Antenna Install

[RadioGrub22]

Hello everyone,

I usually just guest browse these forums when I have a radio issue but wanted to make an actual account to ask this one.

I have a 2m radio for which I am going to mount an NMO in a small plastic housing that mounts at the third brake light location on the rear roof of my pickup truck. I’m assuming ground plane is “ok” as I at least have a nice metal plane forward of the antenna and while not ideal due to the drop in height, I also have a metal tonneau cover that I’m guessing will act as the rear ground plane. Please correct me if I am wrong.

My question is, given that the mount is plastic, do I need some sort of grounding ring run from the NMO to the cab to ground the NMO itself? I know that NMO mounts are usually grounded to metal when they bite in with their teeth. I have heard with a 1/2 wave antenna that it doesnt really matter, but I’m not sure if that’s referring to ground plane in general or referring to grounding the actual mount so that the coax isn’t acting like an antenna.

The mount is very tight so I can’t use a metal plate or anything underneath either.

 

[bchappuie]

It would be greatly improved performance if the mount was metal, and the bolts touch the vehicle, and not just a plastic insert. To get good ground plane the antenna needs to be associated with the plane, not just next to it, as it needs to be connected to the ground side of the antenna, to make it reasonant.

 

[RadioGrub22]

That is what I suspected. I think my plan will be to ground the NMO using a ground strap or as big a washer as I can fit underneath and run it to the roof to tie the two together.

My other question is what about a NGP NMO antenna? Do NGP antennas still require a properly grounded NMO base or can they operate effectively in my scenario above (no ground strap).

 

[SurgePGH]

Hello everyone,

I usually just guest browse these forums when I have a radio issue but wanted to make an actual account to ask this one.

I have a 2m radio for which I am going to mount an NMO in a small plastic housing that mounts at the third brake light location on the rear roof of my pickup truck. I’m assuming ground plane is “ok” as I at least have a nice metal plane forward of the antenna and while not ideal due to the drop in height, I also have a metal tonneau cover that I’m guessing will act as the rear ground plane. Please correct me if I am wrong.

My question is, given that the mount is plastic, do I need some sort of grounding ring run from the NMO to the cab to ground the NMO itself? I know that NMO mounts are usually grounded to metal when they bite in with their teeth. I have heard with a 1/2 wave antenna that it doesnt really matter, but I’m not sure if that’s referring to ground plane in general or referring to grounding the actual mount so that the coax isn’t acting like an antenna.

The mount is very tight so I can’t use a metal plate or anything underneath either.

Your best bet is to just drill the roof. When it is time to sell or trade in the vehicle most people will not even notice it as anything unusual and even if they do it really does NOT affect the resale value of the vehicle.

 

[RadioGrub22]

Your best bet is to just drill the roof. When it is time to sell or trade in the vehicle most people will not even notice it as anything unusual and even if they do it really does NOT affect the resale value of the vehicle.

I totally agree, as I have 2 other NMO mounts where I greenlee punched the roof for, 1 for a GMRS antenna and 1 for a marine XM antenna.

The 3rd brake light mount I have is a 3d printed version that allows use of some nice aftermarket LEDs for bedlights, in addition to having a built in NMO mounting hole which is why I was going to figure out how to get decent performance out of it. I don’t mind a little ding in performance, but I’m not afraid to drop my entire headliner and run a ground strap if need be, it it will lead to much better performance.

 

[citiot]

Your best bet is to just drill the roof. When it is time to sell or trade in the vehicle most people will not even notice it as anything unusual and even if they do it really does NOT affect the resale value of the vehicle.

Agreed. I always get a couple NMO mounts drilled in my new vehicles. New car smell along with the smell of a punch bit going through the paint, clear coat and metal... yummy.

In a pinch, I suppose grounding rings (as OP mentioned) would help.

 

[mmckenna]

Hello everyone,

I usually just guest browse these forums when I have a radio issue but wanted to make an actual account to ask this one.

I have a 2m radio for which I am going to mount an NMO in a small plastic housing that mounts at the third brake light location on the rear roof of my pickup truck. I’m assuming ground plane is “ok” as I at least have a nice metal plane forward of the antenna and while not ideal due to the drop in height, I also have a metal tonneau cover that I’m guessing will act as the rear ground plane. Please correct me if I am wrong.

No, it won't be "OK".
The ground plane needs to extend 1/4 wavelength out in all directions directly under the antenna base to work ideally.
By mounting it on the 3rd brake light, you are going to introduce a couple of issues:
1. Mounting the antenna to the plastic is going to create an issue if you hit a low tree branch, parking garage, drive through, etc.
2. The metal tonneau cover is going to be too low to do much good.
3. With the ground plane of the truck roof all out in front of the antenna, it's going to make your radiation pattern directional.

