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.

"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