Transponder antenna ground plane - Help!

While finalizing the installation of a TRIG TT21 transponder, I am running into less that consistent information regarding the ground plane spec. for the antenna. The plan at this point is to use a RAMI AV-22 rod antenna inside the fiberglass fuselage.

The TRIG TT21 Manual has this to say about the dimension of the ground plane:
"[...] As the ground plane is made smaller, the actual dimensions of the ground plane become more critical, and small multiples of the wavelength should be avoided, as should circles. Rectangles or squares are much less likely to create a critical dimension that resonates with the transmissions. The smallest practical ground plane is a SQUARE around 120mm per side; as the size increases the performance may actually get worse, but will be better by the time the ground plane is 700mm on each side."

From the Cumulus Soaring site, RAMI, the antenna manufacturer reportedly states that "The smallest recommended ground plane would be 12 inches in diameter (305mm)."

Now, when I look at all of the Schleicher TNs, I see that they call out in most case a 130mm ROUND aluminum disk to be used in conjunction with the RAMI antenna mentioned above.

So what gives? Round or Square? Short of a very large plane, what dimension to use for the ground plane? The fact that the information available does not seem to converge on a given dimension or shape for a ground plane tells me that this is probably more than one "OK" answer.

Thanks,
B

Bastoune wrote:
While finalizing the installation of a TRIG TT21 transponder, I am
running into less that consistent information regarding the ground plane
spec. for the antenna. The plan at this point is to use a RAMI AV-22 rod
antenna inside the fiberglass fuselage.

The TRIG TT21 Manual has this to say about the dimension of the ground plane:
"[...] As the ground plane is made smaller, the actual dimensions of the
ground plane become more critical, and small multiples of the wavelength
should be avoided, as should circles. Rectangles or squares are much less
likely to create a critical dimension that resonates with the
transmissions. The smallest practical ground plane is a SQUARE around
120mm per side; as the size increases the performance may actually get
worse, but will be better by the time the ground plane is 700mm on each side."

From the Cumulus Soaring site, RAMI, the antenna manufacturer reportedly
states that "The smallest recommended ground plane would be 12 inches in diameter (305mm)."

Now, when I look at all of the Schleicher TNs, I see that they call out
in most case a 130mm ROUND aluminum disk to be used in conjunction with
the RAMI antenna mentioned above.

So what gives? Round or Square? Short of a very large plane, what
dimension to use for the ground plane? The fact that the information
available does not seem to converge on a given dimension or shape for a
ground plane tells me that this is probably more than one "OK" answer.

Thanks,
B

Does your glider manufacturer have a TN or similar for a transponder
install in this or a similar glider ? if so just follow those directions.
Ground planes are usually better off on the larger side and round. If
making this up a fuselage internal antenna install yourself you *might* be
better off with a dipole antenna that does not need a ground plane.

The Schleicher TN ground plane diameters are often limited by internal
clearance, however again you should follow the glider manufacturers
directions if they exist. There are reasons that an external antenna
install may be preferred to an internal one.

It is fantastic that you are installing a transponder, but I get nervous
with questions like this, if you are having to ask these questions are
there other things as well you are overlooking or misunderstanding? it is a
great idea if you can to have an experienced glider A&P or knowledgable
person check on your plans well before you start the install.

Darryl

When I installed my TT22, I made an aluminum square, 120mm on the side, and
curved it to fit the inside of my fuselage aft of the landing gear. ATC has
no problem seeing me.


"Darryl Ramm" wrote in message
...
Bastoune wrote:
While finalizing the installation of a TRIG TT21 transponder, I am
running into less that consistent information regarding the ground plane
spec. for the antenna. The plan at this point is to use a RAMI AV-22 rod
antenna inside the fiberglass fuselage.

The TRIG TT21 Manual has this to say about the dimension of the ground
plane:
"[...] As the ground plane is made smaller, the actual dimensions of the
ground plane become more critical, and small multiples of the wavelength
should be avoided, as should circles. Rectangles or squares are much less
likely to create a critical dimension that resonates with the
transmissions. The smallest practical ground plane is a SQUARE around
120mm per side; as the size increases the performance may actually get
worse, but will be better by the time the ground plane is 700mm on each
side."