There's a really good reason why people will tell you to bite the bullet and drill the hole in the dead center of your truck roof. It's because it works, and it works well.
Done right, it will not leak and it will not impact trade in value. I've done a lot of NMO installs on leased trucks, the dealers do not care when you turn them in. Using quality mounts (not the Tram/Browning/Chinese crap) and installing them correctly will not cause a leak.

My question is, given that the mount is plastic, do I need some sort of grounding ring run from the NMO to the cab to ground the NMO itself? I know that NMO mounts are usually grounded to metal when they bite in with their teeth.

Yes, it needs to be grounded. But do not confuse a DC ground (simple continuity) with a proper RF ground. They are different things.

I have heard with a 1/2 wave antenna that it doesnt really matter, but I’m not sure if that’s referring to ground plane in general or referring to grounding the actual mount so that the coax isn’t acting like an antenna.

A halfwave antenna does not require a ground plane to work. They will work better with a proper ground plane.
What you are proposing isn't a proper ground plane. You are going to end up with a lopsided radiation pattern and may have issues getting a low SWR.

But, it's your truck, do what you want.

I don't think you are doing yourself any favors by drilling into the plastic third brake light mount.

Drilling the hole in the dead center of the cab roof will likely be easier than trying to mess with a compromised mount into a piece of plastic that is held in place by a pair of sheet metal screws.
Putting in a proper NMO mount will work better. You'll have a proper ground plane under the antenna, you'll have better luck getting the antenna tuned properly, and it's going to perform much better in the long run.

But I do understand that some people don't want to drill a hole in the roof of their vehicle. I get it (well, actually, I don't get it, either the radio is important and you need it to work well, or you don't).
If drilling is absolutely out of the question, then at least consider one of these as a better solution to using the plastic mount:
3rd Brake Light Mounts - Larson Electronics
Drill Free Roof Top Mount - Acari Products
Do understand that neither of these are going to provide a proper ground plane, but they'll be better than what you are proposing.

 

[mmckenna]

As for the NMO mounts and antennas, don't fall for the "I'll save a few dollars by going with this cheap Chinese mount".

Done correctly, an NMO mount will outlast the vehicle it's attached to. The labor is the hard part. Saving a few dollars on a cheap knock off product isn't going to pay off in the long run. I've (unfortunately) been required to install one of the "Tram" mounts on a 2019 Silverado. The brass is soft and it's easy to cross thread. The "pre-terminated" connectors are pure junk. The coax seemed OK, but it certainly was not worth the few dollars they saved over the Larsen, MaxRad, Antennex, Laird mounts. You really do get what you pay for.

Same goes for the antennas. I have Larsen antennas that are 30 years old and still going strong. Worth every penny.

 

[RadioGrub22]

No, it won't be "OK".
The ground plane needs to extend 1/4 wavelength out in all directions directly under the antenna base to work ideally.
By mounting it on the 3rd brake light, you are going to introduce a couple of issues:
1. Mounting the antenna to the plastic is going to create an issue if you hit a low tree branch, parking garage, drive through, etc.
2. The metal tonneau cover is going to be too low to do much good.
3. With the ground plane of the truck roof all out in front of the antenna, it's going to make your radiation pattern directional.

There's a really good reason why people will tell you to bite the bullet and drill the hole in the dead center of your truck roof. It's because it works, and it works well.
Done right, it will not leak and it will not impact trade in value. I've done a lot of NMO installs on leased trucks, the dealers do not care when you turn them in. Using quality mounts (not the Tram/Browning/Chinese crap) and installing them correctly will not cause a leak.



Yes, it needs to be grounded. But do not confuse a DC ground (simple continuity) with a proper RF ground. They are different things.



A halfwave antenna does not require a ground plane to work. They will work better with a proper ground plane.
What you are proposing isn't a proper ground plane. You are going to end up with a lopsided radiation pattern and may have issues getting a low SWR.

But, it's your truck, do what you want.

I don't think you are doing yourself any favors by drilling into the plastic third brake light mount.

Drilling the hole in the dead center of the cab roof will likely be easier than trying to mess with a compromised mount into a piece of plastic that is held in place by a pair of sheet metal screws.
Putting in a proper NMO mount will work better. You'll have a proper ground plane under the antenna, you'll have better luck getting the antenna tuned properly, and it's going to perform much better in the long run.

