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Author Topic: Reinforcing car roof  (Read 4321 times)

Posts: 585


« on: May 24, 2012, 02:11:26 PM »

I have a through panel So-239 and a through panel M8 mount (UK PMR antenna company Panorama, I guess equivalent to an NMO) on my car. The M8 is dead center and used for VHF/UHF, no problem. The SO-239 is near the rear and used for the larger HF antennas.

Now, the contact surface area of the SO-239 mounting is probably no more than a square inch, and the roof sheet metal is very thin, i suspect about 1/32 of an inch if that. You can see the problem - the roof metal flexes, even with the smaller of my HF antennas fitted with a spring, when going at any decent speed. Its pretty clear that over time this will fatigue and eventually fail.

I want to reinforce the area bellow the mount. My local autospares store carries various pieces of 1/4inch perforated, galvanised steel sheets, intended for use making custom brackets etc. I have one about 4x8inch. Im considering installing one of these sheets between the body of the antenna mount and the roof panel, to spread the forces out and to provide additional rigidity. How big a sheet should i use? The forces of course are mostly along the cars axis, so if i used the 4x8 i would install it with the longest side in the axis of the car. Also, whats the best way to bond the sheet to the roof metal? I cant weld it (and i think on such a thin painted sheet welding would do more harm than good anyway) and am loath to install through bolts (unsightly and yet another possible water ingress spot), so can only think to use epoxy cement?


Posts: 1050

« Reply #1 on: May 24, 2012, 02:26:38 PM »

Go to a scrapyard. Ask them to cut a section out of the roof of the same model car you have from the area you intend to have the mount. That way you get the correct curve. Doing it without the curve will just result in lines forming in the roof where the edge of the plate is.

If you're using the mount for a dualbander or <cough> an ATAS, you don't need that big a one - 6" square will be more than enough. No need to bond anything to anything, just go straight through the roof and the plate and wind the nut up tight. As long as you don't drill the hole too big so the mount is a snug fit - ideally so you have to screw it in, there will be contact with the roof panel sufficient for "grounding" and the plate underneath will be superfluous to requirements in this respect. Remove all the paint from the plate you've just got and also clean the underside of the roof where the plate will go if you're really worried however in doing this you remove the rustproofing from the underside of the roof panel.

As for fatiguing and basically being ripped out if you don't, I doubt it will. I had a MK3 Mondeo and used a 3/8 mount on the roof for my Little Tarheel II with a 6ft whip. After 70,000 miles, a lot of it motorway, some of it with the antenna fully extended, hitting trees, the whip being bent over going through low height barriers at drive throughs and car parks it was still fine however when I removed the mount there were three little bumps in the roof where the 3 pronged metal plate that came with the mount gripped into the roof. As I sold it to someone who wanted to put a CB antenna on the car, it wasn't a problem.
« Last Edit: May 27, 2012, 02:42:36 AM by M6GOM » Logged

Posts: 44

« Reply #2 on: May 26, 2012, 06:40:46 AM »

If you do decide to add some reinforcement, make sure it round.  straight lines and 90 degree corners will put unwanted stress in those places.  Also, make sure it fits the contour of your roof.

Posts: 9930

« Reply #3 on: May 27, 2012, 11:22:18 AM »

I knew a fellow who who bought an extra trunk lid for his car at the junk yards.  he punched holes and did what he needed for antennas in the trunk lid and when he is ready to sell the car he will put the original one back on.
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