Transponder antenna - blade vs stub monopole

Alan wrote:
In article brianDG303 writes:
Is there a downside to mounting on a ground plane inside the fuselage
of a non-carbon glass glider?

Since the ground plane should extend at least 23 inches in each
direction around the antenna, it is probably hard to get that inside
the glider.

That sounds like a dimension for the aircraft communication radio,
working in the 120-130 mhz range. The transponder uses 1090 mhz, about
10 times higher. That indicates a 2.3" radius ground plane would be
adequate, or the 6" diameter Marc points out as commonly used.

--
Eric Greenwell - Washington State, USA
* Change "netto" to "net" to email me directly

* Updated! "Transponders in Sailplanes" http://tinyurl.com/y739x4
* New Jan '08 - sections on Mode S, TPAS, ADS-B, Flarm, more

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

What about the transflex antenna?

http://www.lxavionics.co.uk/transponders.htm

Rory

I strongly suggest you DO NOT but the blade antenna's!....
ever if there were some small aerodynamic advantage (I doubt it could be
measured at any glider speeds anyway) but these are very expensive and they
are easily damaged....
I offer blade antennas, the "preferred" simple and cheap stub antenna's and
the composite dipole antenna's..I've had several buyers of the blade
antenna's break them (they aren't typically very happy when this happens).it
doesn't take much effort to damage the composite blade antennas since they
are essentially a tiny wire on a thin metal plate with small (6/32) screws
in a very nice looking shark fin that is mostly filler (putty) with a thin
composite shell..
best regards
Tim
Please visit the Wings & Wheels website at www.wingsandwheels.com

"5Z" wrote in message
...
Any aerodynamic thoughts on the two available choices?

My ASH-26E came with the stub mounted just behind the wheel. Every
now and then it catches on the trailer, so I just straighten it, no
big deal.

An ASW-24B in our club is about to get a transponder and the factory
recommends the installation to be part way up around the fuselage
above the rear gear door hinge - looks like just behind the oxygen
bottle.

The blade looks cool, but is it really any better aerodynamically than
a $20 (http://www.aircraftspruce.com/catalog/avpages/
ted_transponder.php) stub?

-Tom

For what it is worth Schempp-hirth are mounting the transponder antenna in the fin on the Duo Discus. Right next to the
radio aerial. I presume the same applies to the Nimbus 4D that uses essentially the same fuselage.
Apparently there is lots of space for a good dipole antenna, and the foam cored glass structure is not opaque to the
frequencies used.

Nothing external to get damaged but heaven forbid you should ever have to work on it...

Presumably it is not something you could retrofit (for the same reasons you would never want to have to repair it).

Bruce

Tim Mara wrote:
I strongly suggest you DO NOT but the blade antenna's!....
ever if there were some small aerodynamic advantage (I doubt it could be
measured at any glider speeds anyway) but these are very expensive and they
are easily damaged....
I offer blade antennas, the "preferred" simple and cheap stub antenna's and
the composite dipole antenna's..I've had several buyers of the blade
antenna's break them (they aren't typically very happy when this happens).it
doesn't take much effort to damage the composite blade antennas since they
are essentially a tiny wire on a thin metal plate with small (6/32) screws
in a very nice looking shark fin that is mostly filler (putty) with a thin
composite shell..
best regards
Tim
Please visit the Wings & Wheels website at www.wingsandwheels.com

"5Z" wrote in message
...
Any aerodynamic thoughts on the two available choices?

My ASH-26E came with the stub mounted just behind the wheel. Every
now and then it catches on the trailer, so I just straighten it, no
big deal.

An ASW-24B in our club is about to get a transponder and the factory
recommends the installation to be part way up around the fuselage
above the rear gear door hinge - looks like just behind the oxygen
bottle.

The blade looks cool, but is it really any better aerodynamically than
a $20 (http://www.aircraftspruce.com/catalog/avpages/
ted_transponder.php) stub?

-Tom

There is a unique antenna that works fine as a transponder antenna - with no
blade or stub to break off or impede airflow. It is called an annular slot
antenna. Many antenna engineering textbooks contain information on the,
but they are not common. The entire antenna is flush with the aircraft skin
and they are vertically polarized. They would work with fiberglass
aircraft.

I did find a photograph and description of a commercial one at
www.mircrowaveeng.com Look for the L-Band Annular Slot Data Sheet (8 of
10). I suspect this particular unit may be beyond the budget of the average
sailplane owner, but you can see how what they look like.

It would be a good candidate for a slippery sailplane that does not want
stuff hanging out.

Colin Lamb

Whoops, I misspelled the website. It is:

www.microwaveeng.com

Colin Lamb

On 15 Jun, 01:39, "COLIN LAMB" wrote:
Whoops, I misspelled the website. *It is:

www.microwaveeng.com

Colin Lamb

The CAA made some comments about slot antennas in their study on low
power SSR transponders. See page 9 of:

http://www.caa.co.uk/docs/810/Study%...er%201%202.pdf

John Galloway

John gets a gold star. Excellent report. About the only question not
answered is the comparative aerodynamic losses for a glider with the
different antennas.

Colin

On Sun, 15 Jun 2008 02:41:42 -0700, jpg797 wrote:

On 15 Jun, 01:39, "COLIN LAMB" wrote:
Whoops, I misspelled the website. *It is:

www.microwaveeng.com

Colin Lamb

The CAA made some comments about slot antennas in their study on low
power SSR transponders. See page 9 of:

http://www.caa.co.uk/docs/810/Study%...er%201%202.pdf

It looks, from that, as if the optimum for those of us with glass
fuselages would be a vertical dipole mounted inside the rear fuselage. A
14.5cm dipole sounds small enough to fit fairly easily.

However, I have two questions for the experts, as I don't understand RF
radiation patterns or coax losses.

- my glider (Std Libelle) uses pushrods for the rudder and elevator
linkages. IIRC these run along the bottom of the fuselage. If the
dipole was mounted vertically in the upper part of the fuselage
would the control rods wreck the radiation pattern?

- with transponder in the panel and the antenna behind the oxygen
cylinder mount this would put the antenna at the end of 3 - 3.5m
or co-ax. Would that be an acceptable installation?

I hope I never have to fit a transponder, but I'd like to understand this
type of issue in case it becomes a requirement here or I take the glider
some place where one is needed.


--
martin@ | Martin Gregorie
gregorie. |
org | Zappa fan & glider pilot

At 1,000 MHz, RG-58 has a loss of about 15 db per 100 feet. So, a 10 foot
run would represent 1.5 db. That is an acceptable loss. It will reduce the
range a bit.

Generally, if the control cables are not resonant and not in the same plane
as the antenna (horixontal controls, vertical antenna), there should be
minimal interference. There are a number of antenna plotting programs arund
that would allow you to plot the potential interference. to determine the
possible interference.

Colin Lamb