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.