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Author Topic: Reinforcing Sheet Metal for NMO Mounts  (Read 5438 times)
N7SZF
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« on: May 28, 2010, 05:16:01 PM »

I have been lurking for quite a while and have greatly enjoyed reading the forums for the past few months.

I'm putting in a new NMO mount.  The sheet metal is very thin and seems a good candidate for a tree branch tearing it out.  You guys have mentioned using aluminum plates to reinforce flimsy sheet metal before.

How do you adhere the new plate to the existing sheet metal? Please be specific with adhesives and their application.

How do you make the joint electrically sound? I have never heard of a conductive adhesive.

This mount will be used from 6m through 70cm.

Many thanks!

Mike - N7SZF
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N7ZAL
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« Reply #1 on: May 28, 2010, 05:47:05 PM »

As far as a conductive adhesive, there is a silver epoxy out there that we used years ago to connect ground bars....
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Later, Bill N7ZAL (ex. WA2DPB, WB3BOC, N2FWS)
WB2WIK
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« Reply #2 on: May 28, 2010, 05:52:26 PM »

To do this, I never used any adhesive at all (and I have done it).

Just a 16 gauge aluminum backing plate, held in position by the NMO mount itself, which clamps the backing plate to the car's sheet metal.  Once you tighten down the NMO mount, it cannot budge -- the mount itself has great compression strength.

Obviously, for this to work, you pre-punch an NMO (3/4") hole in the backing plate in the appropriate location, and then in the car sheet metal, and line up the two holes while installing the NMO.  I don't see why adhesive wouldn't work, if you want to use some (hot melt glue or epoxy, perhaps) and it wouldn't need to be conductive (although there are silver and copper filled epoxies which are quite conductive -- they're specialty items for the electronics industry and not at Home Depot; Chomerics and a few others make them).

When I've had occasion to do this was when mounting an NMO in the roof or trunk lid of a Corvette.  The fibreglas alone might be strong enough, but it's not conductive and metal needs to be put under the mount to make the antenna work.  I used an aluminum backing plate formed to fit between the ribs of the trunk lid, and that was the only ground plane area the whip had, but it worked quite well and a whole lot better than fibreglas!

NMOs come in different styles for different sheet metal thicknesses.  If you use a thick backing plate, plus the original car sheet metal, a standard NMO might not be "deep" enough to clamp down on it; for such instances, there are "deeper" NMO mounts, see the Larsen catalog for P/N information.
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AF6WI
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« Reply #3 on: May 29, 2010, 12:34:37 PM »

I have a Tarheel Model 100 on the side of my van, and the installer used a steel plate to reinforce the van wall:
http://civex.smugmug.com/Electronics/Tarheel-Antenna-Mount/4328495_EwCwx#253859687_DBNsL
for photos. (Hold your cursor over the picture, and a window will pop up allowing you to select a larger size for your viewing pleasure.)

We didn't glue it or otherwise fasten it to the wall other than the bolts that hold the Tarheel mount on the outside. I did run a grounding strap from a bolt to a van truss after scraping paint off on the truss to be sure I got ground on the plate. (In case it's not obvious, the plate is that black rectangle behind the trusses.) There are other photos of the plate, plus the installed mount from the outside.

I don't hold myself out as an expert on mobile, but my preference is for knowing there's a ground between that support plate and the vehicle - whether it's really necessary or not, I couldn't say.
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W3LK
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« Reply #4 on: May 29, 2010, 06:54:54 PM »

I've never run into sheet metal on a vehicle that's too thin to support an NMO mount, and I have installed a lot of them. I've never had one rip out.

Make sure you are using a US-made mount - from Motorola, Larsen or AnteneX. The imported mounts are NOT standard NMO mounts and they often are not waterproof.
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N7SZF
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« Reply #5 on: May 30, 2010, 11:14:04 PM »

Appreciate the info!

I'm using the Larsen NMO mount, don't think that's going to be an issue. 

Small Japanese vehicle.  I've never had one tear either, but the amount of wiggle on this one is extreme, the roof deforms with the slightest pressure on the antenna base.  With the plate, it's much closer to what I've seen before. I'm a firm believer in prevention.
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K1CJS
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« Reply #6 on: June 06, 2010, 06:34:33 AM »

With some of the newer cars, the sheet metal is no thicker than a couple of sheets of heavy duty aluminum foil.  I wouldn't be worried about the mount tearing out--but i WOULD be worried about deformation of the sheet metal under the mount.

