Topband: Shunt feed - how high is high enough?

[k6se@juno.com]

Pete, N4ZR wrote:

"From answers to a query on Towertalk, as well as some modeling with
NEC2, it appears that I can obtain 50+j250 or so at the bottom of a shunt
feed wire spaced 30' from my tower with the tap to the tower at about 30 
feet, or a considerably higher resistive component (and lower inductive)
if I tap at 60 feet (right below the first tribander in a 2-high stack).

"Convenience of a smaller value series capacitor aside, is there any
reason from a loss or field strength standpoint to prefer the higher tap?
 If so, I had in mind using an UNUN after the cap to convert the
feedpoint down close to 50 ohms.

"The total tower is 97 feet, plus 8 feet of mast.  The tribanders are at
69 and 97 feet, with a Force 12 shorty forty at 105 feet.  I will
initially just be using the tower's lightning ground, laying in radials
on or just under the ground as time and weather permit."
==========
With all that aluminum up at that height, it seemed obvious to me that
the 1/4-wave frequency of the structure is below the 160-meter band, so I
started modeling using EZNEC 3.0 (NEC-2 engine) to try to duplicate the
structure.

I found that the 50-ohm tap point on 15 sections of Rohn 25 (146' 11" -
no top loading) yielded a feedpoint Z of 50 + j250 ohms, exactly what
Pete came up with.  With a shunt wire of #8 AWG, the 50-ohm tap height
was 46.6 feet with 24" spacing.

BTW, this tower was 1/4-wave resonant at 1603 kHz with a series feedpoint
Z of 35.533 + j0 ohms at its resonant frequency.

I then moved the shunt wire up the tower to the 200-ohm tap point.  The
feedpoint Z there was 200 +j410.4 -- a higher inductive reactance, not
lower as Pete said.  The 200-ohm tap point was 69 feet high with a
spacing of 27".

Using the 50-ohm tap requires 348 pF to tune out the reactance.  With 1
kw into the antenna, the RF voltage across the capacitor  is 1121 volts. 
When tuned on the nose at 1830 kHz, the 2:1 SWR bandwidth is from 1755 to
1895 kHz.

Using the 200-ohm tap requires 212 pF to tune out the reactance.  With 1
kw into the antenna, the RF voltage across the capacitor is 918 volts. 
When tuned on the nose at 1830 kHz, the 2:1 SWR bandwidth is 1740 to 1900
kHz.

The radiated field strength is identical with either feed.

It makes sense to me to opt for the closer-to-the-ground 50-ohm tap
point.  That means less climbing and no need for the 4:1 unun at the
feedpoint to match 50 ohms.

When modeling a shunt-fed top-loaded tower, if one uses an equivalent
diameter "wire" to represent the tower gross inaccuracies will be the
result.  I've found that the only accurate method of modeling a shunt fed
tower is to model the actual tower structure, i.e., the three legs with
"rungs" at every four to five feet of height.  All "wire" diameters in
the model must be the same, including, mast, beam elements, etc.  The
correct "wire" diameter to use is the actual diameter of the shunt feed
wire.  Modeling a shunt feed system this way results in very good
accuracy.

73, de Earl, K6SE



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