Looking at the Mini MOXON design, I see the boom length is around the
standard .2 wavelengths for a driven element and reflector. I know for
regular Yagis, beyond .2 wavelength you don't get much more gain.
Adding more elements does add noticeable gain. Of course you cant
easily add elements to a Moxon Yagi unless you add considerable length
to the boom as extra elements can't be added between a 2 element
design. So why then does the 2 element Moxon Yagi designed by W6NL use
a 27 foot boom for two elements? Seems like a lot of extra big time
weight for next to nothing in added gain. I would think maybe the W7XL
mini design could be improved slightly by maybe keeping the boom at .2
wavelength, extending the elements and then reducing the helical
inductors to maintain resonance. Then also maybe using Delta Matching
at the 50 ohm points. Possibly even wrapping the fiberglass tubing
before the helical loading too to improve bandwidth. Just my thoughts
on all this. But, still don't understand a 27 foot boom for the W6NL
design.
Lee, w0vt
On 2/5/2017 10:55 AM, Lee wrote:
I might add, I suspect one could adjust the matching point to 50 ohms
bye using a delta connection running out on both sides from the center
of the driven element provided the center point would give you
something less then 50 ohms. Just seems there should be a better
MOXON program then MOXON Utility. I'm also thinking one could
increase bandwidth bye wrapping the fiberglass tubing with aluminum
foil. Fiberglass intrigues me as it is lighter then aluminum tubing.
(Wrapping it before the helical loading coils.)
Lee, w0vt
On 2/5/2017 10:33 AM, Lee wrote:
Some time ago I decided I wanted to build a full size 40 meter Moxon
Yagi beam. It never happened. Right now I've gotten Cold Feet to the
project. I'm beginning to think the beam would be just too heavy for
my situation. I'm now considering building a 40 meter Mini MOXON
beam as built by W7XA. This rendition uses fiberglass tubing instead
of aluminum tubing and is inductively loaded making it close to a 26
foot x 16 foot rectangle. Has anyone on here built one? I'd like to
hear from you if you indeed have one up and running. Since
fiberglass tubing is shipped in 8 foot lengths, the two elements
would be made up of 4 eight foot lengths. The design has an element
length of 26 feet. That means I'd have to discard 3 foot off two
pieces. I looked at the MOXGEN UTILITY Software and for 7.1 Mhz, the
program shows you all the other beam dimensions. They were as W7XA
described except for he shortened the two elements and made up for it
with loading coils. What I don't know is if I make the two elements
longer to make use of the full fiberglass tubing and then if I reduce
the size of the inductors to maintain resonance, will I upset the
feed point impedance? I'm thinking this will be the case. But, by
altering the length will I be able to find a 1:1 SWR at 7.1 Mhz ?
The bottom line to all this is, will I screw up the feed point
impedance by making the elements longer and have smaller loading
coils? Then too, I'm not sure W7XA actually saw 50 ohms at his feed
point with his design in the first place. Looking at the MOXGEN
software, it does not appear you can alter element spacing with it's
program. It just gives you all the dimensions without loading coils
for a specific frequency. (The only thing I see you can do with the
program is put in the desired frequency and you give the element
diameter. Also, what happens to the impedance if I use a longer boom
like 20 foot instead of 16 foot? I think the impedance would then
change too. The MOXGEN software doesn't allow you to pick different
length booms. Anyone on here brighter then me on this subject? Is
there a better software program for designing a Moxon Yagi beam then
MOXGEN UTILITY?
Lee, w0vt
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