New Transponder for us

In article
,
Darryl Ramm wrote:

A good rough number for steady state consumption for an ACK-30 encoder
(what most of us with Mode C transponders in gliders use today) is a
bit over 60mA @ 12VDC without the heater on. Heater full on they can
draw 400mA @ 12VDC (luckily the heater should not be on often in
usual use - but if you are flying in wave especially make sure the
encoder box is insulated and protected from drafts).

Pardon me for hijacking the thread, but this suddenly made me realize
that I've been wondering about these encoders for a while.

This is probably obvious to those in the know, but alas, it's not coming
to me. Why is it that a transponder encoder requires a heater, when a
mechanical altimeter functions just fine on its own?

--
Mike Ash
Radio Free Earth
Broadcasting from our climate-controlled studios deep inside the Moon