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Help: NMO antenna hole too big

This forum is dedicated to helping people with questions about installing radio equipment in vehicles. This can include antenna installs, electrical wiring questions/problems, and mounting systems. Pictures of installs are welcome.

Note: Discussions regarding lighting, sirens, and other equipment now has its own forum in the 'off-topic' section below.

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Ocean17
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Joined: Tue Jan 16, 2007 2:33 pm

Help: NMO antenna hole too big

Postby Ocean17 » Thu Jul 26, 2007 10:35 pm

Yes, I know, I know..."I drilled it twice and it's still too big!"

So, I had a bunch of "helpers" last night and someone drilled the ambulance roof hole too large for an NMO mount. It is about a 1/8 to 1/4 of an inch to large. (I laid out the whole punch and they grabbed the wrong one) I was using a hole punch as the aluminum roof was too thick for my NMO hole saw. Any ideas or remedies for this? I was considering two oversized washers? Riveting a plate over the hole? Ever hear of an adapter or "filler"?
Thank you kindly...

bellersley
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Re: Help: NMO antenna hole too big

Postby bellersley » Thu Jul 26, 2007 11:19 pm

What size NMO mount were you putting in - 3/8" or 3/4"? If you were hoping for a 3/8" mount, you can probably just make the hole bigger for a 3/4" mount. If not, you might be better off having someone weld a plate in the hole and starting over again.

Ocean17
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Re: Help: NMO antenna hole too big

Postby Ocean17 » Thu Jul 26, 2007 11:37 pm

NMO is 3/4", and the hole is slightly larger. Not to keen on welding on the aluminum roof, but perhaps... I will investigate that possibility, thanks
Thank you kindly...

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Bruce1807
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Re: Help: NMO antenna hole too big

Postby Bruce1807 » Fri Jul 27, 2007 7:07 am

we often rivet a plate to vehicles when radio is removed and being sold.
In this case though I'd look for some big washers or similar.
Lots of silicon please.

tvsjr
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Re: Help: NMO antenna hole too big

Postby tvsjr » Fri Jul 27, 2007 7:43 am

Have a washer welded into place, possibly along with a plate underneath. Fill with Bondo, paint. Drill new hole somewhere else and tell your friends to stay home.

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escomm
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Re: Help: NMO antenna hole too big

Postby escomm » Fri Jul 27, 2007 8:01 am

Motorola sells rubber plugs for the 3/4" holes, they are real easy to use and could be looked as a temporary to intermediate solution. They come in packs of 10 and cost about $5 for the pack. There may be enough play for it to fit a hole slightly larger than 3/4" and stay in secured. It's a pain to get it on a 3/4", to be sure.

Last weekend I was on a service call for a radio whose complaint was no tx/very poor rx. Whoever installed the antenna (a large MSS here in SoCal who shall remain nameless, working for a prior contractor) saw fit to drill a 1 1/4" hole, and when they realized the error of their ways, their solution was to mount the antenna tit to a 2" square steel plate, and then use the antenna to "hold" the plate to the vehicle's body by way of screwing it on way too tight.

Right now my intermediate solution was three layers of electrical tape over the hole until the customer's body shop can bondo it, weld it shut, or whatever the hell they want. Motorola did at one time sell a 1 1/4" hole plug that would work perfect for this, but of course it's NLA according to Parts ID. How they end up fixing it is going to be between them and the previous contractor that operated the vehicles.

It's absolutely amazing what people in this industry try to get away with. It should be noted that this is the same MSS that installs radios with dry-wall screws and doesn't understand that caution must be taken not to drill screws through power cables, but alas I digress....

spareparts
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Re: Help: NMO antenna hole too big

Postby spareparts » Fri Jul 27, 2007 5:14 pm

Ocean17 wrote:Yes, I know, I know..."I drilled it twice and it's still too big!"

