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 Capacity Hat loaded Yagi - needs modeling 
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Post Capacity Hat loaded Yagi - needs modeling
Hi Steve,
I've had some issues with the size of my 2 element yagi for 17m... it's too big for the xyl.

So I'm looking to reduce it to 0.7 x times the original size and add capacity hats to the ends.

That should leave it looking like a yagi for 10m which is acceptable.

I've found a web site that tested capacity hats for 10m and they are about 22" in diameter.
http://w4rnl.net46.net/fdim1.html W4rnl has done some extensive work to prove the capacity hat yagi is nearly as good as a full sized yagi.

Would you be able to estimate the size of capacity hat (square with perimeter wire) 16 foot dipole to resonate on 18.1 Mhz ?
I think it would be about 32" in diameter, but thats just me using a very rough approximation based on the 10m design.

I've also been looking at how I might add 15m by adding an inductive load and
Thanks,
Ray
EI3GD


Fri Jan 04, 2013 9:04 pm
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 Capacity Hat loaded Yagi - needs modeling 
Silent Key

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Post Re: Capacity Hat loaded Yagi - needs modeling
Hi Ray,

With everything modelled as #14 wire, four "radial" wires of 13.75 inches, with their ends connected by four 19.4 inch "perimeter" wires, produced resonance at 18.1 MHz. Things will change with different dipole diameters and "hat" wire diameters, but that should put you in the right ball-park.

73,
Steve G3TXQ


Fri Jan 04, 2013 9:52 pm
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 Capacity Hat loaded Yagi - needs modeling 
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Post Re: Capacity Hat loaded Yagi - needs modeling
Cheers Steve, that's even better (smaller) than I expected.

73's
Ray
Ei3gd


Fri Jan 04, 2013 10:36 pm
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 Capacity Hat loaded Yagi - needs modeling 
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Post Re: Capacity Hat loaded Yagi - needs modeling
Tested it as a dipole to being with...
It turns out to be very close at 18.5 MHz when implementing the ~14 radials"
But it was hard to match the Gamma below 2:1 for some reason.

Also considering the TGM Mini quad concept of adding some L in series with the C to reduce the need for such big radials on the Hat, sacraficing some bandwidth perhaps.

I'm going to replace the Gamma Match with a 1:1 wide broad-band balun which will open up the possibility of adding 15M to the same dipole by adding another L and C Hat section (borrowing from the TGM design).

R.


Sat Jan 05, 2013 5:27 pm
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 Capacity Hat loaded Yagi - needs modeling 
Silent Key

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Post Re: Capacity Hat loaded Yagi - needs modeling
As a dipole the impedance modelled very close to 50 Ohms; that may be why you struggled to get the Gamma Match to work - it didn't need one :)

Steve G3TXQ


Sat Jan 05, 2013 5:51 pm
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 Capacity Hat loaded Yagi - needs modeling 
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Post Re: Capacity Hat loaded Yagi - needs modeling
Ah that makes sense.
I'll go with the 1:1 balun and try to make a dual band 2 element .

Cheers
R


Sat Jan 05, 2013 10:24 pm
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 Capacity Hat loaded Yagi - needs modeling 
Silent Key

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Post Re: Capacity Hat loaded Yagi - needs modeling
Ray,

Not quite sure I understand that.

As a loaded dipole the impedance will be close to 50 ohms - so no Gamma Match needed. But once you turn it into a 2-el beam the feedpoint impedance will drop and you may then well need the Gamma Match.

Perhaps I misunderstood what you intend to do?

Steve G3TXQ


Sat Jan 05, 2013 10:30 pm
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 Capacity Hat loaded Yagi - needs modeling 
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Post Re: Capacity Hat loaded Yagi - needs modeling
Hi Steve,
Sorry, not making my self clear.

I'm goint to attempt a dual band yagi by creating two loads and two Hat's on each end.
Similar to the TGM Yagi (or hybrid Quad as they call it).

But to achieve this I think I'm going to need a broad band unblanced to balanced balun, right?

Tried this today on a dipole and got it resonant at 16 Mhz ( cap hat and Inductive load will need some adjustment to reacg 18.1 Mhz)
VSWR was 1.5:1 and Z was 72 Ohms (measured on MiniVNA).

So if I add another element at say 5ft spacing, I should see the Z fall ... right?

Thanks
Ray


Sun Jan 06, 2013 5:19 pm
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 Capacity Hat loaded Yagi - needs modeling 
Silent Key

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Post Re: Capacity Hat loaded Yagi - needs modeling
Yes, the impedance should drop when you add a second element. How much by, will depend on the spacing. It could drop sufficiently that you need something other than a simple 1:1 balun to get a decent SWR.

73,
Steve G3TXQ


Sun Jan 06, 2013 6:37 pm
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 Capacity Hat loaded Yagi - needs modeling 
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Post Re: Capacity Hat loaded Yagi - needs modeling
When you were talking initially about reducing the size of the antenna I was going to shout try a Moxon/VK2ABQ or Hex beam to give you the reduced physical width however if you're going for more than one band you can still try any of those antenna types or scour the want ads for a Cushcraft MA5B.

I'm suggesting those alternatives in case you're going to butcher an antenna that could be sold on if unmolested.

Cheers, Dave.


