Hi Dan, note the drawing of the folded balun shows the 1/4 wavelength green
cylindrical sleeve connected to one of the dipole elements.
In normal configurations, the sleeve is not connected, just left open so as
to present a high impedance to the dipole elements. Rf flowing on the
feedline shield encounters the short circuit between the shield and sleeve and
gets reflected back to the dipole for reradiation. The length of the sleeve
being 90 degrees, causes the reflected energy to arrive back at the dipole
in phase. That is 90 degees down the sleeve, 180 degree reversal at the
short and another 90 degrees up the sleeve for a total of 360 degrees.
The Pawsey style balun does not use a cylinder. It uses a single 1/4
wavelength conductor to form a parallel line with the coax shield. The Pawsey
stub ideally should be made with similar dimensions of the coax to make the
balun as balanced as possible. In fact, early designs used a piece of the
same type coax xable as the stub, with no connection to the center conductor.
The Pawsey stub design then depends on a 1/4 wavelength balanced line
connected to the dipole elements. Very similar to the beta match used on yagis.
The coax and Pawsey stub are connected together and on a yagi, grounded to
the boom, 1/4 wavelength down the outside of the coax shield. Rf flowing on
the shield encounters the short and reflects the energy back to dipole for
reradiation. The balance between the two conductors act preserve the 180
degree phase between the dipole connections.
There is a variation of the folded balun that works well. The outer
cylinder is split into two halves with two saw cuts. The two halves then work
in
the same manner as a Pawsey stub. I can't see how the folded balun can work
as presented.
73,
Gerald K5GW
In a message dated 9/17/2011 7:28:47 P.M. Central Daylight Time,
n5ardxcc@gmail.com writes:
http://www.antenna-theory.com/definitions/foldedbalun.php
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