868 MHz behaves like light.
FLARM transmits at 868 MHz in Europe — a wavelength of about 35 cm. At that frequency, radio behaves almost optically: it travels line-of-sight, it casts shadows, and anything conductive between two antennas simply blocks the path. A glider that "sees" 8 km ahead but nothing behind doesn't have a weak radio. It has an antenna in a shadow.
Two more properties decide everything about placement. First, polarization: FLARM signals are vertically polarized, so your antenna must stand upright — an antenna lying flat can lose the majority of the link budget before anything else goes wrong. Second, the standard quarter-wave whip is only half an antenna: it needs a ground plane beneath it to work against. No ground plane, no range. A dipole antenna carries its own counterpoise and needs none.
Everything below follows from these three facts: line of sight, vertical, ground plane.
Carbon is a Faraday cage.
Glass fibre is transparent at 868 MHz; carbon fibre is not. A carbon fuselage is, for radio purposes, a metal tube. An antenna mounted inside it — under the panel, behind the seat, along a carbon boom — is transmitting into a box.
In a carbon aircraft the antenna must see the sky through something non-conductive: through the canopy from the glareshield, through a GFRP panel section, or via an external antenna. This single rule explains most of the dramatic range differences between two "identical" installations.
The cable matters as much as the antenna.
Thin coax is expensive at this frequency — RG174 loses roughly 1 dB per metre at 868 MHz, and every 3 dB is half your transmit power. Keep the cable as short as practical, route it in smooth curves, and never kink it: a sharp bend changes the cable's impedance permanently even if it looks fine outside.
Connectors: seat SMA connectors hand-tight, avoid stacking adapters, and treat a connector that has been stepped on during rigging as suspect. A damaged connector can reflect power back into the transmitter and quietly cost you kilometres of range.
Verify with data, not with faith.
An installation that looks perfect can still underperform — so measure it. Your IGC flight logs record every FLARM contact; upload a few flights to the FLARM range analysis tool and you get a polar plot of your real reception range in every direction. Sectors that come back short point straight at the shadow: a canopy frame, a transponder antenna, the carbon side of the cockpit.
Aim for several kilometres of range in every direction that matters, with the strongest coverage ahead. Check it after installation, and again each season — antennas move, cables age, and the traffic you never see is the traffic that matters.