Topband: shunt feed help needed

[k6se@juno.com]

Pete, N4ZR wrote:

"I'm hoping someone can help me understand what I'm seeing.  Setup is a
Rohn 25 tower with 3 yagis -- a short 40 at 105 ft, a C-3E at 97 ft and
another C-3E at 69 feet.  I tapped the tower at about 30 feet with a #14
shunt wire spaced ~24 inches.
 
" With about 400 pf of series capacitance, reactance minimum is 10 ohms
at 1865 Khz, as measured on an MFJ 259B.  However, at that frequency the
measured R is only 3 ohms, where I was expecting more on the order of
12-15 ohms, based on modeling and other people's experience.

"Does this measurement suggest that the tap should be moved up in search
of a higher resistance point?"
==========
Yes, it does.  The "secret" in shunt feeding a tower is to find the
50-ohm point where the shunt wire attaches to the tower.  There will be
some inductive reactance in additon to the 50 ohms of R, so a series
capacitor is need at the feedpoint to cancel that reactance, ending up
with a pure 50-ohm resistive load.

A simple way to visualize the shunt feed tap point is to consider that,
if you shunt fed it very close to the ground, the R would be close to
zero.  If the shunt feed tap point was near the top of the tower, the R
would be quite high.

The 50-ohm tap point is obviously somewhere between these extremes and
can be found by either trial and error or by accurately modeling the
actual tower structure and the top loading involved.  (If you model using
a cylindrical conductor as an equivalent to your tower, gross inaccuries
result no matter what the conductor diameter is.)

I'll be glad to model your tower for you if you provide me with the
following:

1) Exact height of the Rohn 25 above ground.

2) Height of any mast above the tower.

3) Boom lengths, element spacing, element length and height of your HF
beams.

4) Are the HF beam elements DC grounded to their boom?

5) Accurately describe your radial system so that I can use an equivalent
ground resistance.

Modeling has proven to be highly accurate in determining the 50-ohm tap
point (including wire spacing), the correct value of "gamma" capacitor,
and the RF voltage the capacitor will have to withstand (depending on the
power applied).

All bets are off unless your HF beam coaxes and rotator cable come all
the way down to the ground before they leave the vicinity of the tower. 
I assume that all tower guy wires are insulated from the tower and that
no other wires (dipoles, slopers, etc.) are hanging from the tower.

73, de Earl, K6SE