Topics

WIMO SHF 2328 23cm-Yagi 28 Element ?


militaryoperator
 


WIMO SHF 2328 23cm-Yagi 28 Element

?  good, bad, indifferent? worth buying? 
Ben
----------------------------


Interestingly, after it was pointed out to me I did count elements. Seems this 28 "element"
yagi turns out to be a 21 element, 19 directors, radiator and reflector.

Apparently, 8 reflectors stacked count as individual elements.

What if it was a solid plate at the back? 21 element yagi then?

Seems a bit of a rip-off but what do I know, hi.  

Are all commercial antenna misrepresented this way? Is it the norm?


Ben 



 


Paul Randall G3NJV
 

Ages ago I had aTonna 17e for 2m, this had a triple stacked reflector and iirc yes all three were counted. So 15e really. 



Sent from my Samsung Galaxy smartphone.



-------- Original message --------
From: "militaryoperator via groups.io" <Military1944@...>
Date: 21/09/2020 22:17 (GMT+00:00)
To: UKMicrowaves@groups.io
Subject: Re: [UKMicrowaves] WIMO SHF 2328 23cm-Yagi 28 Element ?


WIMO SHF 2328 23cm-Yagi 28 Element

?  good, bad, indifferent? worth buying? 
Ben
----------------------------


Interestingly, after it was pointed out to me I did count elements. Seems this 28 "element"
yagi turns out to be a 21 element, 19 directors, radiator and reflector.

Apparently, 8 reflectors stacked count as individual elements.

What if it was a solid plate at the back? 21 element yagi then?

Seems a bit of a rip-off but what do I know, hi.  

Are all commercial antenna misrepresented this way? Is it the norm?


Ben 



 


KENT BRITAIN
 

I am afraid much of this is the norm.
Like antennas that hear signals, but not noise.   2E0VAA


On Monday, September 21, 2020, 4:17:16 PM CDT, militaryoperator via groups.io <military1944@...> wrote:



WIMO SHF 2328 23cm-Yagi 28 Element

?  good, bad, indifferent? worth buying? 
Ben
----------------------------


Interestingly, after it was pointed out to me I did count elements. Seems this 28 "element"
yagi turns out to be a 21 element, 19 directors, radiator and reflector.

Apparently, 8 reflectors stacked count as individual elements.

What if it was a solid plate at the back? 21 element yagi then?

Seems a bit of a rip-off but what do I know, hi.  

Are all commercial antenna misrepresented this way? Is it the norm?


Ben 



 


Chris Bartram G4DGU
 

Hello Ben,

Quoting element numbers is a good sales pitch for the unwary, but a much more important determining factor in yagi design is the overall length.

73

Chris G4DGU


On 21/09/2020 21:33, militaryoperator via groups.io wrote:

WIMO SHF 2328 23cm-Yagi 28 Element

?  good, bad, indifferent? worth buying? 
Ben
----------------------------


Interestingly, after it was pointed out to me I did count elements. Seems this 28 "element"
yagi turns out to be a 21 element, 19 directors, radiator and reflector.

Apparently, 8 reflectors stacked count as individual elements.

What if it was a solid plate at the back? 21 element yagi then?

Seems a bit of a rip-off but what do I know, hi.  

Are all commercial antenna misrepresented this way? Is it the norm?


Ben 



 


John Lemay
 

Ben

 

Adding elements can have a modest effect on gain, but as Chris says, boom length is the biggest single factor. In the case of the Wimo, the multiple element reflector will have very little impact on gain, but it does improve the front=to-back ratio somewhat. Yes, a wire mesh plate would work similarly.

 

It’s not a rip-off, there really are 28 elements.

 

For readers of a certain age, think back to the 88 element Jaybeam “Multibeam” ………

 

John G4ZTR

 

From: UKMicrowaves@groups.io [mailto:UKMicrowaves@groups.io] On Behalf Of Chris Bartram G4DGU
Sent: 21 September 2020 23:37
To: UKMicrowaves@groups.io
Subject: Re: [UKMicrowaves] WIMO SHF 2328 23cm-Yagi 28 Element ?

 

Hello Ben,

Quoting element numbers is a good sales pitch for the unwary, but a much more important determining factor in yagi design is the overall length.

73

Chris G4DGU

 

On 21/09/2020 21:33, militaryoperator via groups.io wrote:

 

WIMO SHF 2328 23cm-Yagi 28 Element

?  good, bad, indifferent? worth buying? 

