[TenTec] OCF antennas evolution

[Robert Mcgraw]

I agree 100% with Carl and Stuart on this.  One caveat, please use a
balanced tuner an not a poor performing balun to get from unbalanced to
balanced configuration.  Also most likely a 1:1 current balun will
exhibit lower loss, handle higher power than a 4:1 internal balun.

Remember the power ratings on a balun are for MATCHED conditions, which
by the way is highly unlikely in a configuration used for multiple bands.
 If you plan to run anything near legal limit power then a balun power
rating of 5KW to 10KW is reasonable.

73
Bob, K4TAX







> 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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