Transponder Antenna Thought

Two problems...
1. Two antennas fed together would effectively look like a 25 Ohm load
to the transponder, which is designed for 50 Ohm loads. SWR would be
2:1, so some of the output power from the transponder would be reflected
back to the unit. Any power reflected reduces power being radiated out
of the antenna, reducing range.

2. Mounting the antennas on the side would make the radio wave
horizontally polarized. ATC receive site is set for vertical
polarization. There would be a 20 dB loss of signal just due to the
differences in polarities (which means the signal would seem 100 times
weaker at each end, thus reducing range again).

Scott


Gary Emerson wrote:

Had this random thought over the weekend. Figured there were people on
RAS that might have information to say whether this would help or hurt
the transmit "power" of a transponder.

Instead of mounting a single antenna on the belly of the fuse where it
can get beat up in the trailer, off field landings or just rocks and mud
on the runway, what about a pair of antennas mounted on the side of the
fuse on the tail boom similar to static ports. If you ran a single
co-axial cable and then put a T at the end and then ran a short piece to
each side of the boom then you'd have, in concept anyway, a good 360
"view". It may well be that antennas don't like this sort of
arrangement so I figured I'd ask. Also has the obvious downsides of
needing the factory to build this in to the plane and doubling your
antenna costs. The benefit is that it does move the antenna farther
from the pilot.

Two problems...
1. Two antennas fed together would effectively look like a 25 Ohm load to
the transponder, which is designed for 50 Ohm loads. SWR would be 2:1, so
some of the output power from the transponder would be reflected back to
the unit. Any power reflected reduces power being radiated out of the
antenna, reducing range.

No. It is done all the time - feeding two or more antennas. It is simple
to do. If you feed a 50 ohm antenna with a 1/4 wave of 75 ohm line, the
impedance is converted to 100 ohms. Then, place two 100 ohm feedlines in
parallel and you have 50 ohms. To determine the proper 1/4 wave length, you
would multiply a normal 1/4 length time the velocity factor of the coax. If
the determined length is not sufficient for the spacing, you would use 3/4
wavelength line. Not simple - but not difficult.

2. Mounting the antennas on the side would make the radio wave
horizontally polarized. ATC receive site is set for vertical polarization.
There would be a 20 dB loss of signal just due to the differences in
polarities (which means the signal would seem 100 times weaker at each end,
thus reducing range again).

No. The antennas could be mounted vertically, as in a vertical dipole. A
few other configurations could also be used to obtain vertical
polarization - but you could not mount a normal vertical in a horizontal
plane, since it would become horizontal polarization.

Colin

"Scott" wrote in message
...
Two problems...
1. Two antennas fed together would effectively look like a 25 Ohm load to
the transponder, which is designed for 50 Ohm loads. SWR would be 2:1, so
some of the output power from the transponder would be reflected back to
the unit. Any power reflected reduces power being radiated out of the
antenna, reducing range.

It would be worse than that because the 50ohm cable impedance to the
T-Junction would be in series with the 25ohms created by the two parallel
50ohm stubs, which would present 75ohms equivalent to the transponder. This
will reduce the power for radiation whilst increasing the stress on the
transponder's output.

To add insult to injury the reflected wave back from the impedance mismatch
at the junction would have exactly double the voltage (in the opposite
polarity) when it gets to the transponder output and could very likely blow
the finely tuned output amplifier. The beauty of this type of malfunction
(also common in high-speed electronic signals) is that the act of measuring
the wave with say an oscilloscope conveniently causes the reflected wave to
disappear.

2. Mounting the antennas on the side would make the radio wave
horizontally polarized. ATC receive site is set for vertical
polarization. There would be a 20 dB loss of signal just due to the
differences in polarities (which means the signal would seem 100 times
weaker at each end, thus reducing range again).

Scott


Gary Emerson wrote:

Had this random thought over the weekend. Figured there were people on
RAS that might have information to say whether this would help or hurt
the transmit "power" of a transponder.

Instead of mounting a single antenna on the belly of the fuse where it
can get beat up in the trailer, off field landings or just rocks and mud
on the runway, what about a pair of antennas mounted on the side of the
fuse on the tail boom similar to static ports. If you ran a single
co-axial cable and then put a T at the end and then ran a short piece to
each side of the boom then you'd have, in concept anyway, a good 360
"view". It may well be that antennas don't like this sort of arrangement
so I figured I'd ask. Also has the obvious downsides of needing the
factory to build this in to the plane and doubling your antenna costs.
The benefit is that it does move the antenna farther from the pilot.

One thing is for sure, there is a pretty sizable knowledge base
available via RAS on an incredibly wide array of topics.

I think the general concept I had to move to two antennas on the side is
vastly outweighed by the complications and I suspected as much. Think
I'll just stick to the rod type below and either bend it back or replace
it as needed.