Ben

----------------------------

 

 

Interestingly, after it was pointed out to me I did count elements. Seems this 28 "element"

yagi turns out to be a 21 element, 19 directors, radiator and reflector.

 

Apparently, 8 reflectors stacked count as individual elements.

 

What if it was a solid plate at the back? 21 element yagi then?

 

Seems a bit of a rip-off but what do I know, hi.  

 

Are all commercial antenna misrepresented this way? Is it the norm?

 

 

Ben 

 

 

 

 


Virus-free. www.avg.com


Chris Bartram G4DGU
 

So called because of its pretty gross sidelobe levels! As for the initial claim of '20dB' gain ...

73

Chris

G4DGU


On 22/09/2020 08:04, John Lemay wrote:
think back to the 88 element Jaybeam “Multibeam” ………


Colin Ranson
 

Re a 17ele Tonna, mine became a zero element after it went through the next doors bathroom window in 1982..... and I was lowering the mast prior to moving house (for the benefit of G3XDY it was Tranmere Grove) ! when a tensioned guy snapped and the mast went sideways.

 

Colin de G8LBS.

 

Sent from Mail for Windows 10

 

From: Paul Randall G3NJV
Sent: 21 September 2020 22:33
To: UKMicrowaves@groups.io
Subject: Re: [UKMicrowaves] WIMO SHF 2328 23cm-Yagi 28 Element ?

 

Ages ago I had aTonna 17e for 2m, this had a triple stacked reflector and iirc yes all three were counted. So 15e really. 

 

 

 

Sent from my Samsung Galaxy smartphone.

 

 

 

-------- Original message --------

From: "militaryoperator via groups.io" <Military1944@...>

Date: 21/09/2020 22:17 (GMT+00:00)

To: UKMicrowaves@groups.io

Subject: Re: [UKMicrowaves] WIMO SHF 2328 23cm-Yagi 28 Element ?

 

 

WIMO SHF 2328 23cm-Yagi 28 Element

?  good, bad, indifferent? worth buying? 

Ben

----------------------------

 

 

Interestingly, after it was pointed out to me I did count elements. Seems this 28 "element"

yagi turns out to be a 21 element, 19 directors, radiator and reflector.

 

Apparently, 8 reflectors stacked count as individual elements.

 

What if it was a solid plate at the back? 21 element yagi then?

 

Seems a bit of a rip-off but what do I know, hi.  

 

Are all commercial antenna misrepresented this way? Is it the norm?

 

 

Ben 

 

 

 

 

 


G0FVI
 

I was given a badly damaged Wimo last year to repair and use for UKAC. It is the 44 element version and as you say the reflector elements are counted as advertised unlike other beams. I therefore thought I had a 37 element and recently purchased the advertised 44 element only to find it was the same as the one I've already got (no I'd never measured the boom length and estimated it to be round about 8.5 feet so more fool me!). Not to worry they are both going to get stacked horizontally.

If you buy a Wimo there are a couple of possible issues you need to be aware of. Some people have reported the folded dipole element fails due to water ingress (I haven't had this problem). However on the donated Wimo I have, I noticed the 'tuning' was way off, with a very narrow bandwidth. Turns out the through boom elements need metal to metal contact with the boom to work. Water gets in and causes corrosion which affects the metal to metal contact via the self tapping screw that holds elements in place. The solution is to take the screw out (preferably replace it) and rub the elements down with wire wool, then apply some moly grease around areas were moisture gets in (this seems to do the job). 
What I would say is these antennas work very well with an excellent F/B. Spares are readily available from Wimo and the DX Shop. 

Andy G0FVI


Robin Szemeti - G1YFG
 

The through boom in contact elements are the biggest problem in my opinion. Eventually, you will get corrosion poor contact, at which point it goes out of tune. Cleaning them is only a temporary fix, as they will build an insulating layer of aluminium oxide at some point. it is just a question of WHEN.

I prefer insulated elements for this reason, at least once tuned, they should stay tuned.


On Wed, 23 Sep 2020 at 09:41, G0FVI via groups.io <Andrew.gilfillan=ntlworld.com@groups.io> wrote:
I was given a badly damaged Wimo last year to repair and use for UKAC. It is the 44 element version and as you say the reflector elements are counted as advertised unlike other beams. I therefore thought I had a 37 element and recently purchased the advertised 44 element only to find it was the same as the one I've already got (no I'd never measured the boom length and estimated it to be round about 8.5 feet so more fool me!). Not to worry they are both going to get stacked horizontally.

