A "tuned" ground plane is one that is an odd multiple of a quarter wave at the
operating frequency. Odd -- one, three, five, and so on. In practicality (with
the possible exception of GPS) the norm is ONE quarter wave, or a quarter-wave
groundplane.
Having said that, you must understand that you cannot make a "perfect" circular
("disk") ground plane for a transponder, as it must operate on two frequencies
simultaneously -- 1030 and 1090 MHz. What is quarter wave for one is not
quarter wave for the other. While the error is slight and relatively
insignificant for transponders, it DOES become significant when you start to
talk about things like the aircraft COM band, where the instantaneous bandwidth
is on the order of 11%.
My chosen way to make a transponder ground plane is to start off with a square
of aluminum using the following calculations:
w = 11810 / f
s = 0.487 w
where "w" is a quarter wave in air at frequency "f"
and "s" is the length of one side of the square of aluminum.
Then punch a hole in the exact center of the groundplane for the antenna.
Then cut the corners of the square to make a regular octagon.
SOMEWHERE along the periphery of that octagon will be an exact quarter wave at
both 1030 and 1090 MHz. IF you select w to be halfway between these two
frequencies (i.e. 1060 MHz.).
So, in the above scenario, w = 11.14" and s = 5.42".
Jim
(John Tvedte)
shared these priceless pearls of wisdom:
-I have a Velocity - and am planning on using a Comant CI-105
-transponder antenna, and place it out on the wing strake area - as I
-do not want it close to the engine and near my AHRS setup.
-
-As this airplane is a glass airplane, and this antenna requires a
-ground
-plane - I have read about a "tuned" ground plane.
-
-The suggestion was to make a 5.2" disc - @1090Mhz, 1/2 wave is
-5.15229..."
-
-Can someone give me a heads up as to what a tuned ground plane really
-is?
-
-Thanks,
-
-John
Jim Weir (A&P/IA, CFI, & other good alphabet soup)
VP Eng RST Pres. Cyberchapter EAA Tech. Counselor
http://www.rst-engr.com