaints of noise, reduced reception, etc., due to corrosion developing where the antenna made contact with the mount. In 100% of the installs, on my personal vehicles, I've never had an issue with moisture wicking it's way up the threads of the nut when I used one of these gaskets.

If an NMO mount is installed properly there should be no moisture leaking past the rubber "O" ring under the large nut. I use a silicone(?) grease, made by Motorola, on the rubber "O" ring of the nut and the rubber gasket anytime I do an install.

Click to expand...

If you use some high vacuum silicon grease on the threads of the NMO mount, you won't have that problem. The grease that comes with the mount sucks and migrates with heat. The other product you could use is some sort of an anti seize compound that has conductivity like Pentrox or another grounding compound. This will prevent the large nut from becoming non moveable over time if you need to replace the mount for some reason.