If you buy a Wimo there are a couple of possible issues you need to be aware of. Some people have reported the folded dipole element fails due to water ingress (I haven't had this problem). However on the donated Wimo I have, I noticed the 'tuning' was way off, with a very narrow bandwidth. Turns out the through boom elements need metal to metal contact with the boom to work. Water gets in and causes corrosion which affects the metal to metal contact via the self tapping screw that holds elements in place. The solution is to take the screw out (preferably replace it) and rub the elements down with wire wool, then apply some moly grease around areas were moisture gets in (this seems to do the job). 
What I would say is these antennas work very well with an excellent F/B. Spares are readily available from Wimo and the DX Shop. 

Andy G0FVI


--
Robin Szemeti - G1YFG


John Fell
 

Andy ,
I would hope the design does not use the self -tapping screw to provide the grounding point ....
It should be the contact areas between the individual elements and the inside faces of the boom cross holes - the screw applies the clamping force to retain good RF continuity .
Your efforts to improve the ongoing efficiency should be useful if you cleaned the through boom hole surfaces .
If your feed fails , the folded dipole design by DL6WU works well and is easy to construct .
Triagonal reflectors , according to NBS data, do contribute a small forward gain/reduced rear pickup .

73
John
G0API


On Wed, 23 Sep 2020 at 09:41, G0FVI via groups.io <Andrew.gilfillan=ntlworld.com@groups.io> wrote:
I was given a badly damaged Wimo last year to repair and use for UKAC. It is the 44 element version and as you say the reflector elements are counted as advertised unlike other beams. I therefore thought I had a 37 element and recently purchased the advertised 44 element only to find it was the same as the one I've already got (no I'd never measured the boom length and estimated it to be round about 8.5 feet so more fool me!). Not to worry they are both going to get stacked horizontally.

If you buy a Wimo there are a couple of possible issues you need to be aware of. Some people have reported the folded dipole element fails due to water ingress (I haven't had this problem). However on the donated Wimo I have, I noticed the 'tuning' was way off, with a very narrow bandwidth. Turns out the through boom elements need metal to metal contact with the boom to work. Water gets in and causes corrosion which affects the metal to metal contact via the self tapping screw that holds elements in place. The solution is to take the screw out (preferably replace it) and rub the elements down with wire wool, then apply some moly grease around areas were moisture gets in (this seems to do the job). 
What I would say is these antennas work very well with an excellent F/B. Spares are readily available from Wimo and the DX Shop. 

Andy G0FVI


Robin Szemeti - G1YFG
 

The problem with the DL6WU dipole is that it is very difficult to fit over the boom on 1296 and above.

Most articles recommend 50mm  between the top and bottom of the folded dipole as the maximum for 144MHz,  and if you scale that to 1296 you get about 6mm ... if you make the gap too big, it begins to behave like some sort of resonant full wave loop, not a folded dipole.

Most 1296 booms are around 15mm and the "over boom" diploles I see have a height of around 25 to 30mm .. this is far too big in my opinion.  I've spent some time playing with a coupler and a variety of  lengths, I was unable to get an acceptable match and get it to fit over the boom ...


On Wed, 23 Sep 2020 at 09:58, John Fell <john.g0api@...> wrote:
Andy ,
I would hope the design does not use the self -tapping screw to provide the grounding point ....
It should be the contact areas between the individual elements and the inside faces of the boom cross holes - the screw applies the clamping force to retain good RF continuity .
Your efforts to improve the ongoing efficiency should be useful if you cleaned the through boom hole surfaces .
If your feed fails , the folded dipole design by DL6WU works well and is easy to construct .
Triagonal reflectors , according to NBS data, do contribute a small forward gain/reduced rear pickup .

73
John
G0API


On Wed, 23 Sep 2020 at 09:41, G0FVI via groups.io <Andrew.gilfillan=ntlworld.com@groups.io> wrote:
I was given a badly damaged Wimo last year to repair and use for UKAC. It is the 44 element version and as you say the reflector elements are counted as advertised unlike other beams. I therefore thought I had a 37 element and recently purchased the advertised 44 element only to find it was the same as the one I've already got (no I'd never measured the boom length and estimated it to be round about 8.5 feet so more fool me!). Not to worry they are both going to get stacked horizontally.