From the Cumulus Soaring site, RAMI, the antenna manufacturer reportedly
states that "The smallest recommended ground plane would be 12 inches in
diameter (305mm)."

Now, when I look at all of the Schleicher TNs, I see that they call out
in most case a 130mm ROUND aluminum disk to be used in conjunction with
the RAMI antenna mentioned above.

So what gives? Round or Square? Short of a very large plane, what
dimension to use for the ground plane? The fact that the information
available does not seem to converge on a given dimension or shape for a
ground plane tells me that this is probably more than one "OK" answer.

Thanks,
B

Does your glider manufacturer have a TN or similar for a transponder
install in this or a similar glider ? if so just follow those directions.
Ground planes are usually better off on the larger side and round. If
making this up a fuselage internal antenna install yourself you *might* be
better off with a dipole antenna that does not need a ground plane.

The Schleicher TN ground plane diameters are often limited by internal
clearance, however again you should follow the glider manufacturers
directions if they exist. There are reasons that an external antenna
install may be preferred to an internal one.

It is fantastic that you are installing a transponder, but I get nervous
with questions like this, if you are having to ask these questions are
there other things as well you are overlooking or misunderstanding? it is
a
great idea if you can to have an experienced glider A&P or knowledgable
person check on your plans well before you start the install.

Darryl

On Monday, March 17, 2014 10:41:38 AM UTC-4, Bastoune wrote:
While finalizing the installation of a TRIG TT21 transponder, I am running into less that consistent information regarding the ground plane spec. for the antenna. The plan at this point is to use a RAMI AV-22 rod antenna inside the fiberglass fuselage.



The TRIG TT21 Manual has this to say about the dimension of the ground plane:

"[...] As the ground plane is made smaller, the actual dimensions of the ground plane become more critical, and small multiples of the wavelength should be avoided, as should circles. Rectangles or squares are much less likely to create a critical dimension that resonates with the transmissions. The smallest practical ground plane is a SQUARE around 120mm per side; as the size increases the performance may actually get worse, but will be better by the time the ground plane is 700mm on each side."



From the Cumulus Soaring site, RAMI, the antenna manufacturer reportedly states that "The smallest recommended ground plane would be 12 inches in diameter (305mm)."



Now, when I look at all of the Schleicher TNs, I see that they call out in most case a 130mm ROUND aluminum disk to be used in conjunction with the RAMI antenna mentioned above.



So what gives? Round or Square? Short of a very large plane, what dimension to use for the ground plane? The fact that the information available does not seem to converge on a given dimension or shape for a ground plane tells me that this is probably more than one "OK" answer.



Thanks,

B



Antenna design is determined by physics; the size is determined by the transmit frequency. Other than where the antenna goes, the particular aircraft has nothing to do with antenna design or size.

The minimum for a ground-plane antenna is a ground plane of 4 wires (or rods), that are 1/4 wavelength long, and that extend radially from the base of the center vertical element. This is the type of ground plane antenna that you see at an airport ground station. (bear in mind that an aircraft transponder antenna is upside down from a base-station antenna) The transponder transmit frequency of 1090 mhz will give you 1/4 wavelength ground plane rods (or wires) of 68.8 mm length.

The next best thing is a multitude of 68.8mm long radial wires until you have so many that you ultimately end up with a metal disc with a radius of 68..8 mm. A round metal circle requires some degree of accuracy.

The third best thing is a square where the distance from the center to one side is less than 1/4 wavelength and the distance from the center to a corner is more than 1/4 wavelength. That is why one of the examples is a square with a side of 120mm. The square is nice because it is very forgiving of inaccuracy.

The best is an extremely large (as a ratio of the ground plane to the height of the vertical element) ground plane. Considering how short the vertical element is, 700mm is large enough. Due to the large size, this is not suitable in a glider.

bigger is not necessarily better. As you get bigger than the minimum, there are some sizes that are good and some sizes that are bad. As the TRIG TT21 Manual states, anything in between the minimum and very large "may" make performance worse. And that is why you have gotten so many answers.

My suggestion is that you go to to the RAMI site and clicking on "contact tech support". The next best suggestion is that you either use the 68.8mm radius circle or the 120mm per side square. Make sure you round the corners for safety.

