> No, but I wonder how they relate to F/R bandwidth, gain bandwidth and
> SWR bandwidth. A lot of work has been done to get all three of these
> in reasonable balance, and I don't recall any 3-element solutions that
> have actually yielded 50-dB F/R over anything like an entire band,
> much less in combination with acceptable SWR bandwidth and a fairly
> flat gain curve in the range expected for such antennas.
If you want truly wide-band performance...forget a Uda-yagi
antenna. Its primary advantage is simplicity, not bandwidth or
performance.
In a purely endfire array drive all the elements with cross-fire
phasing, and use transmission lines or simulated transmission
lines instead of lumped components.
If the physical shape or style of array is not critical, then use a
broadside antenna with a non-resonant reflector system (like a
screen or grid of wires).
You can get octaves of bandwidth using either method.
For Ham use, a simple yagi or array of yagis is plenty good. I live
in a rural area with very few buildings or uncontrolled metallic
objects around my antennas. Even in my very open environment,
ionospheric scattering and multi-path limits useful antenna null
depth to perhaps 20 or 30 dB. If you shut the door more than that,
it does no good. You still hear the backscattered signals from the
rear.
Unless the antenna occupies a large physical area in terms of the
wavelength, it is impossible to remove radiation from a substantial
area.
Adding elements without increasing occupied area, in an attempt
to force a wide null area with a physically small antenna, quickly
causes the element currents to become excessive. The elements
have destructive interference even in the desired directions, so the
array "fights itself".
This is called "superdirectivity" in antenna textbooks, and is readily
discussed with appropriate warnings about high Q, low efficiency,
and critical operation.
BTW, don't confuse an engineering discussion of superdirectivity
with the amateur radio term "superdirective" used in antenna ads.
The recent term is borrowed from some article in a Ham magazine
(I think it was Comm Quarterly) as sales fluff.
There is no free lunch...and there are very few cheap meals worth
having.
73, Tom W8JI
w8ji@contesting.com
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