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Author Topic: NMO punch thru 2 layers of sheet metal?  (Read 3717 times)
K3GM
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« on: September 30, 2011, 06:37:58 PM »

This is sort of a companion post to my earlier one regarding the FordFusion.  My plan is to install an NMO mount in the trunk lid of my new car.  The ideal location on the lid has two layers of sheet metal, the trunk lid and an inner stiffening skin, about the same gauge as the lid.  The web is flat against the lid, but a piece of paper can be inserted between them.

I get one chance with the punch. Forget the Antennex hole saw.  I only punch.  2 questions:  Will the Greenlee go thru two layers of sheet?  Will the mount accommodate a combined thickness of two layers of typical automotive sheet metal?

http://i9.photobucket.com/albums/a96/TwoSevenRight/2011-09-29_17-18-50_343.jpg

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K7AAT
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« Reply #1 on: September 30, 2011, 07:44:16 PM »


  The bottom line answer to your question,  I do not believe the standard NMO mount will fit on two thicknesses of automotive steel.  I can not say whether or not a green-lee will cut through, either;  possibly,  but it may be risky.

   On the bright side,  there ARE special NMO mounts that are made to mount on thicker materials.  There are also some special thick mount NMO s that mount in a 3/8"  hole and are much easier to install for your particular need.   Some such are available on EBay at:  http://www.ebay.com/sch/i.html?_nkw=thick+NMO+mount  .  ( I had merely Googled  " NMO mount thick"   to find this.    I recently purchased one of these and it appeared to be a rather good quality unit.  It was mounted on an aluminum boat with rather thick decking.  One caveat,  it requires an FME coax connector for the underside.  I also found this connector on EBay, listed for 4 dollars.... for the RG-8X coax I was using.   It was a crimp connector so you will need a crimp tool with proper sized die.

   Ed   K7AAT
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K0BG
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« Reply #2 on: October 01, 2011, 04:20:24 AM »

Most all trunk lids have strengthening braces. If you choose the right location, you can install the NMO from outside, and snake the coax around and through the bracing. If you must go through two layers (I wouldn't, see below), there are NMO mounts made for the purpose.

If you do go through two layers, you want to be careful. The bracing, and the skin of the trunk lid are usually separated by a layer of insulation, or perhaps a glue layer. If there is a gap, and you tighten down the NMO, you run the risk of creating a stress dent.
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K7AAT
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« Reply #3 on: October 01, 2011, 08:46:03 AM »


 Yes,  I fully agree with Alan's comments.   The picture originally posted is a good one but unfortunately I am having difficulty 'getting my bearings'.   It appears to me that the inner bracing material is actually spaced away from the actual external trunk sheet metal in the mounting area you were speaking of and that you possibly could install the antenna normally with an external 3/4" antenna hole drill bit of the type made for that purpose and insert the NMO mount from the top as is normal,  and IF the spacing underneath is at least half an inch ,  you could route the coax between the external sheet metal and that inner brace material without drilling a hole in it.   The NMO mount was designed to be installed this way.  I think using a GreenLee punch may make your installation more difficult.    Perhaps a couple more pictures of both the inside and outside could give me a better perspective.  Looks like some kind of small 2 seat sport car from what I can see now.

Ed   K7AAT
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K3GM
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« Reply #4 on: October 01, 2011, 09:48:35 AM »

Ed, the vehicle is a Ford Fusion.  Image is trunk up, so the top of the image is the rear most edge of the trunk.  I'd like to place the mount between the two triangular cutouts.  There the 2 sheets are lying against one another and are not separated, but perhaps I should take the safe route and place it towards the rear edge of the lid where i can access the lid sheet only.

Note that I could place it inside one of the triangular cutouts, but the antenna would be slightly off-center.
« Last Edit: October 01, 2011, 10:02:31 AM by K3GM » Logged
K7AAT
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« Reply #5 on: October 01, 2011, 10:12:59 AM »


  OK.  Its clear to me now.  I do think you could experience unforeseen issues with both installation and antenna mount mechanical problems if you tried to mount a standard NMO mount on those two sheets of metal,  as Alan also mentioned.  If you don't want to use the special mount I pointed out,  then perhaps your suggestion of moving it back just beyond the stiffening layer would be the way to go.   While visualizing it best I could I think that is what I would do.   Good luck.   I was going to suggest a low profile trunk lip mount available from Antenex,  but in reality your hole mounted antenna while outperform such,  considerable.

   Ed  K7AAT
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K3GM
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« Reply #6 on: October 01, 2011, 11:02:07 AM »

Ed, "the hole" in my new vehicle is almost tradition for me, and they perform so much better too.  This is the first Ford product for me in a long time.  All of my previous GM vehicles had small oval cutouts in the stiffening ribs or sheet metal to allow an NMO mount.
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