On Monday, March 17, 2014 7:41:38 AM UTC-7, Bastoune wrote:
While finalizing the installation of a TRIG TT21 transponder, I am running into less that consistent information regarding the ground plane spec. for the antenna. The plan at this point is to use a RAMI AV-22 rod antenna inside the fiberglass fuselage.



The TRIG TT21 Manual has this to say about the dimension of the ground plane:

"[...] As the ground plane is made smaller, the actual dimensions of the ground plane become more critical, and small multiples of the wavelength should be avoided, as should circles. Rectangles or squares are much less likely to create a critical dimension that resonates with the transmissions. The smallest practical ground plane is a SQUARE around 120mm per side; as the size increases the performance may actually get worse, but will be better by the time the ground plane is 700mm on each side."



From the Cumulus Soaring site, RAMI, the antenna manufacturer reportedly states that "The smallest recommended ground plane would be 12 inches in diameter (305mm)."



Now, when I look at all of the Schleicher TNs, I see that they call out in most case a 130mm ROUND aluminum disk to be used in conjunction with the RAMI antenna mentioned above.



So what gives? Round or Square? Short of a very large plane, what dimension to use for the ground plane? The fact that the information available does not seem to converge on a given dimension or shape for a ground plane tells me that this is probably more than one "OK" answer.



Thanks,

B

If you have a fiberglass fuselage I'd suggest using the L-2 antenna.
It doesn't require a ground plane and is easy to install
Dan

On Monday, March 17, 2014 7:41:38 AM UTC-7, Bastoune wrote:
While finalizing the installation of a TRIG TT21 transponder, I am running into less that consistent information regarding the ground plane spec. for the antenna. The plan at this point is to use a RAMI AV-22 rod antenna inside the fiberglass fuselage.



The TRIG TT21 Manual has this to say about the dimension of the ground plane:

"[...] As the ground plane is made smaller, the actual dimensions of the ground plane become more critical, and small multiples of the wavelength should be avoided, as should circles. Rectangles or squares are much less likely to create a critical dimension that resonates with the transmissions. The smallest practical ground plane is a SQUARE around 120mm per side; as the size increases the performance may actually get worse, but will be better by the time the ground plane is 700mm on each side."



From the Cumulus Soaring site, RAMI, the antenna manufacturer reportedly states that "The smallest recommended ground plane would be 12 inches in diameter (305mm)."



Now, when I look at all of the Schleicher TNs, I see that they call out in most case a 130mm ROUND aluminum disk to be used in conjunction with the RAMI antenna mentioned above.



So what gives? Round or Square? Short of a very large plane, what dimension to use for the ground plane? The fact that the information available does not seem to converge on a given dimension or shape for a ground plane tells me that this is probably more than one "OK" answer.



Thanks,

B

One way to fabricate a ground plane is to use copper foil tape that uses conductive adhesive. Just overlap it in strips on the inside of the fuselage around the antenna.

Tom

Really...why do you have to make this so complicated?
Get an L-2. Providing you have a pure fiberglass fuselage just slap it on the inside of the fuse and be happy.
I have an LS-8. The L-2 is on the side of the fuse behind the wheel. Air traffic control radar reads me just fine no matter what my orientation to the radar. Commercial aircraft that are above me read me no matter what our relative orientations.

On Wednesday, March 19, 2014 6:34:12 AM UTC-7, wrote:
Really...why do you have to make this so complicated?

Get an L-2. Providing you have a pure fiberglass fuselage just slap it on the inside of the fuse and be happy.

I have an LS-8. The L-2 is on the side of the fuse behind the wheel. Air traffic control radar reads me just fine no matter what my orientation to the radar. Commercial aircraft that are above me read me no matter what our relative orientations.

"Providing you have a pure fiberglass fuselage"

Need I say more?

Thanks for the info. An A&P & experienced avionic installation centre will be doing the install and xpdr test so it will be their call at the end. I am just trying to understand why there are varying ground plane recommendations between the trig manual, RAMI antenna mfr and what Scheichler typically recommends. Mpa-Thank you for the explanation. I know understand a bit better the trade-offs in the ground plane sizes. I saw the L-2 antenna. Seems nice.

Don't let them cut your fiberglass, use the internal dipole described earlier glued/velcroed to an internal verticle brace. We did that for an Ls4, Grob 103 and Grob 104.

BT