Mon Jan 07, 2013 9:13 am
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 Capacity Hat loaded Yagi - needs modeling 
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Post Re: Capacity Hat loaded Yagi - needs modeling
Thanks Dave,
The 2 element yagi is a home made from some other beam parts, so nothing lost there.

Hexbeam is too large for my neighbourhood, I need stealth and low profile!
The more like a TV antenna the better.
Moxon is a possibility...


Mon Jan 07, 2013 4:09 pm
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 Capacity Hat loaded Yagi - needs modeling 
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Post Re: Capacity Hat loaded Yagi - needs modeling
Would be a piece of duff to simply bend the ends of the elements towards each other although bending alloy tubes would be very much a one way process so you want to work out the Moxon dimensions using the online claculator to at least get in the general ball park for where the bend should occur.

http://www.ac6la.com/moxgen.html

That should give a downloadable programme that will make allowance for conductor diameter mind you in the past I've built a 17 metre Moxon from 18 and 22mm aluminium angle using this software so it's a fairly tolerant antenna type. :blush:

I use mine on three bands 20,15 and 10 via three feeds although in all fairness the 20 metre band is the best and 10 is not so good... pretty poor actually :blackeye: but works none the less... shame you couldn't try the Hex which is the same sort of performance but all bands from one feed however the Moxon is positively stealthy by comparison and mine disappears, almost :blink: , against the large trees at the end of the garden.

Good luck, Dave.


Mon Jan 07, 2013 6:01 pm
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 Capacity Hat loaded Yagi - needs modeling 
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Post Re: Capacity Hat loaded Yagi - needs modeling
Intresting stuff Dave,

Researching the Moxon and VK2ABQ compared to an extended 18ft wide TGM type (end loading) I found the following...
Some data gathered here http://w4rnl.net46.net/mu2a.html

Based on 17M design:
Moxon would be 18' x 7'
TGM style with end loading would also be about 18' x 7' (my version would be 18', the actual TGM is only 10', I expect will be more efficient and have better gain)
VK2ABQ would be about 12' x 12' square

But the gains would be quite different:
VK2ABQ about 4.5 dbi
Moxon about 5.5 dbi
TGM claims 5 dbd (so ~7 dbi) for a 2 element (if we accept the published data)

The arguement seems to be that the Moxon and VK2ABQ have more side radiation, with the moxon having less than the VK2ABQ.
This seems to make sense since both designs have radiating side components.
Whereas the TGM design creates it's load in non-radiating components, thats the theroy anyway.

So the TGM style is looking more favourable in terms of front gain, though it is probably going to be far harder to build and tune I suspect.
The VK2ABQ would be easiest to build using standard X configuration, but tuning might be a challange.
The Moxon would be slightly harder than the VK2ABQ given the legnth of the elements but still fairly easy.

Bandwidth might be an issue on the TGM design though, but I'm hopefull that 9' elements vs 5' elements in the original will allow for greater BW given that less loading will be needed.

Steve, am I on track here?

Cheers
Ray.


Mon Jan 07, 2013 10:04 pm
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 Capacity Hat loaded Yagi - needs modeling 
Silent Key

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Post Re: Capacity Hat loaded Yagi - needs modeling
I reckon the TGM Gain figure is a tad optimistic; a mid-band figure for a full-size 2-el monoband 20m Yagi is about 6dBi or 3.9dBd, and you're not going to beat that with a small, loaded, beam. Your Moxon and VK2ABQ Gain figures look right.

As you say, the biggest issue with the TGM approach is performance bandwidth; SWR bandwidth doesn't always reveal just how narrow the other bandwidth parameters are - particularly the F/B bandwidth. But as you say, you have less loading than the TGM so that will help.

73,
Steve G3TXQ


Mon Jan 07, 2013 10:43 pm
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 Capacity Hat loaded Yagi - needs modeling 
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Post Re: Capacity Hat loaded Yagi - needs modeling
I swapped out an MA5B for this multi band Moxon which is probably not as good as that antenna was on 10 metres but runs rings around the Cushcraft product on 15 and 20.

Without wishing to knock the performance of the TGM antenna as there's one currently in the for sale section I have to say owning the MA5B was intensely frustrating at times as on 20 for example the SWR sweet spot was from memory only 100kHz wide before going into the danger zone so in practice on that band I had the antenna set between 14.150 and 14.250 in other words just US extra class operators and the unuseable area around 14.230 where the digital QRMers hang out. ;)

You can see my Moxon on my QRZ pictures although it looks a bit like a Quad due to the effort I put into making the mast look taller but does sit parallel to the ground. :blush:

It was originally a kit of parts from Sandpiper for a 20M Moxon and as you can see sits on a framework of four telescopic fishing poles, the 17M Moxon I had previously though was built from scratch using aluminium angle and would be in the same traditional Yagi style with parallel elements as yours might be altered to, obviously suspending the ends between the two elements and maintaining the correct spacing is a minor task using "unobtainium" insulating sections or a small length of white plastic overflow pipe.

You do realise the antenna will "only" be 24ft wide if it works on 20M too ad that 24ft will look like about 18ft if it sits at 60ft above ground. :D

Cheers, Dave.

PS. Just had a thought and the boom on the 17M Moxon was wood treated with fibreglass resin so you would have to swap the metal boom out for an insulated equivalent... minor hassle.


Tue Jan 08, 2013 9:12 am
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