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NMO Externally Mounted

motormayhem

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Joined
Jun 11, 2012
Messages
7
Hi All,
I have a NMO antenna mounted outside my vehicle on a bracket where the bottom (coax) side of the connector is outside the vehicle. I'm worried the coax isn't weatherproofed on that side of the connector as when it is installed in a vehicle lid that section would be inside the vehicle. Does anyone have experience with the best way to address this?

IMG_0626_400x.jpg
 

a417

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does it still work?

I see tons of corrosion there, a pinch point between body panels. I'd look for a better location.
 
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Oosh. Get this rubber grommet set from Harbor Freight for $5...


This video will show you what rotten coax looks like on the inside...

 

mmckenna

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Hi All,
I have a NMO antenna mounted outside my vehicle on a bracket where the bottom (coax) side of the connector is outside the vehicle. I'm worried the coax isn't weatherproofed on that side of the connector as when it is installed in a vehicle lid that section would be inside the vehicle. Does anyone have experience with the best way to address this?
Long term, you have several issues that are going to cause headaches.

The underside of those style NMO mounts are not designed to be weatherproof. They are designed to be mounted through the roof of the vehicle where the coaxial connection is protected from the elements, warm and dry. Water, road salts, etc. is going to get in there and corrode things. Water + copper + salts = a hot mess of nastyness. You could try sealing it, but from the looks of things, it's already too late.

Solution:
Drill the freakin hole and mount it like it's supposed to be. The hammy bracket mounts are a short term solution.

Or, if you really cannot bring yourself to do a proper installation, at least use the Larsen NMOHF style mounts. They used an enclosed connection on the underside that should work a bit better. It's a really good idea to to add some marine grade heat shrink tubing to the point where the coax enters the mount to seal it up. These are not really designed to be externally mounted, but if you maintain it, it'll work a bit better.

As for coax routing, yeah, it's going to get pinched and that's going to create some more issues:
-smashing the coax like that will change it's characteristic impedance. Probably not going to make a big problem, but it ain't right.
-Damage to the outer jacket will let water in. Water + copper = green corroded mess.
-Damage to weather stripping can let water into the car.


Get rid of that mess and drill a hole dead center in the roof and do a proper installation. Then you don't have to worry about any of this stuff.
 

motormayhem

Member
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Jun 11, 2012
Messages
7
So it does look like there are some options for NMO connectors that are better weatherproofed on the underside, that's good. The two below mounts look like much better options than the connector that is on there. It's hard to tell from the image but the other issues discussed are not really present in this setup. The coax fits freely between the hatch and the body panel so it's not pinched there or even fretting the outer jacket. The coax then routes down the inside of the hatch before the hatch seal and goes into the body. I'm pretty confident the coax is not suffering in this configuration, but the exposed end and solder joints certainly has me worried. Thanks for all the feedback.

These are looking like my best options. Any other suggestions?

IMG_4851.JPG
 

Firekite

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Apr 2, 2019
Messages
346
I have switched to the LArsen mount as its a much better mount for outside on a bracket.


You may want to swap to one of these.
Which? None seem to be explicitly designed for external mounting use. Just glancing at them, it looks like maybe the Maxrad one is the most sealed up?

 

a417

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I'm pretty confident the coax is not suffering in this configuration, but the exposed end and solder joints certainly has me worried. Thanks for all the feedback.
The coax is suffering, due to the end of the cable where the mount is being corroded and allowing moisture and gunk under the jacket, and most assuredly into the core.

If it was replaced with a new NMO mount, new cabling and a drilled hole, you wouldn't have to worry about it at all. @mmckenna has beaten this topic to death before, I'm in his corner.
 
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mmckenna

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I'm pretty confident the coax is not suffering in this configuration, but the exposed end and solder joints certainly has me worried.
Minimum bend radius is a thing with some types of cable. Coaxial cable, fiber optic cable and some twisted pair cable have limits on how tight a bend you can make in them. Bending coax to sharply can cause the center conductor to migrate towards the outer shield. That will result in a change in impedance, and worst case, result in a short between the center conductor and outer shield.

Looking at a random Belden RG-58, they recommend no more than 2" of bend radius:

Your cable routing looks like it's less than 2" radius.


