I felt that the following was worth adding to our list. It explains quite a
lot that was puzzling me.
Graham
>[The Canard Aviators's Mailing list]
>Dan (and the rest of the group),
>
>A little more here on how transponders are built and tuned....
>
>I used to be the business manager for BendixKing's transponders, so perhaps I
>can provide us all with some more insight.
>
>You commented on the transponder being "out of tune" from the factory. A
>transponder should receive interrogations from ground-based radars and
>airborne TCAS systems on 1030 MHz, then send out a reply at 1090 MHz. Because
>things such as coax length, bends, ground plane anomalies, and antenna
>capabilities, the frequency put out by the transponder tends to get "pulled"
>down.This is the same regardless of whether the plane is a homebuilt
>composite or factory Spam can.
>
>Because of this frequency pull, the transponders leaving the factory are
>actually tuned to put out a signal approximately 1 MHz above the desired
>optimal; ~1091 MHz. In most installations, this results in the installed unit
>transmitting at the right point. Because of variations in installations,
>however, many will require newly-installed transponders to be modified.
>
>And additional source of variation is that the lower-priced transponders
>utilize a cavity which helps control the frequency of the signal output.
>These cavities offer the advantage of being low-cost (relative to all-digital
>technology), but they also have drawbacks, such as having their frequency
>drift with time (even if not used), they may be sensitive to temperature
>fluctuations, and they are sensitive to shock.
>
>There now are digital transponders on the market (BendixKing offers a
>high-end version, while Garmin offers a low-end version). Beware that the
>digital transponders, especially the low-end ones, have their share of
>problems and are still susceptible to installation-specific problems.
>
>The Terra (later Trimble) transponders are in a class all of their own (it
>ought to be one in reform school)....As many of you may unfortunately
>remember, their transponders required expensive upgrades, and many others
>have had numerous maintenance problems. When we bench tested them in our lab,
>we found that even the best of them did not meet the minimum expected
>performance specs.
>
>Failing to meet these specs can be more than annoying: it could cost you your
>life. TCAS, the airborne collision avoidance system on most jets and
>turboprops, requires you to have a fully functioning transponder in order to
>be "seen". Having a transponder that doesn't work properly might lead the
>crew of one of these fast-moving planes to assume that there are no planes
>(i.e. you) in front of them. It was this type of assumption a few weeks ago
>that caused the Lear in Florida to collide with the Extra 300. I don't know
>if the Lear was TCAS-equipped, but the ground-based radar system wasn't able
>to keep the two planes separated.
>
>One last note: although some out there may think I am biased because of my
>employer, please realize that I try to leave my job at the office. I just
>want to enhance everyone's flying safety.
>
>Tom Staggs
>Long-EZ N13YV
>
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