Ben, it suggests an antenna/antenna lead problem if the transponder is
working properly on the bench. Your continuity check presumably confirmed
that the core conductor was correctly connected to the antenna rod, and the
coax screen was correctly connected to the ground plane. Ideally, it would
be useful to do a check with a VSWR meter.
The amber light flashing at irregular intervals while airborne shows that
the system is responding to secondary radar interrogations. Non-receipt by
the ground stations suggests that your transmitter pulses are either weak or
not getting through. Ground testing would have shown if the transmitter
power was down, so suspicion lies with the antenna or antenna lead.
An antenna is normally regarded as a "reciprocal" device; ie if it is good
at reception, it will also be good at transmission. However, there is an
exception when high-power radio frequency pulses from the transmitter bridge
the gap between conductors and cause a short-circuit. This does not show up
with the weak received signal, but the powerful transmitter signal may cause
a breakdown.
A practical way of testing for this would be to borrow another antenna,
complete with the ground plane, and connect it with a fresh lead directly to
the transponder output. With the Europa's plastic fuselage, it should work
just fine if placed loosely in the baggage compartment but restrained so
that the antenna element is vertical (up or down) and the ground plane is
horizontal.
If you do not have ready access to another antenna, you could try simply
making a fresh coax lead to connect the transponder to your existing
antenna. For test purposes this could be loose, without having to dismantle
your carefully crafted aeroplane to put it through all the hidden recesses
to replace the existing lead.
If neither of these works, but the set still operates correctly on the
bench, then you should ask your friendly radio engineer to apply an external
transponder test set that will test the aircraft system on the ground by
means of measuring the response to a weak interrogation signal.
Hope this helps
Mike
-----Original Message-----
From: Ben
Subject: Transponders
Another one for an avionics whizz!! I have a King KT 96 transponder.
The set has been checked out on the bench and pronounced OK. When
powered up it warms up normally and then the ident and test settings
have the usual effect(constant light). However both on the ground and
in the air on "norm" or "alt" the ident light flashes erratically (but
on average about 40 times a minute). Once airborne ground radars
report "no signal" . The aeriel is a brand new rod type with an 8" ally
disc as a ground plane. I have double checked continuity of the coax
screen and of the conductor and checked for shorting between the two.
All appears ok. If the aeriel is diconnected all the flashing ceases.
Any helpful suggestions???
|