[TowerTalk] 40 Meter MOXON Yagi

[Lee]

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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