When using a gamma match you are usually going up on the tower beyond the 50
ohm point. From that point down the gamma rod and the tower is acting like a
parallel wire transmission rod, transforming that impedance down to a lower
value, hopefully near 50. You will see the 50 ohm R plus inductive reactance at
the feed point. The series gamma capacitor tunes out the inductive reactance
leaving just the R. The way to change the R is with a shunt capacitor (as in
an Omega match) the series capacitor only compensates for the series inductance.
I do not think taking a reading at the bottom of the gamma wire with the MFJ is
giving a valid R value, especially since adding a series capacitor seems to "
change the R".
Matching a tower with a gamma match is just like matching a Yagi with a gamma
rod.
If the bottom of the tower (center of the Yagi element) has an impedance of Z
then a point up the tower or out on the Yagi element X degrees will be Z
divided by (cosine X) squared. ( plus a factor due to unequal diameters of the
element and gamma rod must also be considered)
For example if the base of the tower is 35 ohms, moving up 30 degrees (40 some
feet) will give a Z at that point of about 45 ohms. But the gamma rod and the
tower act like a step up transformer. Remember how a folded dipole works. Two
same size wires will give a step up of 4 times what a single wire has. Single
wire dipole 75 ohms and folded dipole 300 ohms. If the wires are not the same
the ratio is different. In the case of the tower being larger than the gamma
wire, the ratio is much higher. So the Z at the top end of the gamma will be
several times higher. I usually guess this number and plot on a Smith chart.
Then move around x degrees. This brings me to some value of R and inductive X.
Then the gamma Capacitor tunes out the inductive reactance. Measuring this
with the MFJ 259 should give good results. If the reading is like 100 ohms you
can make the gamma rod shorter, bringing down the R. If low, make the gamma rod
longer. (I may have this just reversed, again, I like
to use a Smith chart because it is easy to see what happens when one
parameter is changed. It can be very confusing otherwise and I am relying on
only my old memory as I write this)
I know some of the old ARRL antenna books have good info on designing gamma
matches. For all my towers I have gotten close enough with a #12 wire about 20
to 30 foot long about 18 inches off the tower.
One other easy method that seems to always work is to come down to an L
network. I have used a slant feed wire running from an L network on the ground
up at a 30 to 45 degree angle to a tap on the tower. That particular
installation was a Rohn 45 tower at 70 ft top loaded with a full size 4 element
20 meter monobander. The shunt C on the output of the L network changes the R
and the coil compensates for the reactance. Remember shunt element changes the
R and the series element does not change the real R. Sometimes you have to turn
the L network around because you want the shunt C on the high impedance side.
73,
Don
N4DJ
Sent from my iPad
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