Transponder Antenna Thought

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.

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

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

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.

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.

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.

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

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

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

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.

--
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Eric Greenwell - Washington State, USA

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