GAMMA MATCHES
• Ozaukee 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 coax
Balanced feed; Tee match: split feed with 4:1 coax balun:
My experience with a modified CushCraft
A50-5
• This 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 peck’
• Back 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 match
1. Get the antenna clear of surrounding metal
or ground effects.
2. Make sure you have a decent analyzer, and
a half wavelength (or multiple) cable.
3. 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 antenna
In 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 analyzer
Step 2
• 1. I used an MFJ 259B, good for the bands from 1.8 MHz up to 144
MHz
• 2. Other analyzers are similar (earlier OZ club presentation)
• 3. Display will show: frequency, R, X, SWR
• 4. Connection to antenna will need to be made with a short run of
coax, electrical length must be a multiple of half wave
• 5 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 1
• Following 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 2
• Using 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 3
• Next, 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 edges
• If 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 length
– Match better at lower edge, shorten DE a little
– Match 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)