icelord wrote:
Gain is also a major requirement...

Gain implies directionality. You probably want low gain, unless you can
maintain a constant aim from The Suit. Think about it.

400 meters with line of sight.
+10 dBm Tx Power and a 6 dB whip antenna
Suit side: -96dBm sensibility [spelling!]
freq: 433 MHz

Ask the current US president about radio receivers hidden under
ill-fitting suits. I believe that he has some recent hands-on
experience with the subject...

This seems to be a trivial problem. Any old 6-inch wire should work. If
not, why not use a higher gain antenna at the "server" end of the link?
Also - police "wires" (mics) work, you shouldn't have any problems.