On 12/13/13 6:48 AM, Ian White wrote:
Jim Lux wrote:
On 12/13/13 4:07 AM, john@kk9a.com wrote:
There is only one driven element on an OWA antenna. I think the
biggest
advantage is the direct 50 ohm feed. No match is needed making it
easier
to build and there are no match losses. It is easy to design a yagi
with a
wide bandwidth with >50 impedance however that is not referred to as
OWA.
and, the trade, as always, is that the gain is slightly less, and the
F/B & F/R (aka sidelobe levels) will be worse, particularly at band
edges
That tradeoff isn't universally true. It may well be true for HF Yagis
because of the relatively small number of design variables (element
lengths and spacings) that are available to be optimized. But VHF/UHF
long Yagis offer many more variables, making it possible to have all the
desirable features at the same time with a truly negligible reduction in
forward gain.
yes, true..
ALthough you do get to a "manufacturing tolerances" issue.
it's not just the number of elements: There's also relatively few folks
making 20 meter Yagis 5 wavelengths long, while there's quite a few
144MHz 5 lambda boom designs out there, and even more at 440.
As computer optimization has grown towards a mature science, we have
come to realise that the cut-and-try methods of the old days produced an
unhealthy obsession with maximizing the forward gain. This was
forgivable because forward gain was the only parameter that could be
quickly measured at each step along the way, but it often produced
antennas that were 'ill-behaved' in almost every other way, and failed
to deliver the promised gain in real-life situations.
That's an interesting insight as to the why. I'd extend it to an
obessession with VSWR and forward gain, but for the exact same reason:
both are easy to measure.
And going for max gain leads to superdirective arrays which are very
sensitive to the relative phases and amplitudes of the elements, have a
lot of stored energy in the antenna, and are very tolerance sensitive.
The designs for VHF and UHF tend not to be pushing the superdirective
thing as much, either: plotting gain vs boom length for all those VHF
and UHF designs shows them all laying pretty much on the same gain
proportional to length line without any real big outliers.
With enough computing power to carry out multi-factor optimizations -
and a large enough number of design variables available within the
antenna itself - the most profitable way forward is usually to optimize
the radiation pattern and feedpoint impedance across a wide range of
frequencies, because those are the factors that lead towards a
'well-behaved' antenna. Squeezing the unwanted minor lobes will
automatically redirect the energy into the main lobe, so the forward
gain pretty much takes care of itself.
The lessons to take home:
1. In real life, an 'optimum' design requires a much richer set of
features than the maximum possible forward gain.
2. Be careful what you ask for, because a computer will deliver it.
And the difficulties with measuring the parameters other than forward
gain and match mean that there will be plenty of opportunity for
"discussion" about antenna design, well into the future.
(although.. the wide availability of small UAVs that are easy to control
means that doing full patterns of HF antennas by amateurs isn't far off. )
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