If you buy a Wimo there are a couple of possible issues you need to be aware of. Some people have reported the folded dipole element fails due to water ingress (I haven't had this problem). However on the donated Wimo I have, I noticed the 'tuning' was way off, with a very narrow bandwidth. Turns out the through boom elements need metal to metal contact with the boom to work. Water gets in and causes corrosion which affects the metal to metal contact via the self tapping screw that holds elements in place. The solution is to take the screw out (preferably replace it) and rub the elements down with wire wool, then apply some moly grease around areas were moisture gets in (this seems to do the job). 
What I would say is these antennas work very well with an excellent F/B. Spares are readily available from Wimo and the DX Shop. 

Andy G0FVI


--
Robin Szemeti - G1YFG


Andy
 


It is the 44 element version and as you say the reflector elements are counted as advertised unlike other beams.

The old 17ele 2m F9FT Tonna Yagis were the same. They were really a 15 ele antenna and the reflector was 3 ele. So 15ele plus 2 extra elements in the reflector. ISTR it was much longer than their 13 ele classic design, much longer than just adding 2 more elements would suggest.

Andy


Chris Bartram G4DGU
 

Hello Robin,

I looked at the problems with folded dipoles (or loops as most of them become at >1GHz) some years ago using a proper E-M analysis tool (not NEC derived software) when designing antennas for my old QTH in SW Wales. While folded dipoles/loops with a half-wave balun could be made to work with some fiddling, I decided to use simple split dipoles with driven by a so-called Pawsey Stub(1) balun. That showed good balance, despite a friend suggesting that the structure was unbalanced 'by inspection'. The losses were very low indeed when measured on my HP net. an. at that time. I've come to the conclusion that favouring the 'folded dipole' feed has no real engineering validity.

FWIW, my VHF/UHF yagi antennas were designed around 4m lengths of 20mm od, 1.5mm wall alloy tube - I forget the alloy type, maybe 6082. These were self-supporting at 20m agl even in the windy conditions of NW Carmarthenshire. I used polyprop. standoff insulators manufactured for supporting a commercial garden trellis. Unfortunately, these are no longer available. I have used the simple dipole/'Pawsey' stub balun arrangement very sucessfully on various antennas from 82 - 1296MHz.


73

Chris G4DGU

(1) Pawsey was a pioneer Australian radio astronomer working from the mid-1940s. He described the balun structure which carries his name sometime around 1950, however, the idea was patented by EMI in 1936, in the name of an engineer named White who was working with Alan Blumlein on the development of high power TV transmitters.

On 23/09/2020 10:15, Robin Szemeti - G1YFG wrote:
The problem with the DL6WU dipole is that it is very difficult to fit over the boom on 1296 and above.

Most articles recommend 50mm  between the top and bottom of the folded dipole as the maximum for 144MHz,  and if you scale that to 1296 you get about 6mm ... if you make the gap too big, it begins to behave like some sort of resonant full wave loop, not a folded dipole.

Most 1296 booms are around 15mm and the "over boom" diploles I see have a height of around 25 to 30mm .. this is far too big in my opinion.  I've spent some time playing with a coupler and a variety of  lengths, I was unable to get an acceptable match and get it to fit over the boom ...


Robin Szemeti - G1YFG
 

My current 70cm antenna is a "Pawsey stub" balanced dipole feed, I tried replicating it at 1296, but the mount above the boom when all the directors are through boom messes it up.   I will be cutting away a section of boom and fitting a bracing piece, maybe that will solve it.

On Wed, 23 Sep 2020 at 14:18, Chris Bartram G4DGU <chris@...> wrote:
Hello Robin,

I looked at the problems with folded dipoles (or loops as most of them
become at >1GHz) some years ago using a proper E-M analysis tool (not
NEC derived software) when designing antennas for my old QTH in SW
Wales. While folded dipoles/loops with a half-wave balun could be made
to work with some fiddling, I decided to use simple split dipoles with
driven by a so-called Pawsey Stub(1) balun. That showed good balance,
despite a friend suggesting that the structure was unbalanced 'by
inspection'. The losses were very low indeed when measured on my HP net.
an. at that time. I've come to the conclusion that favouring the 'folded
dipole' feed has no real engineering validity.

