[TowerTalk] XM240 element spacing

[Tom Georgens]

I built two of these antennas, but they were on TH6 and 3L  Hy-gain booms,
so I don't have an answer to Dave's question.  However, I must say that this
is a phenomenal antenna for its size.  Not only a low SWR, but a good
pattern and gain across the entire band.  It is my favorite antenna.

Logistically, the T-bars make it tricky to install on a guyed tower, but can
be done by experienced people, or with a crane.  Also, other than intense
corrosion, the 8P environment does not see stress from ice or very high
winds, so I cannot comment on those use cases.  In my case these antennas
have been maintenance free.

Sadly, my 8P station had to be dismantled when the plantation owner sold the
farm where my station was located.  If I get to rebuild, this will certainly
be my choice on 40 meters.

Thanks Dave for this fine piece of engineering

73, Tom W2SC 8P5A

-----Original Message-----
From: TowerTalk <towertalk-bounces at contesting.com> On Behalf Of Leeson
Sent: Friday, January 7, 2022 11:03 AM
To: towertalk at contesting.com
Subject: [TowerTalk] XM240 element spacing

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/sli
des/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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