RST Engineering wrote:
As I remember, the 7AC has a metal fairing between the fuselage and the wing
to join the fuselage, windshield, and wing. Please confirm.

If this is so, I will design and send you the parts for an external antenna
that you can mount by drilling a half inch hole in that metal fairing. In
essence, it is a 22" brass welding rod soldered into (and hot glue filled) a
BNC-M connector. A BNC-FF bulkhead connector is inserted into a half-inch
hole drilled into the metal fairing just aft of the leading edge of the
wing. A coax cable with BNC-M connectors on either end then connect the
bulkhead connector with the radio. Simple, cheap, and effective. VSWR
bandwidth is nothing to write home about, but it will work one hell of a lot
better than the rubber resistor.

The "ground plane" for this antenna is the metal fairing. If you want to
make a marginally better ground plane, you can make a short jumper wire
between the fairing and the metal fuselage tubing. Use a metal cable clamp
to connect to the tubing and a screw-nut-crimp connector to connect to the
fairing.

Pretty much what we did, only with a different connector and attachment.
The coax and mount cost $20.
This is what I use.
You will want to rivet a doubler plate to the fairing where you attach
the antenna base. You can then screw on the antenna and cut the whip to
the frequency you desire. The coax enters the cabin at the wing leading
edge meets the top of the windscreen. This is also where the coax is
coiled and stowed when not in use.

http://www.antenna.com/lm_cat/lmrpg37.html

Motorola Style Mounts
Model KRM66
Similar to K-66 except Motorola style mount and 14 ft (4.3 m) of
RG-58/U cable and PL-259 connector.
Model KRMX66
Similar to K-66 except Motorola style mount and 17 ft (5.2 m) of RG-58/U
for ASP-7450 and ASP-7650 series. Connector available separately.
Model KREMX66
Similar to K-66 except Motorola style mount and 17 ft (5.2 m) of
PRO-FLEX PLUS low loss cable. Connector available separately.