Hello,
I am in the process of building a 4 element 2m yagi, centred in 146 Mhz and
will be utilising a folded dipole for the driven element.

I came across a commercial folded dipole, fed with coax that had been
threaded inside of the driven element. the coax exits through a hole in the
centre of the dipole opposite the feed point.
The coax is connected directly to the ends of the dipole, but on closer
inspection it is different coax on the feed side to the coax tail, so
obviously it is a matching transformer.

Would this be a type of a trombone balun? Assuming it is, then there would
be a 1/4 w/l of 75 ohm coax connected to the 50 ohm tail.

Are my assumptions correct?

Has anyone had any experience with this type of matching transformer? I like
the look of it as it is certainly tidier that an external 4:1 coax balun.
73
John Vk2KC

VK2KC Gmail Inscribed thus:

Hello,
I am in the process of building a 4 element 2m yagi, centred in 146
Mhz and will be utilising a folded dipole for the driven element.

I came across a commercial folded dipole, fed with coax that had been
threaded inside of the driven element. the coax exits through a hole
in the centre of the dipole opposite the feed point.
The coax is connected directly to the ends of the dipole, but on
closer inspection it is different coax on the feed side to the coax
tail, so obviously it is a matching transformer.

Would this be a type of a trombone balun? Assuming it is, then there
would be a 1/4 w/l of 75 ohm coax connected to the 50 ohm tail.

Are my assumptions correct?

More or less :-)

I've tended to prefer a gamma match... its far more flexible in terms
of matching and does the balanced to unbalanced conversion as well.

Has anyone had any experience with this type of matching transformer?
I like the look of it as it is certainly tidier that an external 4:1
coax balun. 73
John Vk2KC

73's
--
Best Regards:
Baron.

On Dec 23, 9:39*pm, Baron wrote:
VK2KC Gmail Inscribed thus:

Hello,
I am in the process of building a 4 element 2m yagi, centred in 146
Mhz and will be utilising a folded dipole for the driven element.

I came across a commercial folded dipole, fed with coax that had been
threaded inside of the driven element. the coax exits through a hole
in the centre of the dipole opposite the feed point.
The coax is connected directly to the ends of the dipole, but on
closer inspection it is different coax on the feed side to the coax
tail, so obviously it is a matching transformer.

Would this be a type of a trombone balun? Assuming it is, then there
would be a 1/4 w/l of 75 ohm coax connected to the 50 ohm tail.

Are my assumptions correct?

More or less :-)

I've tended to prefer a gamma match... *its far more flexible in terms
of matching and does the balanced to unbalanced conversion as well.

Has anyone had any experience with this type of matching transformer?
I like the look of it as it is certainly tidier that an external 4:1
coax balun. 73
John Vk2KC

73's
--
Best Regards:
* * * * * * * * * * *Baron.

A T match with a 1/2 wave phasing line between the two sides of the T
is what Telrex used to use, very simple to build and rugged.

Several years ago I pulled apart a commercial 160MHz yagi antenna as you
describe. It had a very thick folded dipole driven element. The 50 ohm
coax tail was feed in at the boom through the inner of one side of the
folded dipole & was connected to quarter wave length of coax (120/123 ohm
from memory) & the other end was connected to the folded dipole feed point
(centre to one side & earth to other).

I pulled it apart to have a look because I was interested in how the very
wide bandwidth specification of the antenna was achieved (specification
covered whole upper VHF commercial band).

Essentially just a quarter wave length matching section of coax from 50 to
300 ohms.& very thick driven element

Best wishes for Christmas to all on the group.

Might find this Christmas YouTube of interest

http://www.youtube.com/watch?v=GkHNN...layer_embedded


Regards

Eric R Gray (VK3ZSB)



"VK2KC Gmail" wrote in message
...
Hello,
I am in the process of building a 4 element 2m yagi, centred in 146 Mhz
and will be utilising a folded dipole for the driven element.

I came across a commercial folded dipole, fed with coax that had been
threaded inside of the driven element. the coax exits through a hole in
the centre of the dipole opposite the feed point.
The coax is connected directly to the ends of the dipole, but on closer
inspection it is different coax on the feed side to the coax tail, so
obviously it is a matching transformer.

Would this be a type of a trombone balun? Assuming it is, then there would
be a 1/4 w/l of 75 ohm coax connected to the 50 ohm tail.

Are my assumptions correct?

Has anyone had any experience with this type of matching transformer? I
like the look of it as it is certainly tidier that an external 4:1 coax
balun.
73
John Vk2KC

"Baron" wrote in message
I've tended to prefer a gamma match... its far more flexible in terms
of matching and does the balanced to unbalanced conversion as well.


The gamma match does not seem to be thought of as working very well at vhf
and above. Also it defeats some of the purpose of a balun on a beam. The
balun is to keep the current in the antenna balanced which the gamma match
will not to. This tends to skew the pattern of the antenna if the current
is not ballanced on the antenna elements.

Hi Ralph,
Ralph Mowery Inscribed thus:


"Baron" wrote in message
I've tended to prefer a gamma match... its far more flexible in
terms of matching and does the balanced to unbalanced conversion as
well.


The gamma match does not seem to be thought of as working very well at
vhf and above.

I've never had any real problems even at 23cms. Certainly non even with
other feed techniques that I've tried.

Also it defeats some of the purpose of a balun on a beam.

I agree that it wont necessarily prevent common mode currents on the
co-ax.

The balun is to keep the current in the antenna balanced which the
gamma match will not to. This tends to skew the pattern of the
antenna if the current is not balanced on the antenna elements.

I've not really noticed any problems with pattern skew, though I have no
doubt that it occurs. I suspect that it would be more of a issue with
phased arrays.


OT. Merry Christmas to all. (*)

--
Best Regards:
Baron.

On 12/24/2010 9:16 AM, Baron wrote:

The balun is to keep the current in the antenna balanced which the
gamma match will not to. This tends to skew the pattern of the
antenna if the current is not balanced on the antenna elements.

I've not really noticed any problems with pattern skew, though I have no
doubt that it occurs. I suspect that it would be more of a issue with
phased arrays.


Works well enough on EME arrays when I've built them from Cushcraft
Boomer parts kits.

tom
K0TAR