[TowerTalk] Antenna Gain and Reality
Roger
roger at rogerhalstead.com
Thu Dec 11 19:48:30 EST 2014
There are several absolutes in antenna gain and many probably, might,
maybe, could be attributes.
The two absolutes are the 2.1 db dipole to Isotropic source (I've been a
ham since 61 and the books have always listed that figure) and gain
measurements are only true for the test range where they were made.
Hence even though the manufacturer may be honest and make an honest
attempt at measuring gain figures in all axises, comparing one
manufacturers antennas against another manufacturers antennas from
advertized data only gives ball park figures and tells me little about
how they will perform in my installation.
Doubling the number of antennas theoretically doubles the gain which is
an additional 3 db for each doubling although it's unlikely due to
losses in feeding and matching that this will be achieved. Losses in the
coax m
Even with computer modeling of the installation, the inputs must be
accurate and few if any know the ground characteristics near or far to
any great precision.
The radiation pattern at vertical angles is a crap shoot. You are
playing the percentages for atmospheric conditions that will match the
maximum signal radiated will be at the most desirable angle to match
those conditions. That's why the contesters have stacks that allow them
to select the antenna height that lets them put the maximum signal into
the desired range/distance.
As most of us have both budget and land constraints that limit us to the
number, size, and height of our antenna(s) It's unlikely we can depend
on equaling the performance in the advertized figures. Often for 40,
75, and 160, a simple wire antenna, or vertical will out perform all but
the largest and/or sophisticated antenna or array.
I've had extremely good luck with sloping, center fed, half wave
dipoles. compared to some stations running Yagi antennas at reasonable
heights. Many times the first word in the response to my call, is
"Wow". Yes, I do run QRO, but the best sounding signal out of the
tetrode amp is at, or around the legal limit so there is little
incentive for pushing beyond the legal limit and those tubes are expensive.
--
73
Roger (K8RI)
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