I assume from the post you made
a little later, that you don't want your car to
look like it has a billboard on the roof! what I
would suggest (please note I am a bit new to the
RF game and more than a little daft) is to use a
mesh type screen between the two antennas, which
you should be able to get from your local hardware
store. however what may work better is a type of
repeater setup, where instead of using one antenna
for transmit and recieve, use two, but switch between
the two of them.
Hope that helps!