A Moxon antenna is also pretty much a dual-driven array. Instead of
both elements being current fed at the center, the driven element
couples to the second element capacitively at the element tips. You can
almost think of the second element being "end fed" by the driven
element, but the net effect is that both elements are driven rather than
the reflector being exclusively parasitically coupled.
And not surprisingly, the the Moxon has a wider bandwith and greater
front/back than a 2 element Yagi ... just as you describe.
Extending that principle to 3 or more elements gets rather problematic,
though, since I believe you have to distribute currents unevenly among
elements for optimum performance ... and although I haven't tried to
model it I suspect that optimum current phase gets hairy as well.
73,
Dave AB7E
On 11/16/2012 11:42 AM, Rick Karlquist wrote:
I have a MonstIR with 3 full size elements on 34 foot boom. Without
readjusting the element lengths, it has a usable bandwidth in terms
of not losing appreciable gain of 100 kHz at the most. So a shortened
element beam will have even less. The 2:1 SWR bandwidth is irrelevant;
what is the bandwidth over which you are within 1 dB of peak gain?
Probably less than the 2:1 SWR bandwidth. I know it is on my MonstIR.
Since the MonstIR elements can be individually adjusted, I tried QSY'ing
across the band by varying only the driven element. This is like using
a tuner to make the rig happy. The result was predictable; you lose
considerable gain by not varying the parasitic elements.
The simplest bandwidth improvement is to replace the linear loading
with coils. The inductance of a linear loading assembly increases
with frequency; exactly what you don't want. However, this won't
help much.
The real fix for this problem is to go to a driven array,
as opposed to a Yagi. I have modeled them and the bandwidth is much
better. For example, see the CAL-AV 2D-40 driven 2 element beam. This
happens to have full size elements and covers the whole band, but you
could make a driven beam with shortened elements and it would beat a
similar Yagi.
They cite US Patent 6,411,264, which is very readable. If you
read the patent you can see what they do to get the bandwidth.
The patent also discusses prior art in this area, in case you
want something in the public domain, like the ZL special.
BTW, 2 element driven arrays have much better F/B than Yagi's.
If I didn't have the MonstIR, I would not hesitate to buy the
CAL-AV 2D-40. I'm surprised it isn't more popular.
Rick N6RK
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