These are looking like my best options. Any other suggestions?
Larsen NMOHF:

Seal the end of the NMO mount where the cable enters with some of this:

The marine grade heat shrink tubing has a hot melt adhesive on the inside that will melt and flow sealing up the connector and cable pretty well. The underside center button where the cable connection is made would probably benefit from a light application of silicone sealant just to be sure it's protected.
 

kb4mdz

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Minimum bend radius is a thing with some types of cable. Coaxial cable, fiber optic cable and some twisted pair cable have limits on how tight a bend you can make in them. Bending coax to sharply can cause the center conductor to migrate towards the outer shield. That will result in a change in impedance, and worst case, result in a short between the center conductor and outer shield.

Looking at a random Belden RG-58, they recommend no more than 2" of bend radius:
Soooo, anyone ever notice how most NMO-mounts with attached RG-58 cable, the cable is wound up in a hank like it's a rope? With sometimes a bend radius of less than 2 inches?

And I've also been concerned about unprotected shield underneath NMO's mounted on the various L-brackets & such. Great place for water to enter & wick down and ruin the cable. But hey, when you're contracted to do so many car installs in a day for that PoPo agency, and a protected NMO mount costs a little more, and any future failures can just be chalked up to normal wear & tear but billed out.... eh, what are some radio shop managers gonna do?

Anyone? Anyone? Buehler??
 

mmckenna

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Soooo, anyone ever notice how most NMO-mounts with attached RG-58 cable, the cable is wound up in a hank like it's a rope? With sometimes a bend radius of less than 2 inches?
I think it might depend on the brand. I use Larsen, and while it's wound up in the package, I've never seen it too tight.

And I've also been concerned about unprotected shield underneath NMO's mounted on the various L-brackets & such. Great place for water to enter & wick down and ruin the cable. But hey, when you're contracted to do so many car installs in a day for that PoPo agency, and a protected NMO mount costs a little more, and any future failures can just be chalked up to normal wear & tear but billed out.... eh, what are some radio shop managers gonna do?

Anyone? Anyone? Buehler??
I've never seen an exposed bracket mount done by a professional shop, only hobbyists. But I don't doubt it happens. A properly installed NMO won't have this issue.
Unfortunately there are too many that have aversions to doing a proper antenna install.

Bigger issue I ran into was the dang Motorola 800MHz antennas for the late 80's/90's that had the plastic base. The water would get in there, corrode and split the base open. If not fixed, it would start corroding the NMO mount. Got tired of fixing those and swapped out all those P.O.S. Motorola antennas for Larsens.
 

motormayhem

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The biggest issue I have with mounting it though the body panel here is this is on our camping vehicle. Even with the best of intentions we have had an antenna snag a tree and get ripped off. Its much easier to make a new bracket than have a buckled/ripped body panel. Maybe for this setup it would have made more sense to use an SO-239 connector.

I think for the time being I'm going to swap to one of the heat shrunk / crimped versions of the NMO base and see how it holds up. It looks like I can get those with ATX195 coax or RG58A/U. Since my run is only 3ft in length I'm thinking the difference in loss is negligible and getting the RG58A/U with the solid PE dielectric vs the ATX195 with a foam PE is going to be my best foot forward with regards to bend radii / conductor migration? Is that correct thinking?
 

a417

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The biggest issue I have with mounting it though the body panel here is this is on our camping vehicle. Even with the best of intentions we have had an antenna snag a tree and get ripped off. Its much easier to make a new bracket than have a buckled/ripped body panel. Maybe for this setup it would have made more sense to use an SO-239 connector.
I have taken many a public safety vehicle thru some really inappropriate areas and never lost an antenna to a tree branch, lost a light bar once...but not an antenna.

And if that bracket bends and warps enough to deform it, it's most likely going to deform the underlying metal.
 

