Funny timing.
I've been playing with a M2 40M 2L conversion. last year I had expanded it to a
full size 2L with bent tips vertically, but just
changed that last weekend to a Moxon. Still testing things. SWR/bandwidth looks
great. I originally wanted tubing on the sides,
but had too much droop and switched to wire. Then, playing with 4nec2x, I
decided that droop was not a bad thing. I just modelled
the droop as an optimizing variable, along with the wire lengths. The boom is
about 19-1/2', I added about 40" tubing to each
element half over a stock 40M2L. Wire droop on the sides can be 2'-4'..tip
spacing about 16". Taller center truss support. two
truss lines out to the longer elements on each side.
With droop, there is less horizontal pull on the elements, so less deflection.
I used a wire sag calculator to figure out what
amount of droop gave a reasonably minimal side pull. (for the weight wire). The
current is low at the ends, which so using wire
isn't bad.
It's an interesting 3D question. W6NL has shown you can play with
loading/coupling. By allowing droop, you can have different
interactions of gap, wire length, boom length. No one legislated that
everything has to be inline.
I had already bent 90 degree tubing angles for the ends, so my wire is attached
to the aluminum tubing elements without bending
it. (it attaches to the arm of the bent tubing with hose clamps). I twisted the
90 angle so it points inline with the drooping
wire. So it's a straight line for the wire. Bending 6061 tubing takes some
attention/technique but I got good results.
I was going to use some 12 gauge aluminum wire, but right now it's just 12 awg
thnn copper, with the ends tinned where they clamp
to the aluminum.
I found with modelling things, that there is a lot of flexibility in terms of
what dimensions give the desired SWR/bandwidth/gain.
In terms of model vs real life, I found that my model seems to be higher in
freq than real life, so I optimized for 6.9Mhz and
things seemed about right. The swr curve not symmetric either, so it pays to
optimize low. I had noticed when trying to sim the
W6NL moxon conversion, that it seemed to center lower for me too, so I just
assumed something must be consistently off.
I don't have much wind and not much time in the air with this yet, but that's
what I've got.
When I had the 19-1/2' boom as a yagi with bent tip elements, I used a copper
tubing coil as a hairpin. Found I couldn't get
enough inductance out of a normal hairpin design to get a match.
-kevin
ad6z
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