You are only sort of correct.

At HF where the local world is as large as your antenna, that statement
is true, at 6m it starts to become less of an issue as the outside world
becomes big and somewhat ignorable. At 2m it is quite ignorable, and at
70cm it pretty much isn't there. Yagis I have designed for 432 perform
exactly as predicted by the software, with the exception of the ones too
long for the antenna range, where the only discrepency was the gain
tests a bit lower than the model predicts. And then it was .3 dB on an
18.4 dBd prediction.

It is possible to predict antenna gain and pattern very accurately if
you do it at a frequency that does not interact with the local
environment to a any large degree. It is also possible to build those
antennas and have them perform the same time and time again. I've built
dozens of long boom yagis for 2, 222, and 432, and after the first one,
you don't even have to measure the SWR before setting the T match shorts
unless you are anal or an EME're. Oops, same thing, I guess. And
every antenna I've ever range tested (thank you Central States VHF) has
been within .1 dB of predicted except one.

The designs I have built over the last 15 years didn't need to be
adjusted at all from the model plus predicted boom corrections. The
only variable was the matching system on the driven element, which none
of the modeling programs handles very well.

And for you picky folks, yes, the losses from foliage, etc. aren't
ignorable, but the antenna system still performs as predicted unless you
mount it in the center of a tree.

And I do realize I'm preaching to the non-choir here, since this group
is (apparently) not very interested in anything above 10m.

tom
K0TAR

Reg Edwards wrote:




====================================

Danny, there's nothing wrong with antenna pattern-prediction progams. There
are good nunber-crunched side effects. It's only their practical application
by people in their own back yards which is unreliable. It cannot be helped.
It's a fact of life. The environment and performance of an antenna cannot
be accurately predicted unless it is at a height of several wavelengths
above and away from obstructions.

Not a single one of my programs predicts a radiation pattern. Only an
exceedingly few professional antenna design engineers would ever make real
use of such facilities even if I could write them. I don't like wasting
time. I havn't all that amount of time anyway. Nobody has produced a new
type of antenna for many decades. (Not even Fractal). It's all been done
before.

But, as I say, radiation prediction programs are entertaining, educational
and satisfying. Enjoy them while you can.

Nevertheless, unreliable is a fair practical description.
----
Reg, G4FGQ