Topband: Modeling a shunt fed tower with EZNEC

[Ford Peterson]

Gregg 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. Can anyone provide me with some guidance? Better yet,
> If you have successfully modeled a shunt fed tower can you please forward me
> a copy of the EZNEC file that I can use as a reference?
> 
> Thanks
> Gregg
> W6IZT at aol.com

There are a number of problems with modeling a shunt feed.  If the goal is to verify pattern, then it is fairly easy.  If you are looking at designing the matching circuit, well, good luck.  The variables are many and any of them will alter the feed point quite a bit.  For example...

A grounded tower is connected to the model's "perfect ground."  There is no such thing as perfect ground in real life.

Connecting wires of dissimilar sizes will produce unreliable results.  E.g. connecting an 1/8" wire to a 1' diameter tower is a modeling problem.

Another common modeling mistake made is that the individual wires used in the model have to be oriented such that the segments all line up when the wires are in parallel.  E.g. The vertical tower portion of the model needs to be broken up into several wires so that the segments appear on the tower at identical heights as the segments appearing on the shunt wire.  I solve this problem by using software other than EZnec.  I use Nec2Go   www.nec2go.com  In Nec2Go, the author has taken the time to help you manage this issue.

Ford-N0FP
ford at cmgate.com