[TenTec] OCF antennas evolution

[Carl Moreschi]

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
> _______________________________________________
> TenTec mailing list
> TenTec at contesting.com
> http://lists.contesting.com/mailman/listinfo/tentec
>