With a good modeling software and proper skill is nowadays possible to
realize even very complex (yagi) antennas (like those with open sleeve
tapered elements) and predict results, even SWR dips within KHz.
With cut and try method it's practically impossible to obtain
reasonable results or optimize when more than one variable is present,
not counting that SWR at TX side, typical effort, is often only a
transformated impedance (by cable lenght) of an antenna that's not
resonant, not a cure.
When a computer model is correct but practical results doesn't appear
coincident with predicted ones is almost sure that the antenna in
object is not working because suffering by interactions or detunings
caused by objects or other antennas, not because the model is incorrect.
A cut and try procedure that would minimize SWR doesn't solve the
problem at all, likewise wrong is modeling too close from ground, from
other antennas elements or objects.
Both will not make a poorly installed antenna efficient or properly
---------- Initial Header -----------
>From : firstname.lastname@example.org
To : <email@example.com>
Date : Wed, 26 Jun 2002 04:17:32 -0400
Subject : [Towertalk] 468/f
> >And for how long did 468/f rest resplendent in the books,
> >unquestioned, until Joe Common Ham got his mits on antenna modeling
> When I was a kid, the ARRL handbook offered 468/f as an approximation
> of the length of a dipole @ 1/2 wave height. It was stated rather cle
> that trimming would be required to accomodate site-variables.
> Last week, I cut an 80m dipole using 468/f as a starting point. Went u
> an inverted Vee. Took perhaps 10 minutes with a meter to trim off a f
> from each end, precisely as expected. (I always add a foot on 80 & 40
> just to give me room to trim. THAT they didn't put in the handbook!)
> Evidence suggests that 468/f is still a workable approximation.
> It would seem that Joe Common Ham may suffer from an over-reliance on
> model data, or a lack of common sense in applying it.
> Perhaps it's the expectation that you can predict unseen environmental
> variables with any precision whatever.
> Jim Jarvis, N2EA
> Essex, VT
> Towertalk mailing list