So, I had a bunch of "helpers" last night and someone drilled the ambulance roof hole too large for an NMO mount. It is about a 1/8 to 1/4 of an inch to large. (I laid out the whole punch and they grabbed the wrong one)


I used to keep a bunch of aluminum stiffener plates 8 x 8 x 0.0508 (16 gauge) inches with holes for an NMO mount. You would be astounded how many ambulance boxes were drilled wrong or had damage from antenna's striking obstructions. The problem is there always was further damage from electrolysis between the steel washers, brass base, and aluminum truck body.

One plate inside, one outside, with a thick surface NMO if the original mount was too short for full thread engagement BTW, There are several 2-part auto body adhesives that work exceptionally well in this application. Avoids drilling more holes for pop rivets. I can e-mail a PDF / DXF of the plate so a local fabricator can make a few up for you.

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FireCpt809
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Re: Help: NMO antenna hole too big

Postby FireCpt809 » Fri Jul 27, 2007 6:40 pm

I second Spareparts idea.. A few private ambulance here use type II van style ambulances. They have a fiberglass roofs. We used an 8"x8" steel plate 1/8" with the NMO hole pre drilled in the center. Then drilled a 1 1/4" hole in the center of the fiberglass roof then attached the plate with some silicone and pan head sheet metal screws.

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jban
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Re: Help: NMO antenna hole too big

Postby jban » Fri Jul 27, 2007 8:44 pm

FireCpt809 wrote:I second Spareparts idea.. A few private ambulance here use type II van style ambulances. They have a fiberglass roofs. We used an 8"x8" steel plate 1/8" with the NMO hole pre drilled in the center. Then drilled a 1 1/4" hole in the center of the fiberglass roof then attached the plate with some silicone and pan head sheet metal screws.


Idea from an old tech friend. For fiberglass roof use a thick mount NMO and a $3.00 pizza pan. Drill hole in pizza pan and sandwich to the inside. Use a little silicone to prevent vibration of the pan. Used several times and never had a problem.
John B

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kf4sqb
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Re: Help: NMO antenna hole too big

Postby kf4sqb » Tue Jul 31, 2007 7:01 am

I would say that, short of welding something over the hole to close it and moving the antenna elsewhere (if possible), you can bolt/rivet something over the hole and move the antenna elsewhere. If bolted or riveted, make sure you use either stainless steel bolts, or stainless steel or aluminum rivets, and make sure you put star washers on top and bottom of at least one of the fasteners (preferably a bolt, as rivets can work lose with time). If the plate has a poor electrical connection, it can cause all kinds of strange problems, and possibly cause corrosion. The key either way is to use plenty of de-ox on all surfaces. If the plate fits the roof tightly, with no gaps, and is bolted/riveted properly, there should be no problem with leakage. Make sure all surfaces are clean and dry, with no foreign substances or burrs anywhere. I would certainly be leery of using any type of RTV silicone, as it releases acetic acid (vinegar) as it cures (that's why it smells like vinegar, if you didn't know), and could very well etch the aluminum and accelerate corrosion. You may, if you feel like you really need a sealant, want to consider running a bead of a polyurethane caulk around the outside edge of the plate after assembly. I would recommend a product called Vulkem. I've personally seen this stuff used to seal a hole that was completely submerged in water at the time, and it held. Just be careful of what you get the stuff on, as gasoline won't hardly take this stuff off before it cures!

Also, as was mentioned earlier, let your friends stay at home next time! :roll:

BTW, let us know what method you use, and how you make out with it.
kf4sqb "at" wetsnet "dot" com



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Bruce1807
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Re: Help: NMO antenna hole too big

Postby Bruce1807 » Tue Jul 31, 2007 7:08 am

On the nice vehicles we rivet a plate underneath the hole and then fiberglass the top .
When done properlly, no one knows the difference if the painting is done correctly.

Cavscout
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Joined: Sun Sep 09, 2001 4:00 pm

Re: Help: NMO antenna hole too big

Postby Cavscout » Sun Aug 19, 2007 9:01 pm

Try this... it is not pretty, but can get you out of a jam

http://www.maxrad.com/product_overview_ ... &styleid=8

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EVModules
was EVConcepts
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Re: Help: NMO antenna hole too big

Postby EVModules » Thu Aug 30, 2007 9:58 pm

I'd just drill it out to 1", put a plastic sheet metal hole plug with sealant and pop it in place, and drill a new spot.
Sean Barr
EVConcepts

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FireCpt809
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Joined: Sat Jan 19, 2002 4:00 pm
What radios do you own?: Alot..