FWIW, my VHF/UHF yagi antennas were designed around 4m lengths of 20mm
od, 1.5mm wall alloy tube - I forget the alloy type, maybe 6082. These
were self-supporting at 20m agl even in the windy conditions of NW
Carmarthenshire. I used polyprop. standoff insulators manufactured for
supporting a commercial garden trellis. Unfortunately, these are no
longer available. I have used the simple dipole/'Pawsey' stub balun
arrangement very sucessfully on various antennas from 82 - 1296MHz.


73

Chris G4DGU

(1) Pawsey was a pioneer Australian radio astronomer working from the
mid-1940s. He described the balun structure which carries his name
sometime around 1950, however, the idea was patented by EMI in 1936, in
the name of an engineer named White who was working with Alan Blumlein
on the development of high power TV transmitters.


On 23/09/2020 10:15, Robin Szemeti - G1YFG wrote:
> The problem with the DL6WU dipole is that it is very difficult to fit
> over the boom on 1296 and above.
>
> Most articles recommend 50mm  between the top and bottom of the folded
> dipole as the maximum for 144MHz,  and if you scale that to 1296 you
> get about 6mm ... if you make the gap too big, it begins to behave
> like some sort of resonant full wave loop, not a folded dipole.
>
> Most 1296 booms are around 15mm and the "over boom" diploles I see
> have a height of around 25 to 30mm .. this is far too big in my
> opinion.  I've spent some time playing with a coupler and a variety
> of  lengths, I was unable to get an acceptable match and get it to fit
> over the boom ...
>






--
Robin Szemeti - G1YFG


alwyn.seeds1
 

Dear Chris,

Thanks for the note about the “Pawsey Stub”.

Like Uda, White did not receive the recognition due to him, although at least in Uda’s case Yagi’s involvement in the work was considerable.

I have a number of the ‘30s patents from Blumlein’s group- their innovations were absolutely remarkable- and the patents easier to read than subsequent textbook descriptions.

Regards,

Alwyn


_____________________________________________________

Alwyn Seeds, Director
114 Beaufort Street (Management) Company Ltd.,
114 Beaufort Street,
London,
SW3 6BU

114 Beaufort Street (Management) Company Ltd., Registered in England and Wales: No. 02797775 Registered Office Address: 114 Beaufort Street, London, SW3 6BU
______________________________________________________


Alan Beard
 

Hi Robin,

This is a real "hashup". The last 1/4 wave to the feed point is an impedance transformer.
Thus it was quite broadband, and that's what I was looking for.

And, how to do a circular polarisation feed that has low coupling between the H and V.

Question is then, now it's matched, how close should the first director be?

On my loop above, this is an old old trick used in thousands of VHF and UHF dipoles
the coax goes inside the loop (an oblong), a "cold" point and around to a grounding plug
where the coax outer is terminated and the coax inner, now in a larger diameter tube goes
the last 1/4 wave, to a plug in the other side tube, this all covered in heatshrink, often
with a fiberglass tube as stiffener.


