Gain isn't caused by lower element impedance or more element
current. That effect is actually an artifact of the process
that creates gain. The element fights other elements to
radiate, and the radiation resistance decreases because
current (and voltage) has to increase to radiate the same
applied power. It's similar to why a small loop antenna
(where each part of the loop opposes radiation from the
other parts) has extermely high voltages and current, even
at low power.
Gain is acheived because of pattern multiplication
(interference) as multiple elements radiate energy. In
certain directions the radiation partially or completely
cancels, and that means the applied power has to go
elsewhere where it is "less cancelled".
A yagi, like any endfire antenna, forces nulls that remove
energy from certain directions. The wider the null area, the
more gain it has. If you don't force a null where there is
substantial energy, you don't increase gain because you
haven't moved anything. If you increase current in losses
too much by having too wide a null area, losses can easily
consume the power removed from radiation that would have
been in the area of the wider nulls.
When an element isn't free to radiate in all directions and
supplies power for all the other elements current has to
increase. (Voltage around the outer halves of elements
increases also, so we have to watch dielectric losses.)
There are a dozen ways to look at why this happens. ,You can
look at it as a Q (stored energy) problem, a mutual coupling
problem, or many other ways. Basically the element has to
"fight" radiation from other elements, so current and
voltage increases. It's more like restricting a nozzle or
squeezing a balloon than having more current cause a gain
increase.
All that aside, we have to be just as fussy when considering
a few dB. Even though a model can be tweaked for optimum
gain, as far as I know neither AO nor Eznec allows for
ribbon shaped conductors. A ribbon has most of the current
pushed out to the edges, very little flows in the center.
While inductance of a ribbon is lower, resistance is a bit
higher.
I'd also be willing to bet neither Eznec nor AO models
contained other losses, like dielectric losses and
connection losses in high current areas (where ribbon
contact is made). I sure can't model the element housing in
my versions of Eznec or AO, and I have no idea what the
connection (or matching) losses are.
ALL of those losses that are probably not accounted for
very well if at all in the model increase quickly as the
radiation is confined to a smaller hemisphere of space
around the antenna, because voltages and currents increase.
How do we even know the thing in the backyard is tuned
optimally?
None of this means anything about the quality or performance
of the product. I'm just pointing out how silly it is to
take a model that doesn't include exact characteristics and
argue whether the extra dB is real, when you are not likely
to be within 2dB anyway.
One thing for sure, a wide spaced tubing element antenna
with lower currents is less critical for small series loss
resistances, and doesn't have the dielectric loss concerns
because high electric field areas are surrounded only by
air.
If one antenna is 7 dB and the other is 8dB, we'd best find
another much better reason to sing praise. It could easily
be the 8dB is a really 6dB!!
73 Tom
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