RE: NEC-LIST: Software to model effect of boom on elements for UH F yagis?

According to Gunther Hoch in his article "Extremely Long Yagi Antennas"
published in VHF Communications, about 66% of the metallic boom diameter
must be added to the grounded overall element lengths in yagis.

The boom has a profound influence on the performance of a yagi, as does the
element thickness. Don't neglect the specific resistance of the material
either.
If the first two parameters particularly are ignored, one could end up at
half-gain with poor polar performance.

This can be modelled with NEC2 - construct the boom, then attach the
elements to the boom. NEC2 assumes that when the coordinates coincide/nearly
coincide, they are in physical contact. The documentation mentions this.

In commercial practice elements are floated on insulators for one reason:-
corrosion on the joint in grounded elements will destroy the performance of
a yagi. In RF terms these floating elements are raised above ground
potential and are subject to static build up, generating noise.

Ian Roberts.

-----Original Message-----
From: Ed Troy [mailto:etroy_at_aeroconsult.com]
Sent: 21 March 2002 08:56
To: nec-list_at_ece.ubc.ca
Subject: NEC-LIST: Software to model effect of boom on elements for UHF
yagis?

Does anyone know of any software that will accurately model the effect of
the boom on the length of the elements of a UHF yagi? Obviously, something
free or inexpensive is preferred, but I am wondering if anything models
this effect.

Of course, for most yagi antennas, the effect is almost insignificant,
since the boom diameter is such a small fraction of the element lengths,
even if they pass through, and are physically connected to, the elements.
But, if you are designing a long yagi array at 2.5 GHz, using, say, a 1/4
inch diameter boom, and 1/8 inch diameter elements, the boom diameter is
more than 10% of the total length of the elements, and thus a NEC
simulation is grossly inaccurate.

I have found rough guidelines and some programs that supposedly allow you
to correct element lengths for the effect of the boom diameter, or that
even design a yagi given the boom diameter and element diameters. But, I
would like something that would allow me to accurately predict the
performance before going through the time and expense of actually
fabricating the antenna.

TIA,
Ed

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