175 or 250 watt transponder?

In article Darryl Ramm writes:
On Jan 8, 12:21=A0pm, Richard wrote:
On Jan 8, 12:01=A0pm, Itsaplane wrote:
Needing a replacment, my transponder shop told me that above 15k' the
FAR's require a 250 watt transponder (125w below 15k').

I'm pretty sure that sailplane owners are mostly installing 175 watt
units and flying them to 18k' (and higher in wave windows). =A0Right?

Bottom line -- 175 watts or 250 watts?

Eric Rupp
ER



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...TSO.nsf/0/ACB=
BA541FF4C071D86256DC10067F209?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 =3D 70.8W to 501W power at coax output
at the antenna
above 15k feet 21dbW to 27dBW =3D 126W to 501W power at coax output at
the antenna

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

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


I am sure I quoted way too much.


The guide says you can use the real cable loss if it isn't 3 dB. That makes
life easier for most with gliders, as a short length of low loss cable will do
better than 3 dB.

175 watts is 1.43 dB above the 21 dBW requirement for use above 15,000 feet.
So you simply need to get the loss below 1.43 dB in the feedline. That appears
to be not very difficult. For example, Times Microwave LMR240 cable shows up
as a total loss of 1.07 dB when connected to a load with an SWR of 1.5:1. It
takes 16 feet of this cable to get the loss to 1.42 dB.

If that is cutting it too close, you can get a better matching antenna, or
use a lower loss cable. At 1.2:1 SWR, that same 16 feet of LMR240 has a total
loss of 1.36 dB. LMR400 cuts the loss to 0.74 dB even with a 1.5:1 SWR. (It
has 0.69 dB loss for 16 feet with a perfect match at the end.)

Unfortunately, many aircraft installations use smaller lighter and more
flexible coax -- such as RG58, which will give 1.9 - 2.9 dB loss (too much).
(There is a lot of variety in types of RG58.)


Alan
wa6azp