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

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

On Jan 9, 5:35*am, Darryl Ramm wrote:
On Jan 9, 12:17 am, (Alan) wrote:
In article Darryl Ramm writes:
[snip]

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.
[snip]
Darryl

And I'll answer my own self-doubt about how Microair can have a Type
1A 200W transponder. Microair state in their installation manual "The
T2000SFL allows for 1.5dB cable loss from the unit to the antenna.".
That gets incorporated as a part of the TSO and supplants the generic
3dB assumption in the TSO and so I'm sure they are talking about
actual output power from the transponder being 200W.

Darryl

the transponder manufacturer (Becker) "recommends" 250 Watt above 15,000'
but to the best of my knowledge there is no FAA requirement for transponder
power
tim

Please visit the Wings & Wheels website at www.wingsandwheels.com


"Richard" wrote in message
...
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

Richard
www.craggyaero.com

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?)

In practice a 175W Becker appears to work fine above 15,000'. That's
what I'd use.

I'd worry more about getting something reliable, easily serviceable
and has an readout of pressure alt/flight level (ie. replace the
Terra) than whether 250W is important.


Darryl

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,

There must be no soaring anywhere that RAS members live. I am amazed
at where this thread has gone.

Buttom line the vast majority of the transponders sold in the USA for
sailplanes are Mircoair T2000, Becker 175w and Becker 250W (less of
these).

IMHO any one of these will be sufficient and relaible at the altitudes
that sailplanes fly, use with an ACK A-30 encolder (less power draw
than the Amer-King encoder and smaller). The Microair is shorter that
the Becker. Both fit in a 2.25" hole. My customers have had good
results with all.

Decide what you want to pay and any of the above will be
satisfactory.

The Garrecht is not a good idea in the USA because it is not certified
in the USA, also the lead time is 6 months.

Mode S is more expensive and I would not spend the money. Becker Mode
S are available in the USA.

Richard
www.craggyaero.com