The other day I received an email requesting info on my Moxon 40m mod
for the Cushcraft XM240, as used at K3LR, 8P5A, K1KP, WE9V and others. A
specific question was, "What is the exact element spacing you get if you
use the boom holes and clamps on the XM240?" This dimension is needed to
duplicate the spacing on a home-built boom.
Easy, I thought, I'll just look through the many versions of the design
I've written and published. But no, I don't seem ever to have had that
exact number!
No problem, I'll just look to see what I and others have used in antenna
modeling files. Well the answer varied wherever I looked. So I replied,
"A model I've used for XM240 boom length is 264 - 3.5 = 260.5", but I
find that I used 254" in my Moxon computer models on K6STI AO, and K1KP
and others have used 258". I fired up my old DOS PC and checked to see
how much difference this makes. The answer is, it doesn't much. So all
the antennas that have been built with the XM240 boom holes where they
are have worked out fine."
I went one step further: "I downloaded a manual xm240.pdf at
https://store2.rlham.com/shop/catalog/man2/cushcraft/xm240.pdf But
there's no information about the boom tip dimensions to the mounting
hole, although the boom segment dimensions are given (BA is 59 15/16 and
BB is 71 15/16), so the overall boom length is 263.75".
But both the U-channel and the bracket are 4" across, so we only need
the dimension from the boom tip to the edge of the bracket. I haven't
found any Eznec or other model files other than K1KP's online, but I did
find this photo:
http://www.cn2r.net/Photo/Station/Manufacturing/Yagi%20construction%2040/slides/2%20El%2040M%20boom%20to%20element.html
I blew it up and printed it so I could estimate the dimensions with a
caliper, and it appears the tip sticks out exactly 2" beyond the clamp
edge. So the dimension from the tip to the mid-clamp hole is 2 + 2 = 4",
and the element spacing from this is 263.75 - 8 = 255.75"
So with this long-winded "shaggy dog story" finished, my question is,
does anyone have ax XM240 boom section handy so you can confirm this 4"
offset of the element clamp bolt hole at either tip?
BTW, I've exchanged some 800 emails with folks needing info for building
these antennas, and I'm tickled to see the commercial versions as well.
This all started with visits with Les Moxon here, and then in the UK in
around 1990, plus in 2003 applying the wonderful capabilities of K6STI's AO.
73 de Dave, W6NL/HC8L
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