...We want a capable IFR airplane...

Dean -

You are correct about the diplexer - it requires external power to operate.
If power lost or it fails your transmitting power is reduced by 3dB (50%)
and you can only use Comm 1. But before you settle on two radios I would ask
you - What makes a capable IFR airplane? We are taught from early on:
Aviate, Navigate, Communicate. How big a deal is it if one of your comm
radios fails? Not very big at all. Having redundancy in your navigation
sources is far more important than talking to ATC. Adding a second NavComm
radio will give you two VOR receivers, but who uses VORs as a primary source
of navigation these days? I would much rather have redundant GPS receivers
with VOR as a backup. Yes, you need a Nav radio to shoot ILS approaches but
aren't you going to have a WAAS receiver with vertical guidance? My advice
is to save weight and panel space and just install one NavComm.

If you really want two comm radios, the standard solution is to install a
second comm antenna on the bottom centerline of the fuselage. It should be
at least 3' away from the other comm antenna. The idea of putting all the
antennas on the centerline is that you pay a smaller drag penalty since each
antenna is flying in the disturbed air from the antenna ahead of it.

You'll have to do a 'release' to create a recessed mount for the antenna
base. Trace the outline of the antenna base onto the fuselage and carefully
cut away the outer skin. Then use a chisel to remove the exposed honeycomb
core. Then fill the hole with an expoxy-flox mix. Wrap the antenna base with
clear packing tape and press it into the hole. Allow to cure overnight and
then pop out the antenna. You should now have a hole with a solid floor
which perfectly fits your antenna. This procedure is also described in
Chapter 24 section D of the builder's manual.

Best of luck, and remember less is more! (avionics that is :-))

-Adam Molny