How to tune a gamma match in 12 steps with a dozen loops and many decision points, made easy!
Remember, there are really 5 adjustments to tune a gamma,
and they
all interact with each other:
1. the capacitor - tunes out the
reactance left at the end of
the gamma after the parallel tubing
transmission line transforms
it from the tap point.
2. the tap point - selects the
impedance fed into the parallel
tubing transmission line
3. the spacing between tubes -
affects the impedance of the
tubing transmission line
4. the relative diameters of the
tubing - also affects the impedance
of the tubing transmission line
5. the length of the driven element
- affects the impedance at
the tap point
Usually 3 and 4 are fixed early in the design and are
ignored.
the best procedure i have found is:
1. set driven element to calculated
length
2. pick a tap point
3. set capacitor to mid range
4. sweep band, find minimum swr
5. if minimum is out of band
lengthen or shorten driven element to
get it in the band.
6. tune to min swr
freq, sweep capacitor to find minimum swr
7. adjust tap point a small amount
8. sweep capacitor to find minimum,
if its higher than before
adjust the tap the other direction
9. repeat adjusting tap and
sweeping capacitor for minimum until
swr is
reasonable (NOT PERFECT!)
10. sweep band and plot swr curve
11. if you like curve return to 9
and continue until you get as
low as you have time for, then do
10 again. adjustments
should
keep getting smaller, if not
something is wrong. lengthen or
shorten driven element a bit and
start at 2 again.
12. if the curve has moved from where
you want adjust the driven
element length to move minimum and
go back to 6.
the one most people forget is the
use of the driven element
length to move the minimum. you can move it
with the tap point
or capacitor a bit, but it will
usually take much larger
adjustments and often won't allow
adjusting for low swr at the
same time.