----- Original Message -----
From: "Fred Fillinger" <n3eu@comcast.net>
Subject: Re: Europa-List: Transponder issues
|
| > 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.
Understood, Fred,
......and thank you for the time and trouble you took to clarify
my cloudy knowledge - used the b*******rs for over sixty years and never
knew what made them tick. My first wooden jet trip over Air Defence Group
radar, the communicator said, "Shilo 27, make your cockerell crow". I
replied, "Sorry, I think I'm the only living tissue aboard." * I can almost
see the raised eyebrows and knowing looks in the control room - especially
since the loudspeaker gives everyone an ear. This interchange was superceded
by "Sqawk XXXX" ["make your parrot squawk.."] but that's where it came
from......
Again, much obliged, suh.
Ferg
A064
* - I now suspect that wasn't true. Kerosene gave rise to strange tiny
creatures which ate various units in the 'plane. Also, by our losing about 5
pounds of weight an hour (in summer), our flying suits were home to pounds
of salt and other various living organisms. But I digress...............
|