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douglist(at)macnauchtan.c
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 Posted: Fri Sep 30, 2011 8:33 am Post subject: Transponder troubles |
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At 18:50 -0600 9/11/11, Kelly McMullen wrote:
Mode A and mode C are distinguished by a time spacing between pulses received in the interrogation from the ground. Mode A inserts twelve bits as entered into you code selector. Mode C sends twelve bits of altitude information. There is also a mode B for which the details are "protected" by the US military.
Those settings, in an AT50 are set with analog adjustments inside your transponder and might well have drifted out of spec. If you're sending mode C information in response to a mode A request the ground will get a code they don't expect. It might also look intermittent because it could depend on the received signal amplitude of the interrogation pulses. (How far away is the transmitter?)
Ant at one time there was an AD that required transponders to reply to mode C requests even if they had altitude responding turned off. Reason: collision warning detectors is other aircraft. I believe the AT50 was one of those that was considered not worth making the modification to. That is from long term memory though and may not be quite right. Watch out, though, if you ask for a bench test. You might discover you have an illegal transponder. I'm pretty sure there was an AT-50A that came out.
--
--> From the U S of A, the only socialist country that refuses to admit it. <--
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Kellym
Joined: 10 Jan 2006
Posts: 1705
Location: Sun Lakes AZ
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| Posted: Fri Sep 30, 2011 11:51 am Post subject: Transponder troubles |
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Yes, your suspicion was correct, re the alignment. It was a bit out of
spec. Once adjusted back in spec, it tested perfectly both on the bench
and in the aircraft. Yes, there is an AT50A, which I think has more
output power, but otherwise is pretty much the same box, and they are
interchangeable, like all Narco units...same tray, same pins, etc.
On 9/30/2011 9:28 AM, Doug McNutt wrote:
| Quote: |
At 18:50 -0600 9/11/11, Kelly McMullen wrote:
>
>
> A Narco AT-50.
Mode A and mode C are distinguished by a time spacing between pulses received in the interrogation from the ground. Mode A inserts twelve bits as entered into you code selector. Mode C sends twelve bits of altitude information. There is also a mode B for which the details are "protected" by the US military.
Those settings, in an AT50 are set with analog adjustments inside your transponder and might well have drifted out of spec. If you're sending mode C information in response to a mode A request the ground will get a code they don't expect. It might also look intermittent because it could depend on the received signal amplitude of the interrogation pulses. (How far away is the transmitter?)
Ant at one time there was an AD that required transponders to reply to mode C requests even if they had altitude responding turned off. Reason: collision warning detectors is other aircraft. I believe the AT50 was one of those that was considered not worth making the modification to. That is from long term memory though and may not be quite right. Watch out, though, if you ask for a bench test. You might discover you have an illegal transponder. I'm pretty sure there was an AT-50A that came out.
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_________________
Kelly McMullen
A&P/IA, EAA Tech Counselor # 5286
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