Gary


Gary Emerson wrote:
Had this random thought over the weekend. Figured there were people on
RAS that might have information to say whether this would help or hurt
the transmit "power" of a transponder.

Instead of mounting a single antenna on the belly of the fuse where it
can get beat up in the trailer, off field landings or just rocks and mud
on the runway, what about a pair of antennas mounted on the side of the
fuse on the tail boom similar to static ports. If you ran a single
co-axial cable and then put a T at the end and then ran a short piece to
each side of the boom then you'd have, in concept anyway, a good 360
"view". It may well be that antennas don't like this sort of
arrangement so I figured I'd ask. Also has the obvious downsides of
needing the factory to build this in to the plane and doubling your
antenna costs. The benefit is that it does move the antenna farther
from the pilot.

My ASH-26E was in the final stages of assembly a couple weeks after
9/11/01 and as I got worried about the need for a transponder here in
the US, asked the factory to install the antenna. The transponder is
still on my birthday/Christmas wishlist....

In any case, I've been "stress testing" the simple rod antenna mounted
just aft of the gear doors since the first flight at the end of 2001.
It occasionally catches on the tail strap and sometimes the rear of the
trailer if the ramp is too low. I've straightened it a few times, but
now just leave it "streamlined" at about a 30 degree angle to vertical.
Overall it's pretty sturdy and replacement is cheap, with excellent
accessibility.

Since the location is aft of the gear doors, they're producing some
turbulence already, so I doubt the antenna as adding anything to the
total drag picture.

Might even be possible to cut the antenna near the base and put rejon
with a strong spring to make it semi flexible. Any of you RF engineers
care to comment on this?

-Tom

Reading the thread, I have two questions:

1. Is the signal picked up by ground stations and other
planes significantly diminished if the antenna is mounted
vertically somewhere along the top of the rear fuselage
of a carbon glider, rather than behind the gear doors?

If the ground station is off to the side, then it should
pick up the signal OK, and manoeuvres should give a
signal.

Have any tests been done?

This would be further from the pilot and less prone
to trailer rash.

2. The thread discusses vertical polarisation. If
a glider is thermalling at 45 deg or even 60 deg, with
a vertically mounted antenna, is a suitable signal
still picked up?

A typical cross-country flight involves 20-50 percent
of the time thermalling at a bank angle of over 30
degrees. Not much good if invisible for all this part
of the flight!

Rory

Gary Emerson wrote:
Had this random thought over the weekend. Figured there were people on
RAS that might have information to say whether this would help or hurt
the transmit "power" of a transponder.

Instead of mounting a single antenna on the belly of the fuse where it
can get beat up in the trailer, off field landings or just rocks and mud
on the runway, what about a pair of antennas mounted on the side of the
fuse on the tail boom similar to static ports. If you ran a single
co-axial cable and then put a T at the end and then ran a short piece to
each side of the boom then you'd have, in concept anyway, a good 360
"view". It may well be that antennas don't like this sort of
arrangement so I figured I'd ask. Also has the obvious downsides of
needing the factory to build this in to the plane and doubling your
antenna costs. The benefit is that it does move the antenna farther
from the pilot.

Besides the impedance matching problems, you will also be created
unpredictable radiation patterns. This will probably create nulls (dead
spots) in numerous directions. Might as well not install the
transponder. Check out this web site for some examples:

http://www.ee.surrey.ac.uk/Personal/...pantarray.html

Tom

Might even be possible to cut the antenna near the base and put rejon
with a strong spring to make it semi flexible. Any of you RF engineers
care to comment on this?

Bad idea. The spring is a very lossy coil. You would have to install a
flexible wire inside of the spring to allow bending. That flexible wire is
a bad antenna at this frequency, since rf flows on the surface. And, it
would be inside of a lossy conductor. You can make the antenna flexible
enough to withstand occasional bending, yet solid enough not to bend
excessively in flight.

The thread discusses vertical polarisation. If a glider is thermalling at
45 deg or even 60 deg, with
a vertically mounted antenna, is a suitable signal still picked up?

The quoted loss for using the wrong polarity only applies if you are exactly
90 degrees from the desired polarity. Anything in between is significantly
less of a problem. As a matter of fact, when you are out thermaling, that
very heat that keeps you up will also rotate the polarity of the transmitted
signal. In addition to vertical and horizontal polarization, there is
circular polarity. Circular polarity has the advantage that it is never
more than 3 db down from either horizontal or vertical.

Best advice is to just install the vertical so that it is more or less
vertical and enjoy yourself. Gliders do not stay in one spot, so if you are
nulled out for a second, you will probably be in the clear a second or two
later.

Colin

The next issue of the ASA Newsletter will have an article
on how to construct a removable transponder rod antenna
with a BNC twist lock connector for bottom mounts.
Info on joining ASA can be found here -

http://tinyurl.com/4c2sw