[TowerTalk] OWA Inventor?

[Ian White]

Jim Lux wrote:

>
>On 12/13/13 4:07 AM, john at 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.

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. 

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.


73 from Ian GM3SEK