[TenTec] OCF antennas evolution

[Stuart Rohre]

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