K1TTT Technical Reference


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K1TTT Gamma tuning procedure:

 

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.

 

 

 

 




David Robbins, K1TTT K1TTT@arrl.net