I have used NEC4 to model my short 50 ft tower for shunt feed on 160,
80, and 40 meters. It has a small triband beam with all elements
grounded. The 160m shunt wire goes to the top, of course, and NEC gave
a good estimate of the impedance. On 80m, my first guess was pretty
close to 50 Ohms. It took 3 tries on 40m where I was 3-4 ft off.
I could extend the model to your height and lengthen my 3 elements to
roughly simulate your beam & estimate the 160m shunt length. I assume
that you have enough radials.
On 10/10/2017 8:22 AM, bob burton via Topband wrote:
Looking for some shunt feed wisdom from the list. Trying to shunt feed 25G tower ( 8-10ft
sticks, 8ft flat top, base 18" out of the ground = total tower 89.5 ft) has a F12 C31XR
just above the thrust bearing (@90ft) with 6ft 2" mast above the tribander with a F12
D140 dipole (@96ft). Based on various archive reading I can't get a good sense of how much top
loading I may have (some say only the boom, others say much more). That said I've first tried
a cage wire ( four 14ga wires spaced 6" apart) all the way at the top of the tower spaced
about 2ft out. Trying to dip this with an Autek RF-1 yielded about 1.15 Mhz which didn't seem
right. Cage was taken down and tried just a single wire at a lower tap height of 60ft.
Strangely enough I get about the same frequency but the Z dropped. Moving farther down to 50ft
again about the same frequency but again Z dropped. I'm assuming the frequency readings are
completely off and I'm getting overload from nearby AM broadcast as I am surrounded by them.
I'm unsure now if I should continue trying tap points farther down the tower just looking for
the best Z or use some other approach. If anyone has experience with how much top loading a
C31XR could represent I could use the graphs in ON4UNs book to perhaps get close, but I'm
unsure what the loading looks like and found no real answers searching the archives. Any help
appreciated to help eliminate many more trips on the tower looking for the nearest 50 ohm
spot. Tnx 73's Bob - N4PQX
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