On our recent trip from TX to NY I wanted to mount my computer's magnetic cell phone antenna and a magnetic Sirius Radio antenna to the roof of my '84 Excella 270 MH.
Now, we ALL know a magnet will not be attracted to the aluminum of our rigs, so what to do?
A quick trip to the hardware store and a 30 cent investment yeilded 2 large FENDER WASHERS. I simply asked for the largest they had and received two that were about 2 inches across.
I affixed DOUBLE SIDED FOAM TAPE to one entire side of each washer, then mounted them on the roof of the Excella. The mild steel in the washers provided enough attraction to keep the antennas in place for the entire trip.
The small wires I ran into the upper corner of the sliding passenger window and was able to close the window after gingerly allowing the wires for both antennas to get used to their bent condition.
The fenderwashers and their tape are invisible from the ground. All you notice are the the two neatly mounted antennas with small wires to the window corner.
It was CHEAP [like me], and it WORKS [not like me]!
Hi, N2XBW. You answered your own question; I was going to say the same thing except for cutting a piece of sheet metal [steel] to the size and shape you need and use double sided tape. I'm still looking for an Aluminum Magnet.
A good solution for cellular and satellite radio. That method worked well for receiving and for the cell phone which is relatively high frequency and the washers provided the ground plane. However it may not work for amateur radio transmitting at lower frequencies since at 2 meters or 146-148 mhz the antenna works best if centered on a metal surface that is at least 38 inches wide that is connected to the braid side of the coax. The double stick tape might insulate the base of the mag mount from the roof. Making sure that the radio is well grounded to the body of the Airstream at some point with a short piece of wire braid would eliminate any problem.
__________________
Don (KD6UVT) & Gail Williams
What do you want to be in life, a spectator or a participant?
I've hit upon a possibly simple solution for 2m ham antenna mounting.
I plan to use an electrical junction box cover plate for the ferrous metal base. These are about 83 cents each at Home Depot. To the plate I will solder a short [1/8 wave=25cm] bronze brazing rod radial into a hole drilled at each corner to furnish an acceptable ground plane. I may give the whole business a coat of metal primer and then flat black to prevent future rust streaks and give it a finished look.
Then the assembly will be foam taped to the Excella's roof, then set the magnetic antenna on it. It should work.
Another option would be to omit the radials and internally run a ground wire from the antenna braid directly to the airstream's body as suggested above.
It's pretty cold here right now [+8F last night] so I'll wait till we get a warmer spell to test any of this.
I may be missing something here, but if the DC voltage is coming from the TT, won't the radio "see" the aluminum as a ground plane via the coax shield?
__________________ Jeff & Cindy Remi & Hunter '81 International 31'CB WBCCI #7026 Air #17054 "Fus-A-Lodge"
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Magmount Antennas on an Aluminum Airstream? 
Hi, N2XBW. You answered your own question; I was going to say the same thing except for cutting a piece of sheet metal [steel] to the size and shape you need and use double sided tape. I'm still looking for an Aluminum Magnet. 
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