Transponder Antenna Thought

[Gary Emerson]

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.

[COLIN LAMB]

Well, you can split the output between two antennas, but it cannot be
haphazard. The connecting cables must be cut a specific length. The
additional connections reduce reliability slightly, though.

The more serious problem, as I understand the installation, is that you will
no longer be ominidirectional. You will have a null straight ahead and
behind you - two critical areas. The antenna would still need to be mounted
vertical, so the new install might be difficult.

Colin

[Bob Greenblatt]

And, in addition to what Colin said, the transponder signal is vertically
polarized. That is why those film antennas mounted on a gear door will not
work properly.



--
Bob
bobgreenblattATmsnDOTcom <--fix this before responding

[Doug Haluza]

You can use multiple antennas, but this creates an antenna array, and
design of antenna arrays is a non-trivial pursuit. So you probably do
not want to go there. If you were thinking of using two quarter wave
stubs on the sides of the fuselage, that will not work, since the
polarization will be wrong.

Best bet with a glass fuselage is a dipole antenna inside the fuselage.
With carbon, you just have to deal with the stub on the outside. It
neds to be on the bottom for maximum visibility to ground stations. You
can use faired "fin" version as well, but this is going to be more
expensive to replace if it is damaged.

[[email protected]]

Doug Haluza wrote:
[snip] Best bet with a glass fuselage is a dipole antenna inside the
fuselage.
> With carbon, you just have to deal with the stub on the outside. It
> neds to be on the bottom for maximum visibility to ground stations. You
> can use faired "fin" version as well, but this is going to be more
> expensive to replace if it is damaged.

Could antennae be installed with a velcro or other easy fix/release
attachment, and some sort of connector allowing break-off and
replacement without other damage?

(If they have to go beneath a carbon fuselage, they are bound to get
knocked some time.)

Chris N.

[Robert Backer]

The blade antenna on top works just fine. I used that installation on
my asw22 for 5 years and both ATC and airliners picked it up no problem.
Pretty much the same installation that Tom Knauff had on his duo that
was on the cover of Soaring a few years ago.

Bob

wrote:
> Doug Haluza wrote:
> [snip] Best bet with a glass fuselage is a dipole antenna inside the
> fuselage.
>> With carbon, you just have to deal with the stub on the outside. It
>> neds to be on the bottom for maximum visibility to ground stations. You
>> can use faired "fin" version as well, but this is going to be more
>> expensive to replace if it is damaged.
>
> Could antennae be installed with a velcro or other easy fix/release
> attachment, and some sort of connector allowing break-off and
> replacement without other damage?
>
> (If they have to go beneath a carbon fuselage, they are bound to get
> knocked some time.)
>
> Chris N.
>
>

[COLIN LAMB]

It is possible to eliminate the vertical antenna, yet have vertical
polarization. I was going to install one on my motorglider, but eventually
installed a simple vertical. My motoglider is fabric and not ideal for the
flush mount antenna.

The flush mount antenna is described as a flush disc with annular slot. It
is described in John Kraus's book "Antennas". He designed the first of the
antenna arrays that probe outer space.

Basically, the 1/4 ground plane antenna is replaced with a top loaded
vertical. The top of the vertical is flush with the fuselage and the base
is 1/4" below the surface. The diameter of the disc is about 3". This disc
is the top loading for the shortened 1/4 wave antenna. In addition, there
must be a ground plane at the bottom of the hole.

It would be a good antenna for a plastic airplane - if you are willing to
cut about a 4" hole in the fuselage and then run a ground plane on the
surface about 6 inches in each direction.

It could be molded into a plastic airplane.

To this point, I do not know anyone that has done so and of course no one
makes them. Perhaps it is used in the Stealth aircraft, but generally they
are the last pilots who have a transponder on.

Instead of going to all that work, I opted for a standard quarter wave
antenna. It was destroyed during a less than ideal landing in 3 foot weeds.

Colin

[[email protected]]

Coupling multiple antennas to the same transmitter will likely create
an impedance mismatch, which will introduce standing waves and reduce
power output (kind of negating the original intent). This would have to
be properly engineered by the transponder manufacturer to be a legal
installation. I would direct the question to them, but I expect the
answere will be to stick with a single antenna.

Tom Seim

[[email protected]]

Coupling multiple antennas to the same transmitter will likely create
an impedance mismatch, which will introduce standing waves and reduce
power output (kind of negating the original intent). This would have to
be properly engineered by the transponder manufacturer to be a legal
installation. I would direct the question to them, but I expect the
answere will be to stick with a single antenna.

Tom Seim

[Eric Greenwell]

wrote:

> Could antennae be installed with a velcro or other easy fix/release
> attachment, and some sort of connector allowing break-off and
> replacement without other damage?

I don't know of any method that would provide proper functioning and
break-off without damage, so you'll probably have to invent something.
>
> (If they have to go beneath a carbon fuselage, they are bound to get
> knocked some time.)

It can happen, but the antenna can likely be bent back into position,
and if it can't be, it is easy to replace (15 minutes on my glider,
since it's mounted just behind the gear doors) and cheap ($20-$30). I'm
talking about the rod style antenna - the blade type might take a bit
more knocking, but I think there is a chance of damaging the fuselage if
they get hit. They are much more expensive, also.

--
Change "netto" to "net" to email me directly

Eric Greenwell - Washington State, USA

www.motorglider.org - Download "A Guide to Self-launching Sailplane
Operation"

[Scott]

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.

[COLIN LAMB]

>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

[Tom Lucas]

"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.

[Gary Emerson]

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.

[5Z]

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

[Rory O'Conor]

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

[[email protected]]

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/D.Jefferies/pantarray.html

Tom

[COLIN LAMB]

>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

[Gary Evans]

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