Bill, W4ZV wrote:
"Gregg, I'll post this since it may be of interest to others. I've had
good success with modeling my Rohn 45 shunt-fed triangular tower using a
cylindrical model. My model is based on the following:
Equivalent Cylindrical Diameter = 2 * CUBEROOT [(D * F^2)/2]
where D = tube diameter and F = Face width. For Rohn 45, D =1.25" and F
= 18", so the cylindrical diameter equivalent = 11.7446".
----------
When I started modeling shunt fed towers, I also tried using the same
equation for an equivalent cylindrical conductor to represent the tower
(I also forget the name of the engineer who developed the equation back
in the 30s). I found that it is only accurate for towers without any top
loading (i.e., Yagis, etc.). One can experiment by trial and error, as
Bill did, by changing the cylindrical tower diameter and come up with
something that matched his particular situation, but it will not be
accurate for other towers with other top loading.
The only way to accurately model a shunt fed tower is to model the tower
structure itself, i.e., the legs in a triangular configuration with rungs
every four or five feet of height. There are several other rules that
apply to modeling a shunt fed tower that I will forward to Gregg, W6IZT,
who posed the question. Others that want to know these rules, please
e-mail me direct for a copy.
If you follow these rules, you can very accurately design a shunt feed
system. Of the many requests I've had, one I remember best is Bob's,
K3UL. He asked me what the parameters might be for his tower (shunt wire
spacing, tie point and value of gamma capacitor), so I modeled his
structure and e-mailed the details to him. Bob installed the shunt feed
on his tower and e-mailed back to me that it was right on the money.
What was so gratifying about this is that Bob is a blind amateur and did
the work himself. He now has one of the biggest signals on Topband from
the east coast here on the west coast.
Another memorable modeling session I had was for Bill, W4ZV. He wanted
to "test" my modeling and gave me all of the details for his Rohn 45
tower (height, top loading and ground resistance) and asked me to model
it so that he could compare my modeling with his already-installed shunt
feed system. When I sent him the results, he compared them to the real
world, and e-mailed back the he was amazed at the accuracy of my
modeling. He even measured the value of his gamma capacitor and said it
agreed quite closely with my model.
73, de Earl, K6SE
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