Thanks for the insightful reply. Looks like there is no free lunch here- I
can't get better performance by simply using a different circle array
element. Bummer, I will have to stick with the verticals.
Not with a simple element.
If you have a significant null performance shortfall in some direction,
which could be caused by an array error or abnormal noise levels in a
certain direction, directive cells for elements can help provided they force
a null where you need the additional help. But this is a lot of work and
hardware if you want to maintain all directions, and you would have to be
careful to add nulls where you need them. As K9DX John said:
There would be little advantage with the loops because once the side/rear
response is 20 or so db down, the >>RDF is determined by the width of the
forward lobe.
John K9DX
Which I often say the same thing in a different way....
Antennas obtain directive gain by creating nulls. They do this by pattern
multiplication of the directive cells into an array. We can't add
directivity by forcing a null unless it is in a direction of significant
radiation. If there is nothing to remove from an area, attempting to remove
something more won't help.
On receiving we talk about directivity, on transmitting gain. They are
different because directivity does not include efficiency. This is why two
close-spaced broadside lossy antennas, like Beverages, can have 3 dB gain at
almost any spacing without any significant change in pattern or directivity.
The same applies to other arrays, so for receiving we should always look at
directivity change and not gain change.
If space is not an issue, will enlarging the circle diameter make any
difference? Note, however, that I am considering the 3-band commercial
solutions (160, 80, and 40m 8 circles).
The diameter limitation is where the circumference causes end-fire spacing
to be around 1/4 wavelength. There are some complicated tricks than can
obtain another ~1 dB directivity where more elements are used inside that
circle, but they make phase and level distribution much more complex and
critical. Because of that, the small change rarely materializes in the real
world and can easily go negative.
When the circle gets larger than that diameter limit, then the obvious next
step is more elements inside the circle. The problem is this drives
complexity up.
I experimented here with very large arrays years ago by watching signals and
watching phase, and I found skywave 160 signals commonly have phase and
level variations between arrays centered about 1-2 wavelengths apart. This
is at the root of the very reason we use spatial diversity, and why spatial
diversity works. It stands to reason if phase and level were stable, spatial
diversity would not work.
I could not combine very large arrays in fixed phase with the stable,
predictable, results I hoped to have. I decided the most reliable system was
to combine in stereo and use my brain to ignore the slow phase errors,
although it may not be beyond DSP technology now.
I used the Drake R4C, because it had access to external oscillator ports.
This easily allowed phase locking of both channels. The K3 is the only
receiver I'm aware of that comes close to doing this in an ideal fashion.
With a locked oscillator the signals could be summed in audio by shifting
phase to correct errors, and the results would be identical to shifting
phase at radio frequencies. But this requires locked oscillators and
identical channels, something most receiver designers miss or do not
understand.
The K3 is less than ideal, but much closer to ideal than anything else.
73 Tom
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Topband Reflector
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