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