The part of the discussion about UK use of transponders has missed out
what I consider the main point, which is the complex regulatory
requirements placed on us for fitting transponders.

First, you can now only install approved kit, and our CAA imposes more
stringent requirements for approval than other aviation regulators.

Second, installation can only be done, as I understand it, if either (a)
the manufacture has issued an approved scheme for installation (I
believe none have, other than for motor gliders), or (b) if you pay an
installer to produce a formal modification scheme (fees) and then pay
our CAA substantial "major modification" fees to have the scheme approved.

On top of this there are further substantial fees for annual tests.

Currently, it is unlawful just to fit a transponder yourself or to get a
knowledgeable person to do so for you (or maybe it's lawful to do this,
but then unlawful to fly the aircraft).

Far be it from me to say that preventing voluntary transponder carriage
is foolish, but ...

Darryl Ramm wrote:
I know nothing about UK airspace (besides being stuck in it for far
too much time as a passenger holding over Heathrow), but it sounds
like the existing separation of airliner and jets in UK airspace is a
key point you folks should be arguing.

And again only one of the transponder aircraft needs to be talking to
ATC/radar facilities or have PCAS or higher. I don't know PCAS (and
higher-end systems) adoption in low-end GA aircraft in the UK, but in
the USA it seems pretty high (purely an anecdotal impression). I've
asked before but could not get adoption numbers for the USA.

Companies who make panel mounted transponders usually do so for the
larger GA market and they have to coexist with all sorts of PCAS,
active systems (like the Avidyne) and full on TCAS. I would not hold
my breath for a transponder with integrated PCAS. But maybe now
manufacturers will be making them mostly for UAVs and those UAVs could
do with integrated PCAS :-(

In the Libelle you might be able to make up a mount for a Zaon MRX
under the opaque areas of the front of the canopy, it might be canted
over parallel to the surface, and use an antenna mounted on suction
cups on the canopy. Obviously paying attention to canopy jettison
issues. I'd be kind of surprised it is impossible, but not saying its
going to be trivial.

Darryl