On 6/21/2011 4:31 AM, Owen Duffy wrote:
On Tuesday, 21 June 2011 20:18:16 UTC+10, Helmut Wabnig wrote:

What I learned is, a metallic boom requires a small +length correction
of the dipole elements. YAGICAD does it automatically with a cryptic
value called L+BCF (boom correction factor?)

If you Google for "boom correction factor", you should find a wealth of discussion on the subject. It is not all cut and dried, it remains an area of active research.

Owen

basically, it's all about coming up with a simple algorithm to allow the
use of a simpler modeling code (i.e. model without boom, apply BCF, done).

It all depends on what is "good enough". On an HF beam, the elements
typically sag a fair amount, and wave around in the wind. Furthermore,
the boom is a tiny fraction of the element length so it has a "small"
effect (e.g. 10cm boom on a 10m long element) . So the uncertainties in
performance due to other mechanical factors are bigger than the effect
of a boom.

On a 432 or 1296 yagi using a 2cm square tube as the boom, though, this
is less the case. The elements are rigid, 2cm is a big fraction of a 35
cm long element. And, on VHF/UHF, one typically has a LOT more elements
and a LOT more gain, so the tolerances on everything are much tighter to
get the desired performance. If one were build a 5 wavelength long 20m
Yagi (5 lambda isn't unusual for 2m or 70cm), you'd be worrying about
those sorts of effects too, at least if you wanted to get the 17dBi gain
that might be possible.