On Jan 8, 12:21*pm, Richard wrote:
On Jan 8, 12:01*pm, Itsaplane wrote:



Needing a replacment, my transponder shop told me that above 15k' the
FAR's require a 250 watt transponder (125w below 15k').

True?

I'm pretty sure that sailplane owners are mostly installing 175 watt
units and flying them to 18k' (and higher in wave windows). *Right?

If this 15k' rule is real, I'm supposing that, since gliders aren't
required to have a transponder except in certain airspace (this
airspace is all below 15k' -- except Class A...) that we're generally
free to install anything we want (except 18k' in Class A?).

Bottom line -- 175 watts or 250 watts?

Eric Rupp
ER
(My Terra TR250D transponder is inop and I'm having trouble getting
parts - anybody have a dead one that we might be able to salvage from?)

Eric,

I don't believe that is true. *I think that is from the specifications
from the Becker transponders. * The Microair T2000 *is 200w and specs
say 62,000 feet.

http://www.craggyaero.com/microair.htm

Richardwww.craggyaero.com

Yes it's true, but it's also not...

The above/below 15,000' comes from TSO-C74c see
http://www.airweb.faa.gov/Regulatory...9?OpenDocument

See other specifications there to explain the differences, but for
power output the TSO requirments are

---QUOTE---

2.11 Transmitter Power Output.

a. For equipment intended for installation in aircraft which operate
at altitudes above 15,000 feet, the peak pulse power available at the
antenna end of the transmission line of the transponder must be at
least 21 db and not more than 27db above 1 watt at any reply rate up
to 1,200 per second for a 15-pulse coded reply.

b. For equipment intended for installation in aircraft which operate
at altitudes not exceeding 15,000 feet, the peak pulse power available
at the antenna end of the transmission line of the transponder must be
at least 18.5db and not more than 27db above 1 watt at any reply rate
up to 1,200 per second for a 15-pulse coded reply.

c. The standards of this section assume a transmission line loss of
3db and an antenna performance equivalent to that of a simple quarter
wave antenna. In the event that these assumed conditions do not apply,
the equipment must be adjusted as necessary to provide a transmitter
power output equivalent to that specified.
---END QUOTE---

But how to interpret this? The way I read these TSO requirements are -

below 15k feet 18.5dbW to 27dBW = 70.8W to 501W power at coax output
at the antenna
above 15k feet 21dbW to 27dBW = 126W to 501W power at coax output at
the antenna

below 15k feet 18.5dbW+3dB to 27dBW+3dB = 141W to 1kW power at
transponder output
above 15k feet 21dbW+3dB to 27dBW+3dB = 251W to 1kW power at
transponder output

Where I have assumed the 3dB cable loss in part(c).

I'm not aware of any requirement for what power output manufactures
are required to quote. Are they talking about the power output at the
transponder or the TSO-C74c specified power at the antenna input with
assumed 3dB cable loss? I suspect manufactures normally want to quote
the biggest power output they can however in the case of Microair I
assume they are talking about 200W at the antenna coupling since they
would not meet the ~250W requirement. Maybe one of their dealers can
clarify this. (and yes technically as well there is ambiguity about
peak vs. other power measurements, but again I'd assume manufactures
are going to quote peak to maximize the number and since that is also
what the TSO talks about).

Anybody got a different interpretation of the TSO?


Darryl