A backing plate seems to be the wisest way to prevent that.  If you're worried about the conductivity, lightly sanding the area around the NMO mount hole can be done so the backing plate touches bare metal.  A smear of contact adhesive around the edge of the backing plate will ensure that it doesn't come off or move.
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KI4SDY
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« Reply #7 on: June 12, 2010, 07:39:37 AM »

If the metal is that flimsy and the antenna is as long and heavy as most multi band antennas I have seen, the exterior metal may deform and end up outlining your reinforcement plate. That would be very ugly! I would just use the MFJ 6 and 2 meter magnetic antenna and another short MFJ magnetic 70cm antenna. You can place both on the roof or the longer one on the trunk and the short one on the roof. Wink
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K0BG
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« Reply #8 on: June 12, 2010, 07:55:19 AM »

The truth is, mag mounts are worse than drilling a hole! The main reason is, they collect metallic brake dust, and other debris, which eventually discolors, and scratches the paint surface.
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KI4SDY
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« Reply #9 on: June 12, 2010, 08:10:34 AM »

That might be true for people who never wash their car or don't read the directions that come with the antenna that tells them to remove it when washing. Its also tells them to make sure the surface is dry when replacing it. I have used them for decades with no problem. Roll Eyes
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KC2RFQ
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« Reply #10 on: October 19, 2010, 11:52:03 AM »

I have been lurking for quite a while and have greatly enjoyed reading the forums for the past few months.

I'm putting in a new NMO mount.  The sheet metal is very thin and seems a good candidate for a tree branch tearing it out.  You guys have mentioned using aluminum plates to reinforce flimsy sheet metal before.

How do you adhere the new plate to the existing sheet metal? Please be specific with adhesives and their application.

How do you make the joint electrically sound? I have never heard of a conductive adhesive.

This mount will be used from 6m through 70cm.

Many thanks!

Mike - N7SZF

I doubt that the NMO would rip out under normal conditions.
However, having said that, I noticed that there was just a bit too much flex on the roof of my Chevy Trailblazer. I am constantly putting my 2M/440 Comet antenna into the foldover position and was not comfortable with the amount of flex when I manipulated it to that position.
I used a blank cover for a 2-gang steel electrical box (available from most electrical departments at a home improvement center) as the backing stock material. It's about 4"x4" and perfect for the job. You can easily customize it if needed with a hacksaw and it's inexpensive. Use sandpaer to clean up the surfaces that will come into contact. Use your 3/4" hole saw to cut a hole in the appropriate place on the plate and sister it to the underside of the roof. No need for any adhesive. Line up the hole in the roof with the one in the plate and install your NMO mount. The standard thickness NMO mount will work just fine with a little maneuvering and the compression of the NMO fittings will cinch the backing plate firmly into place. Be certain that the NMO mount claw tabs  go through both metal pieces and engage the underside of the backing plate. The backing plate adds a significant amount of stiffness (and security) to the mount. If grounding is a concern (it shouldn't be, the backing plate makes excellent contact in every installation I've done) you can drill and tap a small hole in the plate for a short screw and run your ground to it. Always give due consideration to your particular antenna/mounting situation. What works in one case might not be appropriate in others.
« Last Edit: October 19, 2010, 01:14:33 PM by Steven Fishman » Logged
W5WSS
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« Reply #11 on: November 17, 2010, 01:02:26 PM »

Avoid mag mounts.
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K1CJS
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« Reply #12 on: November 21, 2010, 08:01:22 AM »

......I used a blank cover for a 2-gang steel electrical box (available from most electrical departments at a home improvement center) as the backing stock material. It's about 4"x4" and perfect for the job. You can easily customize it if needed with a hacksaw and it's inexpensive.......

That type of "plate" may be too small in certain situations, and even though you may stiffen the mount considerably with it, I would hesitate to use something that small--or that stiff.  What you really want is something like an aluminum pizza plate, about 12 inches in diameter, and cut down to fit if necessary.  Such a plate would reinforce, yet it would be flexible enough to allow minimal movement without imprinting the backing plate on the sheet metal of the area where the antenna is mounted.
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KJ1H
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« Reply #13 on: November 21, 2010, 04:57:20 PM »


What you really want is something like an aluminum pizza plate, about 12 inches in diameter, and cut down to fit if necessary.  Such a plate would reinforce, yet it would be flexible enough to allow minimal movement without imprinting the backing plate on the sheet metal of the area where the antenna is mounted.

Seems to me a plate like that could also be used as a groundplane on a surface that doesn't work as one, such as the Corvette previously mentioned (if there's enough roof - wouldn't work so well with a targa top), or the Saturn wagon I had last year with a plastic roof.  I can't imagine why anyone would want to put a 3/4" hole through an expensive custom carbon fiber trunk lid, but if they did it would work there too. Smiley
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73 - Justin
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