GAMMA MATCH
By Bob Archer
The Magic of Grounded Antennas
Most of the antennas I make use a device that is called a “Gamma Match” to feed the antennas and match the impedances. I use this device because I then have control over all the parameters of the antenna and it allows the matching of impedance to a high level across the frequency band. I can control the overall length of the elements, can feed the antenna at the fifty-ohm point of the elements and can vary the inductance and capacitance to the point where they operate at the point of the best possible performance.
Originally, back in olden times, when all RF transmission lines were twin leads the radio folks found that they could match the Impedance of the transmission line to a half wave continuous conductor by spreading the conductors into a “Y” type of device. Early twin lead was 600 ohms and later the 300 ohms we are all familiar with used for TV lead in. It was found that the half wave conductor would resonate at the frequency it was cut to if it were fed with the resonant frequency. This half wave resonant conductor would then have impedance characteristics that would vary along its length. The outer ends of the conductor would have an impedance of infinity because the currents would have no place to go and the center would have zero impedance and the maximum current. So somewhere between the center and the ends would be a spot where the natural impedance would match half of the impedance of the twin lead transmission line. The twin lead conductors were then connected to the proper points on the half wave conductor and the antenna and transmission line were matched and you would have maximum efficiency and energy transmission.
That was fine but then came along coaxial cable and the need for monopole antennas.
The easiest thing to do was, and still is, to connect a coax center conductor to a quarter wave rod conductor and where the rod is connected to the center conductor connect the coax braid to a ground plane, or more properly counter poise, which then becomes the second half of a dipole antenna. Non-symmetrical of course. With this type of antenna the only method of tuning is to trim the rod length to the best center frequency length. This works of course but can be quite a ways off of the best tune.
So we go back and look at the Gamma match. If we cut the half wave conductor in half and fasten one end to a ground plane we have half of the Gamma device described above. It has maximum impedance at the un-grounded end and zero at the shorted to ground end. So we attach the coax braid to the ground next to the rod and come through the ground plane with the center conductor and up to where the impedance is 50 ohms we should have a matched antenna. Except in doing this we have an excess of inductance due to the unshielded center conductor which must be balanced out with some capacitance. When all the parameters are correct we can have an antenna that is impedance matched as perfectly as is possible across its frequency band. To design this type of antenna you must have access to antenna measuring equipment that can measure both phase and amplitude. The parameters are not linear and the only way to get where you want to go is to cut and try and measure. In my wing tip antenna designs I took the Gamma match theory and grounded and bent sheet metal into a swept back design with the Gamma capacitor swept forward and up to the 50 ohm point. I can vary the capacitor value, the overall antenna length and the point at which the capacitor feed attaches to the grounded antenna. The thin sheet aluminum allows the antenna to be flexed to fit the interior of any tip large enough to accommodate it.
I decided early on that wing tip antennas must be grounded to work in the wing tips along with the other stuff that folks have in them. The light wires, other wires, the lights etc. cause standard whip type antennas to not work very well. For years folks have tried to use whips in tips with very poor results. Our first design didn’t work either, it had a null right on the nose of the airplane so we did a redesign and ended up with what we have now.
The results have been very gratifying. I had a Lancair IV pressurized builder tell me he got 200 miles range or more at 20000 ft. on his wing tip Vor antenna. He also had 200 miles range on his vertical stabilizer installed gamma match fed dipole antenna.
Folks that have done direct comparisons of my antenna designs with others have always found mine to be superior.
CHEERS
Bob
Sportcraft Antennas 