On Jan 9, 12:17 am, (Alan) wrote:
In article Darryl Ramm writes:
[snip]
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

Lets drill into this more...

Alan raises a good point by talking about a practical installation.

Since the approval agency doing the TSO (or equivalent) approval won't
know the actual cable loss of a particular installation they are going
to have to assume 3dB. And changing cable losses in a practical
installation won't change the transponder TSO-C47c approval type. It's
called out in the approval doc and required to be marked on the
transponder, etc. So I don't see how a manufacturer can avoid meeting
the +3dB power requirements. Which means that Microair is quoting the
3dB corrected theoretical antenna input power on their TSO Type 1A
approved transponder or I'm misunderstanding something.

A Type 1B transponder with lower power output might be just as good
for our uses in practice as a Type 1A (above 15000' spec) especially
with a good antenna installation and as Alan points out may meet the
power requirements of a Type 1A transponder. Which might all make some
pilots less concerned about worrying about all this in practice,
especially if a Type 1B transponder draws less power battery power
than a type 1A. But would that be legal? And does the transponder
Type approval really matter to us? Well FAR 91.215(a) might come to
our rescue...

---QUOTE---
(a) All airspace: U.S.-registered civil aircraft. For operations not
conducted under part 121 or 135 of this chapter, ATC transponder
equipment installed must meet the performance and environmental
requirements of any class of TSO-C74b (Mode A) or any class of TSO-
C74c (Mode A with altitude reporting capability) as appropriate, or
the appropriate class of TSO-C112 (Mode S).
---END QUOTE---

Note - that's just a "must meet the performance of" not a "must be TSO
approved". So this leaves open the possibility (especially for
experimental gliders, and Eric's glider is experimental) that you can
legally use a transponder outside its TSO Type altitude limits. One
gate to this would obviously be that the tests required under FAR
91.413 (ie. Part 43 Appendix F which circle back to the TSO specs)
must show that it did meet the Type 1A (above 15,000') requiremnts.
Who knows for sure whether the FAA would ever argue that you would
have to do more than that with a modern Type 1B transponder to show
"must meet the [1A] performance". But I'd be pretty surprised to see
this being an issue in practice.

It would be great to hear if other people have different
interpretations of the TSO and FAR requirements.

(I've deliberately kept this to Mode-C, not Mode-S, transponders here
since that what I think Eric is looking at).

The important practical thing, especially in high traffic areas, is to
have a transponder installed and turned on. If a lower power
consumption transponder is important to extend battery life then I'd
personally rather see people install that than turn off the
transponder during flight. Of course there may be options to install
more battery capacity, etc.


Darryl