A while ago, there was a request for the W6NL 40m Moxon EZNEC model. I
completed a model of the 120mph design which I had partially done. This
is the scratch build design, and I think the one that Leeson has at his
QTH. I've built two, one for myself and one for K6KR and both have been
in use for 4 years. I also have one in the shed built by K7ZSD.
Here are the modeling results using EZNEC Pro 4 V5.0 for the antenna at 90'
gain 10.7 dbi @ 7.15 MHz at 22 deg
F/B 9.62 db
swr 7.0 1.26
swr 7.15 1.35
swr 7.3 1.9
My 4 years ago measured swr at 90' with an AIM2170 with the coax
calibrated out are
swr 7.0 1.2
swr 7.15 1.35
swr 7.3 1.8
So I conclude that NEC4 can model this Moxon quite accurately. The NEC2
results show swr minimum well below the band but the pattern and gain
are similar at 7.15 MHz.
At 90' NEC4 3d Moxon plot shows a large vertical lobe. At 70' the
vertical lobe is suppressed (TOA 26 deg), so I might lower my tower 20'
and sacrifice about 0.7db of gain. The F/S looks ok. OTA comparisons
with my 40m 3L 48' boom at 140' and the Moxon at 90' shows about the
same gain (maybe it is 2db delta) but less noise on the 3L, probably due
to the vertical lobe suppression of the 3L. The Moxon weighs less than
100# and the 3L 350# and has guyed elements and probably 3x the wind load.
If any modelers want the EZNEC file, contact me off list. Also, error
fixes, optimization results, or better segmentation are of course
interesting as well. My model average gain (free space, no loss) is
1.081 but I haven't spent any effort to optimize segmentation.
Grant KZ1W
On 2/5/2017 8:55 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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