I have the same problem with the same Bob Archer antenna in my vertical
stabilizer - though I am using RG400 everywhere.  My external belly
antenna is fine but the sensitivity of the antenna in the tail is much
poorer as judged from listening to ATIS 30 miles or so out.  I'd guess
that the reception sensitivity differs by a factor of at least 2.  For
the techies, the SWR of the tail antenna throughout most of the COM
radio range is between 1.2 and 1.3.  The plane is in WLS primer now.

When I was building, I tested both antennas before they were mounted in
the plane - I just suspended them from the ceiling in my garage.
Admittedly not an ideal test, but the whip antenna (without any ground
plane) outperformed the Bob Archer antenna with the same radio and cable.

I'd love to hear of how anyone managed to get a sensitive antenna in
the tail.    ...Clark Still (LIVPT N750S)