"Richard" wrote in message
...
Are these points true?

The following is the simplest way of obtaining a practical yagi design
using
an antenna modeling program:

1 If you use a non conducting boom then you can build a practical model
using the element lengths in the model.

2 For a VHF yagi in the clear you can assume freespace.

3 A simple straight wire in your model that is made source is DE and
is treated as a simple hertz dipole by the program. So the values given
for
R and J are the values you would measure having a hertz dipole as DE.
This is true for the model and would be fairly accurate if the boom was
non conducting and reasonably in the clear in the practical design.

4 It is easy to scale for frequency in antenna programs.

5 It is fairly easy to alter element diameters in antenna programs and
to
obtain a redesign that maintains the original performance
characteristics.*

It seems to me that if one is prepared to use a non conducting boom, a
hertz dipole as DE, then the model is pretty much the practical design.

Not sure about impact of a balun or the feedline.

http://www.cebik.com/scales.html

"Although I have omitted details--since they may vary widely--the general
principles separating the three mounting systems should be clear. The first
version uses a non-conductive plate or other means of insulating and
separating the element from the boom. If the separation is sufficient, the
boom has virtually no effect upon the element. In such cases, one may use
modeled dimensions directly, since the model (whether NEC or MININEC)
presumes that the elements are isolated from unmodeled conductors. An
alternative to the insulating plate is, of course, to use a non-conductive
boom. This technique permits through-boom mounting for the element, but with
no change of the effective length of the element."

(Scaling and Adjusting VHF/UHF Yagis L. B. Cebik, W4RNL)
--------------------------------

So, I could use a metalic boom but make sure the elements are insulated,
then the model is pretty much the practical design.

This means then that designing a practical antenna would be greatly
simplified either by using a non conducting boom or insulating the elements.

I'm still unsure of point 3. But I assume that for a single wire that is
made source in an antenna program, DE is a simple hertz dipole, the
dimensions and the electrical parameters would be reproduced in a
practical design cut to the dimensions as per the model - if the
elements are insulated or the boom is non conducting.