But I do understand that some people don't want to drill a hole in the roof of their vehicle. I get it.
If drilling is absolutely out of the question, then at least consider one of these as a better solution to using the plastic mount:
3rd Brake Light Mounts - Larson Electronics
Drill Free Roof Top Mount - Acari Products
Do understand that neither of these are going to provide a proper ground plane, but they'll be better than what you are proposing.

It looks like some of my subsequent posts from my original are awaiting approval, (as is this one, I guess since I’m newly registered) so I’ve answered some of these points above. And it’s hard responding to everyone when my posts are waiting on approval. I’m guessing no one can see my other posts at the moment.

I already have 2 NMOs on my roof, and while I know a hole in the roof is optimum, I’m not sure how well multiple NMOs on the roof lend to interference with each other if they are close proximity. As it is, even if I was to pop a hole for a 3rd antenna, it would presumptively be placed at a peripheral edge/corner of the roof or toward the rear anyway, which to me isn’t much better than mounted at the brake light, but correct me if I am mistaken. I’m guessing even a little bit of metal plane to the rear is better than nothing though. I’ve also read before that antennas should be spaced out to a certain degree to play nice with each other.

The brake light mount I have is an entire 3d printed housing made of some tough 3M material from a high end $100k+ printer, so I’m not too too concerned about overall strength. In fact I’d bet my aluminum foil roof would tear out from an antenna getting caught on something compared to the brake light lol.

I also only run my antennas when they are needed rather than daily. I park in a garage at home and a parking garage at work so having antennas on every day is not possible. Not too worried in that regard. My center mounted NMO antenna has a spring and a backing plate as well just in case.

 

[RadioGrub22]

As for the NMO mounts and antennas, don't fall for the "I'll save a few dollars by going with this cheap Chinese mount".

Done correctly, an NMO mount will outlast the vehicle it's attached to. The labor is the hard part. Saving a few dollars on a cheap knock off product isn't going to pay off in the long run. I've (unfortunately) been required to install one of the "Tram" mounts on a 2019 Silverado. The brass is soft and it's easy to cross thread. The "pre-terminated" connectors are pure junk. The coax seemed OK, but it certainly was not worth the few dollars they saved over the Larsen, MaxRad, Antennex, Laird mounts. You really do get what you pay for.

Same goes for the antennas. I have Larsen antennas that are 30 years old and still going strong. Worth every penny.

No worries, my current antennas are Maxrad and Larsen, both of which I’m running LMR240 on, though it isn’t the easiest to route due to stiffness. Haven’t had any issues with any of those setups.

 

[mmckenna]

Yeah, the post moderating slows things down. At some point that stops and it goes real time...

Mounting several antennas on a roof can be done.
If you can tell us what you have, what frequencies, if they are transmit or only receive, what power levels, how they are spaced, etc. we might be able to suggest something.

If you've already got NMO's on the roof, then I'd still suggest that as a solution. Ground planes do make a difference. Even with a ground independent antenna, like a half wave, any ground plane near the antenna is going to make it directional in some way.

If one of your existing antennas is RX only, I'd suggest putting your transmit antenna on the roof mount NMO, and move the RX antenna to the 3rd brake light mount.

 

[RadioGrub22]

Yeah, the post moderating slows things down. At some point that stops and it goes real time...

Mounting several antennas on a roof can be done.
If you can tell us what you have, what frequencies, if they are transmit or only receive, what power levels, how they are spaced, etc. we might be able to suggest something.

If you've already got NMO's on the roof, then I'd still suggest that as a solution. Ground planes do make a difference. Even with a ground independent antenna, like a half wave, any ground plane near the antenna is going to make it directional in some way.

If one of your existing antennas is RX only, I'd suggest putting your transmit antenna on the roof mount NMO, and move the RX antenna to the 3rd brake light mount.

So my setup is a GMRS band antenna (it’s like 30 something inches tall, if I recall like 5db gain range) with a 40W midland radio (RX/TX) in center of roof, sort of offset toward the rear, and then a couple feet forward of that is a Shakespeare marine XM radio puck (RX only).

I was considering moving the XM puck to the brake light mount but the puck uses a 9/16” style stud (sort of but not actually a NMO I guess), whereas the brake light is made for a 3/8” NMO mount. I think what I may be able to do is drill out the mounting hole to 9/16 and move the XM puck there and then just place my 2m in the existing hole at the front of the roof and downscale to like a 1/4 wave antenna.

 

[mmckenna]

That's probably a good idea. The XM antenna won't need the ground plane.