Herb, KV4FZ wrote:
"Mauri, Some years ago I feed a 100 foot tower (with 20 meter 4 element
monobander) by running the feed wire inside the tower. (From the rotor
plate to ground as I was in to much of a hurry to mount offset
insulators.)
"This was fed like any shunt with a series capacitor and the wire taped
for best
> match. To my amazement this worked well. Have you ever looked at this
combination or can you analyze its performance?"
=====
I have an EZNEC model similar to this using 13 sections of Rohn 25 (126'
6"), which is very close to quarter-wave resonance at 1830 kHz.
With a #8 AWG shunt wire running up the center of the tower, the tap
point for 50 ohms R is at 39 feet above ground. The inductive ractance
is quite low (30 ohms), so it takes quite a large capacitance to cancel
it (2900 pF).
With the same shunt wire running up outside of the tower and spaced 24"
from it, the 50-ohm point would be 22 feet above ground. A 600 pF
capacitor would cancel the 145 ohms of inductive reactance.
Over "good" ground, EZNEC says the gain of the vertical with the inside
feed method is 0.12 dBi. With the conventional shunt feed, the gain is
1.50 dBi. The difference in gain is apparently caused by the current in
the central shunt wire being enclosed by currents in the tower of an
opposing phase.
BTW, anyone attempting to model a shunt-fed tower, you MUST model the
tower structure (i.e., "legs" and "rungs"). Using an equivalent
cylindrical model of a 3-sided tower is ok for straight verticals, but
NOT for shunt-fed towers, especially when various types of top loading
are involved.
73, de Earl, K6SE
--
FAQ on WWW: http://www.contesting.com/topband.html
Submissions: topband@contesting.com
Administrative requests: topband-REQUEST@contesting.com
Problems: owner-topband@contesting.com
|