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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