40 Moxon questions:
For you guys who have built a 40 Moxon - did you just follow the recipe,
like what's posted on K3LR's page, or actually model it also? Did you model
with AO, or NEC-2, or NEC-4, or several of them? How did your low swr point
and bandwidth compare between the model(s) and the sweep?
I wonder because mine was similar to Leeson's design (thanks, Dave, for
publishing it) but did not use a donor XM-240; slightly larger tubing
diameter, slightly longer wingspan; slightly larger aluminum channel... and
it appears that the actual antenna is resonant about 350 kHz higher than
the model. I had guessed/hoped for 200, but that didn't happen. I'd like to
understand how a 40 Moxon looks/compares using those 3 software engines as
well as a good SWR plot.
My current plan is to change my model to drop the resonant frequency
150-200 kHz while maintaining pattern and gain, until one of you help me
find a more accurate plan to improve my antenna. And then drop the antenna
and modify it (I have a cap arm that doesn't stay put well, so it needs
maintenance anyway).
thanks,
Dave N3AC
On Fri, Jan 7, 2022 at 1:47 PM Tom Georgens <tomgeorgens15@gmail.com> wrote:
> 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@contesting.com> On Behalf Of Leeson
> Sent: Friday, January 7, 2022 11:03 AM
> To: towertalk@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
> <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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