If you're seeking to improve your Flarm range then moving to
a better aerial design is an option. In the crowded Alps you see
many installations using the "SuperFant" antenna; see link
below.
This aerial is both small and slim and so does not impede the
view when mounted on the front of the instrument panel, high
just under the canopy. My experience is that it has much
superior performance to the standard simple Flarm dipole. It's
available with custom length cable.
Of course the position of the Flarm aerial is critical! There are
some truly terrible installations out there....

http://www.gliderdesignparts.de/ant_flarm.html

I always found their service to be fast and relable; usual
disclaimers.