Jon wrote:
"I'm planning on erecting 128 feet of Rohn 25 as a vertical radiator,
with ropes supporting vertical parasitic elements with sloping top-hats
as described in ON4UN's book. (3 elements with Reflector, Driven and
Director).
"What difference will it make whether I series-feed the tower or
shunt-feed it, if any?
===============
I modeled 3 configurations of the 128' vertical without any parasitic
elements in the field:
1) Series fed
2) Shunt fed (gamma match)
3) Folded unipole with 3-wire cage of equal size as tower, spaced 2
feet from the tower.
All three models had virtually the same gain and TOA.
The series-fed 128' tower is very close to resonance at 1830 kHz, with a
feedpoint resistance of about 35 ohms.
Adding the cage to make it into a folded unipole shifted the resonant
frequency down to 1800 kHz, with a feedpoint resistance of about 140
ohms.
The shunt-fed 128' tower, at 1830 kHz, required about 600pF of gamma
capacity. Voltage across the capacitor was less than 1000 at 1kw. Shunt
wire was .25" diameter, spaced 24" from the tower, tied to the tower at
32' high.
All three configurations had a 2:1 SWR bandwidth of better than +/- 100
kHz.
73, de Earl, K6SE
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