Re: Help: NMO antenna hole too big

Postby FireCpt809 » Fri Aug 31, 2007 3:40 am

Cavscout wrote:Try this... it is not pretty, but can get you out of a jam

http://www.maxrad.com/product_overview_ ... &styleid=8


Its on the top of an ambulance..Who is going to see it....LOL

CGK
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Re: Help: NMO antenna hole too big

Postby CGK » Mon Nov 23, 2015 8:13 am

Resurrecting an old thread to see if there are more or better ideas.

I need to mount a new antenna on a race car. The metal roof has no headliner and is open to access. I plan to use a 1/4 wave whip tuned for 450-470 MHz. The current hole in the roof of the car is about 1-1/8” or so. The previous owner had a 3/4" NMO mount installed and made up the difference with a washer on the inside of the car wrapped in electrical tape. That "probably" did not make a good ground for the mount. I bought a 1-1/4” to 3/4” reducing washer (sold for use with electrical conduit boxes) that I plan to mount on the inside of the car and attach the NMO mount to that. I'd like some suggestions how best to do that, or better alternatives. I do not want to fill this hole and make a new, properly sized hole.

I can clean the washer and the inside of the car to make a good electrical connection, but I need to attach the washer solidly because it will support the mount and antenna, and as you know the antenna may get smacked by something even if I unscrew it when the car goes into the trailer and install a rubber plug over the NMO mount. Ideally I would weld or solder the washer to the roof of the car, but it’s in an awkward spot, I don't know how to weld at all and I have never soldered something over my head. I could solder the washer to the outside top of the car after removing the paint to make a clean surface, and then put a rubber gasket between the antenna and the mount to cover the washer/soldering. Another possibility instead of soldering would be to use an electrically conductive adhesive such as those with a lot of silver in them made to repair windshield defogger electrical connections or printed circuit boards. The ones for defogger repair will carry more current.

So, which manner of attaching the washer will be easiest/best from a mechanical and electrical standpoint? Does much current actually flow from antenna to ground? My transceiver is about 45W.

The best suggestion I saw in this thread are from spareparts, and I quote:

"I used to keep a bunch of aluminum stiffener plates 8 x 8 x 0.0508 (16 gauge) inches with holes for an NMO mount. You would be astounded how many ambulance boxes were drilled wrong or had damage from antenna's striking obstructions. The problem is there always was further damage from electrolysis between the steel washers, brass base, and aluminum truck body.

"One plate inside, one outside, with a thick surface NMO if the original mount was too short for full thread engagement BTW, There are several 2-part auto body adhesives that work exceptionally well in this application. Avoids drilling more holes for pop rivets. I can e-mail a PDF / DXF of the plate so a local fabricator can make a few up for you."

Is that a better solution than my reducing washer idea if I do not use such a plate on the outside also because it would be VERY unattractive? Would an aluminum or steel plate instead of my reducing washer on the inside of the car be an adequate ground plane and support the antenna?

Thanks for your thoughts.

CGK
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Re: Help: NMO antenna hole too big

Postby CGK » Tue Nov 24, 2015 8:12 am

Further exploration turned up a mount that should fit my current, too large, hole: https://breedlovemounts.com/NMO_Trunk_Mount.html

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wx4cbh
Posts: 337
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Re: Help: NMO antenna hole too big

Postby wx4cbh » Tue Nov 24, 2015 5:26 pm

Go to your local electrical equipment dealer and ask for an "oil tight hole seal". They are available in various sizes, colors, and finishes, one of which is for a 1and 1/16 inch panel hole size. They have a rubber seal for the outside surface portion, and a threaded stud and specialty contoured fender washer with a nut that goes on the inside surface that centers the device in the hole. I have used the ones from Square D that have a brushed aluminum finish on ambulance roofs because they work well and have a very nice finished appearance.

Hope this might be useful info.
curmudgeon.....and I like it.


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