Posted by
VR2BrettGraham on
Sep 29, 2005; 11:23pm
URL: http://elecraft.85.s1.nabble.com/Two-Band-Moxon-wire-beam-tp382311p382323.html
W1CG replied to K5KVH:
>Thank you for the insight. I calculated the % the wire lengths were
>off by % the frequency was off, and I also "moved" the model up in
>frequency (by shortening wire lengths in the model) by the amount it
>differed from the measurement. In other words, the actual was about
>200 KHz below the model, so I moved the model up 200 KHz. The model
>was at the same height as the measurements. Both techniques produced
>results that were close. One thing I found was to not change the
>wire spacing at the ends, but just to change the length of wires
>between the main supports. I will wait until we hoist it to the
>final height which I estimate to be near 30' and take some more
>measurements then trim based on that. I think we can operate with
>the existing lengths as the top band edge is within the 2:1 SWR
>bandwidth at 21'; it may be a little off at 30' as the resonant
>frequency goes down as the height is increased, but we don't have to
>operate near the top end of the band. We have a LDG tuner, so it
>should be able to tune ok.
I don't recall if you mentioned if you were using bare or insulated
wire.
Between the several percent that contributes & the coupling
between the elements, I think WA1X's (I believe it was) suggestion
on towertalk or somewhere a while back on how he trims
Moxons is the way to go.
With bare wire, I have done quite a bit of mucking around trying
to tune a 10m Moxon with effect of "ground" (reinforced concrete
roof top) a half wavelength up. A decent match with reasonable
pattern I believe is probably much easier to achieve by tuning
each element separately as WA1X described.
The coupling in a Moxon rectangle is pretty intense - it is a good
idea to maintain geometry after trimming the ends, though some
sort of directivity will remain if you don't (up to a point). I have
done all sorts of models of Moxons & all multi-band approaches
other than back-to-back look to be hard to achieve in practice
(nesting, mix with yagi elements, sleeves, etc). Nested Moxon
reflector for the higher band & sleeve around low band driver looks
awfully interesting, but would require a _lot_ of work to trim into
submission in the real world.
GL.
73, VR2BrettGraham
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