> Right-o, Fred,
> If the interrogation system is
> anything like the aerochat freqs, the ATC interrogator probably has
many
> more watts output than does the transponder. That may be reciprocity
in the
> master/slave market, but is it true for the ATC txpdr too?
> cheers, Ferg
I googled up product literature and specs for some of theses
interrogators, and yes they can (user-selectable) put out more power,
but also have 4-6 times the receive sensitivity. And where the radar
site is on an airport, for example, it appears that xmit power would
have to be limited, or the FAA spec on max signal our xponder must
deal with will be busted within, like, 1 mile of the field, meaning
unreliable replies I assume. So, I propose ATC usually has a much
better receiver than transmitter.
For trivia value, I plugged the #'s into my RF calculator spreadsheet.
Our antennas are omnidirectional; ATC's is highly directional (e.g.,
+27dBi, Ferg) and is bolted to the spinning radar antenna. So, if ATC
wanted usability out to 50 miles, and everybody's xponder/antenna can
be assumed to meet FAA spec, then guess how much power ATC
theoretically needs.... 1.1 watt! Since their high-gain antenna also
receives, a similar calculation makes the difference in receive
sensitivity enormous. If you can put out a measly 1 watt out of your
250 at 25 miles at enough altitude, there's a possibility they'll see
you.
Reg,
Fred F.
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