Mtnrider

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Swap out the mount for the above mentioned ......I use electricians puddy on the bottom since i live in a salty area...Mount always stay like new this way
 

mmckenna

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The biggest issue I have with mounting it though the body panel here is this is on our camping vehicle. Even with the best of intentions we have had an antenna snag a tree and get ripped off. Its much easier to make a new bracket than have a buckled/ripped body panel.
I drive a 1 ton 4x4 service body truck at work up into some nasty areas to access our equipment. I've bent the headache rack (1 1/4 square steel) on tree branches, but the two NMO mount antennas on the roof of the truck have not been damaged. And that's an aluminum body Ford. The antennas have springs in the bases, so they just fold over and pop back up. One of the roads was over grown and hadn't seen a vehicle in a few years.

On my personal truck (also a 1 ton 4x4 Ford) I ran through a parking garage in Las Vegas last year and got in a situation with backed up traffic behind me that I couldn't stop to remove the antenna. I had about 3" clearance. The spring in the base of the antenna took a permanent bend, but didn't damage the mount or the aluminum roof.

Maybe for this setup it would have made more sense to use an SO-239 connector.
SO-239's are not waterproof and were never designed to be antenna mounts. The hams use them, but you won't see them in use in any professional application. You could wrap it with sealing tape and it might be OK.

I think for the time being I'm going to swap to one of the heat shrunk / crimped versions of the NMO base and see how it holds up. It looks like I can get those with ATX195 coax or RG58A/U. Since my run is only 3ft in length I'm thinking the difference in loss is negligible and getting the RG58A/U with the solid PE dielectric vs the ATX195 with a foam PE is going to be my best foot forward with regards to bend radii / conductor migration? Is that correct thinking?
I think that is your best shot. As for the cable, check with the manufacturers and see which one has the tighter bend radius. For sharp bends, you want one with a stranded center conductor.
And 3' at VHF and UHF, won't be noticeable.
 

W9BU

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I've bent the headache rack (1 1/4 square steel) on tree branches, but the two NMO mount antennas on the roof of the truck have not been damaged.
What are you using for antennas? Do they have flexible whips? (I think I know the answers to those questions. ;) )

If the OP is using a made-in-Japan antenna built for the amateur radio market, it may not be very flexible over its entire length. Antennas like the Comet SBB5 or Diamond NR770 often have fairly stiff elements which transfer the stress of hitting tree branches and such directly to the mount.

I understand the compromises the OP is making with his antenna mount. I've made them, too. A point that the OP may want to consider is that a flexible 1/4 wave whip on top of the vehicle may perform just as well, or better, than an antenna with 2-3 dBd gain mounted half-way down the vehicle in the rear hatch opening.

User edit: I corrected the Diamond model number.
 
Last edited:

mmckenna

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What are you using for antennas? Do they have flexible whips? (I think I know the answers to those questions. ;) )
Larsen, dammit!

The one I bent the spring on it Vegas was a Larsen NMOWBQC VHF wide band 1/4 wave antenna. I've since switched back to the standard Larsen NMOQ without the spring. The whip is flexible enough to take the hit.

The work truck that gets to go boonie-crashing is the Larsen NMOWBQC with spring and a Larsen NMO800 with spring. Neither whip is super flexible, but the spring is enough to absorb the hits.


If the OP is using a made-in-Japan antenna built for the amateur radio market, it may not be very flexible over its entire length. Antennas like the Comet SBB5 or Diamond NMO770 often have fairly stiff elements which transfer the stress of hitting tree branches and such directly to the mount.
And the ham/hobby grade antennas are often more expensive.
 

Firekite

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I've since switched back to the standard Larsen NMOQ without the spring. The whip is flexible enough to take the hit.
Out of sheer curiosity, you think it’s strong enough to handle pulling into and out of a garage that only has about 6” roof clearance? I’m wondering if I’m just going to be stuck storing an antenna behind the back seat and manually replacing a rain cap if I want to use the radio.
 

mmckenna

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Out of sheer curiosity, you think it’s strong enough to handle pulling into and out of a garage that only has about 6” roof clearance? I’m wondering if I’m just going to be stuck storing an antenna behind the back seat and manually replacing a rain cap if I want to use the radio.
Yes. My dad has a 1/2 ton Chevy pickup 4x4 with that antenna with about 6" of clearance and no issues.
My wife drives a 2009 Ford Escape with that antenna on it It's probably got a foot or so of clearance, but that's been 11 years of pulling in and out of the garage without any issues.
 
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