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 Shrinking the Moxon?? 
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Post Shrinking the Moxon??
Hello All!

I am planning a Moxon for 20 metres at my postage-stamp-sized QTH and I'd like to know if there is any way to "shrink" the size? As it is, I will have to mount it vertically because of space problems.

Maybe by using traps - or would using RG58 or similar coax let me reduce the physical size without affecting performance too much?

I saw a design on t'Internet for a 17 meter Moxon which used RG58 instead of regular antenna wire and claimed a significant reduction in size. But then I read Steve's comments on this 'solution' on his website - and it seems it's all snake-oil.... :scratch:

Does anyone have any suggestions - or am I flogging a dead horse?

Thanks in advance.

73 de

Simon :thumbsup:
G3YXH/9M2XDX


Wed Sep 19, 2012 12:02 pm
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 Shrinking the Moxon?? 
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Post Re: Shrinking the Moxon??
I can't see why it can't be made smaller using induction coils, the workable bandwidth would be tighter but that's something you could probably live with. I haven't seen the comment from Steve you mention but are you sure he's not referring to a multiband antenna and not a monobander as you intend.

Edit; What you're suggesting is really a shrunken version of this. http://www.moxonantennaproject.com/IK0GDH/IK0GDH.htm

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Wed Sep 19, 2012 12:09 pm
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 Shrinking the Moxon?? 
Silent Key

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Post Re: Shrinking the Moxon??
The idea of shrinking it by building the elements out of coax is pure "snake oil"!

Shrinking it by using loading coils - not traps - is sound. But be aware that the effect of the loading on Gain and F/b is more pronounced than would be the case with a conventional Yagi.

I did something similar with the hexbeam:
http://www.karinya.net/g3txq/hexbeam/small_hexbeams/
and would expect similar sorts of results with a loaded Moxon.

73,
Steve G3TXQ


Wed Sep 19, 2012 12:31 pm
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 Shrinking the Moxon?? 
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Post Re: Shrinking the Moxon??
Thanks Steve! Thought so from reading your web-page on the subject.

I actually only want to tune the antenna for (say) 14.150 to 14.300 - so I'm happy to live with any reduced bandwidth.

I looked at a similar idea using coils here: http://ebookbrowse.com/fr5ec-moxon-hybr ... d232714345.

Seems like FR5EC was trying to use coils for a multi-band design but his project was inconclusive and he seems to have given up - but I am happy with a mono-bander.

I'll have a look at the link you gave and see if I can figure out how to use your coil approach to get a smaller 'footprint'.

(Easier for a numpty like me to make coils rather than traps as well!)

Any other thoughts gratefully recieved.

73 de
Simon
G3YXH/9M2XDX


Wed Sep 19, 2012 1:51 pm
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 Shrinking the Moxon?? 
Silent Key

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Post Re: Shrinking the Moxon??
What's your target size?

Steve G3TXQ


Wed Sep 19, 2012 1:56 pm
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 Shrinking the Moxon?? 
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Post Re: Shrinking the Moxon??
Hi Steve,

Well - I just looked at and thought about your notes on the smaller hexbeam in the link you gave and I got the impression the size could maybe be reduced by up to 50% - but I may be wrong and I would have no idea where the coils should go or what the performance degredation might be by such a drastic down-sizing.

If I could get the overall length of the 20 metre Moxon to 18/20 feet without too much of a trade-off in performance - that would be great.

Planning/Council interference is not a problem here - nor are the neighbours - so I'd like to get the maximum out of the available antenna real estate!

73
Simon
G3YXH/9M2XDX


Wed Sep 19, 2012 2:53 pm
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 Shrinking the Moxon?? 
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Post Re: Shrinking the Moxon??
In his book HF Antennas For All Locations Les Moxon shows a design where he has pulled the corners inside the traditional regtangle to reduce the physical size slightly.

When looked at in plan the antenna looks almost as if it has stubs in each corner pointing back to the central axis.

Cheers, Dave.


Wed Sep 19, 2012 8:15 pm
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 Shrinking the Moxon?? 
Silent Key

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Post Re: Shrinking the Moxon??
I tried taking a 20m Moxon model and scaling all wire dimensions by 75%; I set the gaps to their normal 20m values. With a loading coil either side of the feedpoint, half-way along the straight sections, and a single loading coil at the middle of the reflector, I could get a good 50 Ohm match and within 0.5dB of the normal Forward Gain; what suffered was the F/B, and I couldn't do much better than around 12dB.

But that was a very "quick look"; I'm sure a bit more effort would improve the results, but I'm not convinced you would ever get back to the normal excellent rearward pattern.

The "pulling in" of the corners is, of course, just another way of inductively loading the two elements.

73,
Steve G3TXQ


Wed Sep 19, 2012 8:38 pm
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 Shrinking the Moxon?? 
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Post Re: Shrinking the Moxon??
Rather than loading coils Steve couldn't shrinking also be done with say another wire parallel to the leading wire with shorting straps (capacitive loading), you may not get as much shrinkage as coils but would it be more efficient?

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Wed Sep 19, 2012 9:01 pm
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 Shrinking the Moxon?? 
Silent Key

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Post Re: Shrinking the Moxon??
ei9ju wrote:
Rather than loading coils Steve couldn't shrinking also be done with say another wire parallel to the leading wire with shorting straps (capacitive loading), you may not get as much shrinkage as coils but would it be more efficient?

It's an interesting thought, Gerry, not least because capacitive loading is usually less lossy that inductive loading.

However, I tried various capacitive loading approaches when I was trying to "shrink" the hexbeam, and it always seemed to upset the critical capacitive tip coupling that is a vital element of how the hexbeam - and the Moxon - work. I'm not saying it can't be done - just that it seems to involve a degree of complexity that the inductive loading doesn't. When you experiment with inductive loading you can pretty much tweak it to get the correct current flowing in the Reflector, and then independently adjust the capacitive tip coupling - by varying the gap - to give best F/B etc; with the capacitive loading the two things seemed much more inter-dependent.

73,
Steve G3TXQ


Wed Sep 19, 2012 9:30 pm
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 Shrinking the Moxon?? 
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Post Re: Shrinking the Moxon??
Thanks for the input Steve/Gerry etc. I'm thinking now it would be better to go with a hexbeam if I can figure a way to get it over the roof to the back.

I'll tell the neighbours I'm looking for UFOs!


Fri Sep 21, 2012 10:46 am
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