I mounted an (dual-band) antenna for my amateur radio on top of my '89
camper with excellent results. I bought a piece of steel that is approx 8 x
8 in and drilled 4 holes in it to match up with the bolts holding the
pop-top release lever. This serves a a sufficient ground plane when the
pop-top is down and or up. I the drilled a small hole in the top next to the
steel plate to route the cable into the van. I do use a handheld radio, so I
keep a roll of cable behind the drivers seat. This allows me to use the
radio in the front or back of the camper.

The only drawback to this system is the VSWR gets worse when the top is
raised (you lose contact with the body). With my setup it goes from 1:1.5 to
1:4 (approx). That's not really that bad, and I usually don't do much
transmitting when I'm parked anyway. Also, don't forget the extra cable
necessary (about 3 ft.) to raise the top.

Another alternative I played with is a trunk\hatchback\raingutter mount
attached to the back door. It swivels so you can mount it at about any
angle. There are 2 models that I found and have used both. Ham Radio Outlet
http://www.hamradio.com/ carries the Diamond Antennas model K300C or K300KM
and MFJ http://www.mfjenterprises.com/ has the MFJ-345 or MFJ-340. You don't
need to drill any holes with this type mount. The cable can be routed
through the door or the air intake. Also, you don't have to worry about
raising the top.  I had very good results with this method also, I just
preferred to have the antenna mounted on top.

BTW, how many list members are HAMs??

Monty Groff
KC0CNU
'89 Vanagon GL Camper
'62 Type I Sunroof