Topband: Shunt feed short tower

[Kelly Jones]

K6SE said:
"To save a lot of trips up and down the tower, modeling the structure
properly will get you very close to the ballpark.  I've done that for
many hams over the years and have been quite accurate.

The shortest tower without any top loading whatsoever that can be gamma
matched is about 85 feet.  Rohn 25 that high with the tap point at the
top with 36" shunt wire spacing will have a feedpoint Z of 50 + jX.

A tower shorter than that requires an omega match (unless you increase
the shunt wire spacing to rediculous values or decrease the size of the
shunt wire to very small diameters."

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Earl, would you mind modeling my situation?  W6UB plugged some initial
numbers into his model but asked that I supply "real" numbers from the
tower.  I'm interested in seeing what the model looks like with these
numbers.

Here are the natural resonances as it sits today with 1 shunt connected
(based on MFJ259B readings)
1820khz
R=0
X=168

There is a peak at 2065khz where R = 11 and X = 199.  Going up the band
R goes back to 0.  I finally get R=50 at 2.272.  X=355 at this point in
the band.

Adding the second shunt I see the following:
1820khz
R=0 
X=116

Again I see a peak at 2075khz where R = 15 and X = 135.  The R=50 point
with both shunts connected is still 2.272 but X = 250 in this
configuration.

==========

"Increasing the diameter of the shunt wire will slightly increase the
2:1 SWR bandwidth, but it also makes the required 50-ohm tap point
higher. 
Vice-versa on both things is the wire is thinner."

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So you're saying that by having the 2nd 1/2 piece of pipe attached, I
would have to tap higher on the tower than if I only used one?  

==========

If the tap point is at the top of the tower and the resistive component
is greater than 50 ohms, that means you must lower the tap point!!!!!

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Currently my tap point is about 35'.  I started out at the very top of
the tower around 41' but moved the tap point down twice, about 2ish feet
each time.

==========

"Keep in mind (especially for towers less than 1/4-wave
electrically) that:

1) Decreasing the diameter of the shunt wire increases the R.

2) Increasing the shunt wire spacing increases the R.

3) Going higher with the tap point increases the R."

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The way I read this is that I need to move the shunts further from the
tower than they are now.  I currently have both stood off 12".  It would
seem by the above I should go out to perhaps 36"?

Thanks for all of the help with this.  I'm certainly learning a LOT.

73
Kelly - N0VD