What frequency was the "two way radio"?  For VHF (146 MHz) and UHF (440 MHz) and mounting on the rear end of a Vanagon camper, I see no need for the addition of extra sheet metal.  From the rear lift door, you have about 4 feet of sheet metal before you reach the opening for the pop-top.  That translates to about 20 square feet of sheet metal on the roof at that location.  The fibreglass pop-top will be transparent as far as RF is concerned.  VHF and UHF antennas on repeater sites on mountain tops are enclosed in a fibreglass cone for ice and snow protection.  So the pop-top will not interfere. 

Again, I had asked for input from those who have actually installed a VHF UHF antenna on a Vanagon.  The Diamond K-400 trunk mount is the method that I am looking for.  

Usually the issue of ground plains would only be a concern for HF antenna installations.  I will not be installing an HF antenna.

Cheers!
Stuart
http://www.royalsignals.org.uk/photos/Stuart.htm

Sent from my electronic umbilicus

On 2013-06-11, at 8:55 AM, David Bjorkman <ddbjorkman@VERIZON.NET> wrote:

>  You are correct on that one.  I mounted a 2 way radio antenna to the roof of my "84 Westy, but I had to glue a sheet of metal to the roof to do it.  Doesn't need to be a big one, but you have to do the math and trim your antenna correctly.  But it works well and no door interference.  Just watch out in parking garages.  One can take out a whole row of fluorescents pretty easily.  Ask me how I know.
>  
> Dave B.
>  
>  
> On 06/10/13, Rocket J Squirrel<camping.elliott@GMAIL.COM> wrote:
>  
> I have heard that with a fiberglass pop-top there isn't a sufficient
> ground plane to make a vehicle-mounted antenna work well. But perhaps I
> am mistaken.
> 
> --
> Jack "Rocket j Squirrel" Elliott
> KG6RCR
> 1984 Westfalia, auto trans,
> Bend, Ore.
> 
> On 06/10/2013 03:06 PM, Stuart Fedak wrote:
> > I am looking for recommendations from those who have installed a
> > VHF/UHF antenna on their Vanagon. I am considering using the Diamond
> > K-400 trunk mount, and placing it on the rear door. Will this leave
> > enough room for opening the rear door, and will it interfere with the
> > pop-top?
> >
> > I will be using either a Diamond dual band 2m/70 cm or a tri-band
> > 6m/2m/70 cm antenna
> >
> > Your suggestions would be appreciated.
> >
> > Stuart VE3SMF
> >
> > Sent from my electronic umbilicus
> >