Topband: Modeling a shunt fed tower with EZNEC

[Bill Tippett]

W6IZT wrote:
 >I am having problems accurately modeling a shunt fed tower using EZNEC. I
suspect that most of the errors I am seeing result from modeling the tower
as a solid structure.

         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"

John W1FV told me this originally appeared in the IEEE Transactions on
Broadcasting, but I don't have the exact reference.  My current model
uses 11.724" for Rohn 45 and I am not sure why that does not agree with
the above (maybe I made a math error or entered the data incorrectly),
but it is probably "close enough" to prove the model works.

         Using this model, I can predict the 50 ohm tap point and matching
capacitance VERY accurately.  For example, I recently changed my shunt-fed
180' tower to a 3-element parasitic still using the tower as the driven
element.  My model predicted the 50 ohm tap would move from 66' to 86'.  I
put the tap there and voila...I had ~5 watts reflected with 1500 forward, so
it indeed works very well.  I do use two media for the ground, using Eznec's
"salt water" parameters for the initial radius of 150' to simulate my
radial field, and then using Eznec's "average" parameters beyond that.
Eznec gives me a guideline check ***"WARNING: Wire connected to real
non-MININEC ground.  See manual."*** but I simply ignore this because my
model seems to accurately reflect real-world impedance measurements.  No
doubt this will invoke comments by the theorists, but I can tell you it
simply works well for me...especially when I reset my tap on the first try!

         If you have other antennas on your tower, they must also be modeled
accurately for Eznec to work.  Mine has nothing else on the tower so I
don't have that problem, and the cylindrical model above does seem to
work well for me.

                                                 73,  Bill  W4ZV