Alan VK2ZIW


On Wed, 23 Sep 2020 21:03:48 +0100, Robin Szemeti - G1YFG wrote
> My current 70cm antenna is a "Pawsey stub" balanced dipole feed, I tried replicating it at 1296, but the mount above the boom when all the directors are through boom messes it up.   I will be cutting away a section of boom and fitting a bracing piece, maybe that will solve it.
>
> On Wed, 23 Sep 2020 at 14:18, Chris Bartram G4DGU <chris@...> wrote:
>
Hello Robin,
>
> I looked at the problems with folded dipoles (or loops as most of them
> become at >1GHz) some years ago using a proper E-M analysis tool (not
> NEC derived software) when designing antennas for my old QTH in SW
> Wales. While folded dipoles/loops with a half-wave balun could be made
> to work with some fiddling, I decided to use simple split dipoles with
> driven by a so-called Pawsey Stub(1) balun. That showed good balance,
> despite a friend suggesting that the structure was unbalanced 'by
> inspection'. The losses were very low indeed when measured on my HP net.
> an. at that time. I've come to the conclusion that favouring the 'folded
> dipole' feed has no real engineering validity.
>
> FWIW, my VHF/UHF yagi antennas were designed around 4m lengths of 20mm
> od, 1.5mm wall alloy tube - I forget the alloy type, maybe 6082. These
> were self-supporting at 20m agl even in the windy conditions of NW
> Carmarthenshire. I used polyprop. standoff insulators manufactured for
> supporting a commercial garden trellis. Unfortunately, these are no
> longer available. I have used the simple dipole/'Pawsey' stub balun
> arrangement very sucessfully on various antennas from 82 - 1296MHz.
>
> 73
>
> Chris G4DGU
>
> (1) Pawsey was a pioneer Australian radio astronomer working from the
> mid-1940s. He described the balun structure which carries his name
> sometime around 1950, however, the idea was patented by EMI in 1936, in
> the name of an engineer named White who was working with Alan Blumlein
> on the development of high power TV transmitters.
>
> On 23/09/2020 10:15, Robin Szemeti - G1YFG wrote:
> > The problem with the DL6WU dipole is that it is very difficult to fit
> > over the boom on 1296 and above.
> >
> > Most articles recommend 50mm  between the top and bottom of the folded
> > dipole as the maximum for 144MHz,  and if you scale that to 1296 you
> > get about 6mm ... if you make the gap too big, it begins to behave
> > like some sort of resonant full wave loop, not a folded dipole.
> >
> > Most 1296 booms are around 15mm and the "over boom" diploles I see
> > have a height of around 25 to 30mm .. this is far too big in my
> > opinion.  I've spent some time playing with a coupler and a variety
> > of  lengths, I was unable to get an acceptable match and get it to fit
> > over the boom ...
> >
>
>

> --
> Robin Szemeti - G1YFG

---------------------------------------------------
Alan VK2ZIW

OpenWebMail 2.53, nothing in the cloud.


Bob Lockley VK6KW
 

Hi Folks,
I have also seen some Yagis built using Stauff (hydraulic tube to machine) clamps as insulated element mounts. They are available in various sizes and configurations so try here https://www.tom-parker.co.uk/product-request/.

73,
Bob.VK6KW

-----Original Message-----
From: UKMicrowaves@groups.io [mailto:UKMicrowaves@groups.io] On Behalf Of Chris Bartram G4DGU
Sent: Wednesday, September 23, 2020 9:18 PM
To: UKMicrowaves@groups.io
Subject: Re: [UKMicrowaves] WIMO SHF 2328 23cm-Yagi 28 Element ?

Hello Robin,

I looked at the problems with folded dipoles (or loops as most of them
become at >1GHz) some years ago using a proper E-M analysis tool (not
NEC derived software) when designing antennas for my old QTH in SW
Wales. While folded dipoles/loops with a half-wave balun could be made
to work with some fiddling, I decided to use simple split dipoles with
driven by a so-called Pawsey Stub(1) balun. That showed good balance,
despite a friend suggesting that the structure was unbalanced 'by
inspection'. The losses were very low indeed when measured on my HP net.
an. at that time. I've come to the conclusion that favouring the 'folded
dipole' feed has no real engineering validity.

FWIW, my VHF/UHF yagi antennas were designed around 4m lengths of 20mm
od, 1.5mm wall alloy tube - I forget the alloy type, maybe 6082. These
were self-supporting at 20m agl even in the windy conditions of NW
Carmarthenshire. I used polyprop. standoff insulators manufactured for
supporting a commercial garden trellis. Unfortunately, these are no
longer available. I have used the simple dipole/'Pawsey' stub balun
arrangement very sucessfully on various antennas from 82 - 1296MHz.


73

Chris G4DGU

(1) Pawsey was a pioneer Australian radio astronomer working from the
mid-1940s. He described the balun structure which carries his name
sometime around 1950, however, the idea was patented by EMI in 1936, in
the name of an engineer named White who was working with Alan Blumlein
on the development of high power TV transmitters.


On 23/09/2020 10:15, Robin Szemeti - G1YFG wrote:
The problem with the DL6WU dipole is that it is very difficult to fit
over the boom on 1296 and above.

Most articles recommend 50mm� between the top and bottom of the folded
dipole as the maximum for 144MHz,� and if you scale that to 1296 you
get about 6mm ... if you make the gap too big, it begins to behave
like some sort of resonant full wave loop, not a folded dipole.

Most 1296 booms are around 15mm and the "over boom" diploles I see
have a height of around 25 to 30mm .. this is far too big in my
opinion.� I've spent some time playing with a coupler and a variety
of� lengths, I was unable to get an acceptable match and get it to fit
over the boom ...