I totally agree with you Stuart, the 135 foot dipole fed with open wire
line and a balanced tuner is the best all band antenna I can think of.
Carl Moreschi N4PY
121 Little Bell Dr.
Hays, NC 28635
www.n4py.com
On 7/10/2013 4:11 PM, Stuart Rohre wrote:
Many hams have used some form of OCF antenna. Not all are horizontal.
For example, My Gap Titan vertical is technically an OCF antenna, since
electrically it is longer on one side of the feedpoint than the other.
The original OCF was probably the "Windom", which was fed with one wire
to the rig. Balance was not a concern as most rigs had single wire feeds
against Earth. Enough power was used to radiate some and work stations.
Later in the application of the antenna, coax was adapted to feed the OCF.
Well, the first problem was Windom was an out of balance antenna in
that, unequal currents would be found in the differing length
(resistance) radiators.
To feed with coax, you had to step up to the impedance of the tap point
which was considered to be about 300 ohms, or that was the line used to
feed an OCF converted from Windom feed of single wire to parallel feed
in the 50's.
Now, using balanced 300 ohm line, you had still, unequal currents in
each radiator leg. (The legs were differing impedances with more copper
on one side).
Later, coax became popular. Attempts to feed the OCF dipole with coax
and step up transformers, (balun), still faced the unequal length
radiators and hence unequal currents. Coax feeding a balanced antenna
will have some added radiation on the shield which encloses the center
conductor. The shield can be shown to consist of two conductors, the
outside of the shield and the inside of the shield. Mismatching at some
frequencies resulted in radiation from the outer shield, but also pick
up of vertically polarized local noise.
To further "fix" the OCF, cable chokes were added (also called coax
isolators), usually cores applied to the outside of the coax. Finally,
the OCF might become quiet in an urban noise environments. But, it still
might radiate a little vertical component, and still was feeding an
inherently unbalanced point having unequal currents in the dipole wires
of unequal length.
I just like the inherent simplicity of the equal legs dipoles of 135
feet total, fed with parallel line, and a tuner; hopefully a balanced
tuner like a double PI Net, which would finally afford the chance to
have equal currents in all parts of the antenna. These have given good
accounts on all bands, and are simple for home construction, with less
weight, typically, than an OCF with its added matching and choking
components.
I would expect an OCF to have some directionality toward one end vs. the
other, but have never seen this written up. Refined versions like the
"Carolina Windom" (which is not single wire feed, and hence not a
"Windom"), do work well for many folks, but you seem to have to spend
more money and have more weight issues to support the OCF version of
dipoles.
Stuart Rohre
K5KVH
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