[TowerTalk] Log Periodic Choice
jimlux at earthlink.net
Sun Sep 14 14:11:26 EDT 2008
Ralph Matheny wrote:
> NO disrespect intended, but I frankly don't put a lot of stock in one set
> of antenna "measurement" over another set. Indeed one might be able to
> make comparisions between several antennas and find a dud that is 10 db
> below the rest, but to resolve 2-3 db over a dipole with accuracy just
> won't happen to my satisfaction. There are too many things that can
> change and and go wrong in the worst way.
I will grant that getting 2-3 dB accuracy in an afternoon of testing in
someone's back yard using the s-meter on a 30 year old rig is dreaming.
However, with some moderately careful technique, and simple equipment
(calibrated step attenuator), it's very possible to get 1dB sorts of
precision. There's a lot of work involved, though, in hoisting antennas
into place, etc.
If the antennas a reasonably simple, modeling can easily give you
relative performances to better than a dB, but, of course, can't readily
account for the detailed effects of a specific ground/soil environment.
On the other hand, the impact of the ground will be quite similar for
antennas that are generally similar (i.e. comparing a horizontal dipole
against a horizontal yagi).
The other thing performance related that's quite valuable, but for which
fairly low precision measurements are useful, is understanding the
rear and sidelobe behavior. A fairly crude measurement can distinguish
between 3 dB F/R and 10dB or 20dB F/R
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