GAMMA MATCHESOzaukee Radio club November 2013
A quick overview of the steps needed to adjust an antenna matching network usually seen on many older single band yagi antennas By Ken Boston W9GA
What is a gamma match ?This type of feed-line connection is commonly seen being used to allow a coaxial cable to be coupled to the driven element of a yagi beam antenna
It is intended for use on a mono band yagi, (14 meg and up to VHF) and is a single ended termination, as opposed to a balanced connection. It is also commonly seen being used to shunt feed HF vertical antennas.
Gamma match components on a 6 meter beam.
Other feedline matches: Balanced coaxBalanced feed; Tee match: split feed with 4:1 coax balun:
My experience with a modified CushCraft A50-5This antenna as designed has 5 elements on a 12 food boom. The W5WVO mod consists of adding boom material and resetting the element configuration for a 17 foot boom, resulting in a +1.5 dB gain and a cleaner pattern.Upon redesigning the antenna, I tried to set the matching components, and after a couple hours, decided there must be a better way than just hunt and peckBack to the trusty internet, but just became frustrated, with tips ranging from very vague, to a large technical tutorial on antenna feedpoint methods.
3 steps to tuning the matchGet the antenna clear of surrounding metal or ground effects.Make sure you have a decent analyzer, and a half wavelength (or multiple) cable.Make changes to 3 parameters on the driven element to set the match. Driven element length, distance to the shorting bar, and the amount of capacitance.
Step 1: Clear the antennaIn order for the match to be correct when the antenna is raised into the air, you want little interaction from the nearby ground, and any other metal objects. (Near field interference)
For this yagi, we ran poly rope between a tower and a tree, with a pulley tied in the middle, then raised the yagi with a second rope tied near the front of the yagi antenna, thru the pulley and off to the side, and pointed the yagi straight up. The reflector (back end) was set on a pair of wooden horses.
Using the antenna analyzerStep 21. I used an MFJ 259B, good for the bands from 1.8 MHz up to 144 MHz2. Other analyzers are similar (earlier OZ club presentation)3. Display will show: frequency, R, X, SWR4. Connection to antenna will need to be made with a short run of coax, electrical length must be a multiple of half wave5 Connecting with short jumper results in your body being to close to the antenna, skewing the match numbers, use a length.6 Using a half wave (or multiple) will repeat the impedance from the match at a distance.7 Connect a cable which is not terminated at far end, and add (or subtract by cutting if using cable with no connector on end) length until the infinite R value is seen at the analyzer. Do this at the frequency that we are interested in matching.
Adding coax length to get high Impedance
Trim cable to length
Analyzer showing high Z
Step 3; adjusting the match, part 1Following the guidelines from the antenna manufacturers manual, set the driven element length to the needed length for the center frequency in the band desired, (if DE length is adjustable)Connect the cable from the analyzer to the match and set the frequency of the analyzer to the desired center frequency.For the purposes of this talk, some pictures were created using the 10 meter dipole I have brought out to our Field day the last couple years. (part of a 3 element CushCraft beam)Since we are adjusting a conjugate match, we need to set both the real part and the imaginary part of the match; Z = R +/- jX( I was told there would be no math)
View of the match components, (from a 144 beam)
Equivalent circuit for gamma match
Step 3 matching part 2Using the analyzer, first try for a 50 ohm reading for the R value (real part of the impedance) by setting the shorting bar distance out from the boom. Sliding further out should increase the R, closer to boom should reduce the R.
R=52 ohms, X still off (jX=47)
Step 3 matching part 3Next, with the analyzer, adjust the distance of the matching tube from the boom, sliding in or out over the inner conductor mounted to the connector, until the X value is at or near zero. You are effectively varying the capacitance in a piston capacitor. (on the MFJ, there is no +J or -J, so you will have to play with this parameter to see if you are going to zero, or increasing to a higher value.)
R=51 ohms, X=9 ohms, SWR=1.2
Check match at frequency edgesIf after setting shorting bar, and gamma tubing to a good match at the center frequency, check at the band edges, and if one band edge looks a little better, re-set driven element lengthMatch better at lower edge, shorten DE a littleMatch better at upper edge, lengthen DE a little
4 bay EME array going together
Questions?Ask now, or please ask anytime.Thanks for your undivided attention. (wake up Tom)