Guys,
Carl, Stuart, Bob, you are all correct, but not correct in every instance.
Partially correct, mostly correct, but not totally correct.
I agree with everything you all are saying, BUT, one cannot always run
openwire, AND the OCFD is not as bad as you guys are depicting it here! In
the past 2 years I have built over a dozen of these and run all kinds of
tests on them.
First of all, most people who resort to using an OCFD are doing it not out
of choice but out of necessity. ME! I'm a good example. I ran openwire
from 1963 until 2011. Wouldn't use anything else for the low bands. Can't
do it here at this QTH. Had to go with coax and only have permission for
one single antenna.
I'm antenna handicapped.
I had to come up with one antenna that would work all bands and openwire was
totally out of the question (for cosmetic reasons).
The OCFD is a GOOD ANTENNA with Nasty Side Effects! IT IS NOT A BAD
ANTENNA.
First of all I want to categorically state that the antenna works well. My
contests scores consistently prove that.
However I have had to take several steps to cope with the common mode
current (CMC) that we all know exists around these antennas.
For starters, read Jim's RFI-Ham. It is outstanding and gives you a clue on
how to begin to address the problem.
However for more practical application of his concepts, read GM3SEK's
documents on how to build good, cheap RF chokes.
This is based on the concept described by W2VJN in the 2010 ARRL Handbook,
which ultimately falls back on what Jim wrote in RFI-Ham.
If you use the right balun at the feedpoint (not just any balun), but the
right balun, then it forces equal currents into both legs of the antenna,
even though they are not symmetrical.
YES that can cause CMC in the coax because one side might be inducing more
RF energy into it than the other, but every dipole that does not run its
coax away at right angles has potentially the same problem.
Next, you must use the right type of RF Choke. Using the wrong one can
actually increase CMC, not reduce it. See Steve's (G3TXQ) page on RF
Chokes. He explains why this is.
Next, you have to insert the chokes at the right places.
For this see again RFI-Ham or GM3SEK's presentation on managing common mode
current.
And see again Steve's page on RF-Chokes - particularly the bottom of the
page.
If you do your homework, deploy the right hardware, you can get great
service out of the OCFD without all the problems you guys are talking about.
UNFORTUNATELY most people don't have a clue about any of this, don't do
their homework, and have problems. Thus the bad reputation of this antenna.
This is only a bad antenna if implemented wrong. But most of the time it is
implemented wrong.
Most people trashing the OCFD have never even used one. They are basing
their opinion on what they have heard. As I said, most of these antennas
are bad but only because of a bad implementation, not because it is
impossible to get it right! So the horror stories you hear are probably
mostly true, but they don't have to be.
For at least 20 years, people were running these antennas with Voltage
Baluns. That of course caused problems. Lots of problems.
Most of the commercial OCFD's are using current baluns, but BAD ONES.
They are winding both transformers onto a single toroid. This actually
forces an imbalance.
Ask Steve to explain this. He can do it better than I can. But it's fact.
Thus the bad stories around this antenna!
Again, to summarize this, "if" you can use openwire, that is the preferred
way to go.
If you can't, don't be alarmed by the horror stories around the OCFD (alias
Windom).
Do your homework.
Understand what all you must do to get reasonable performance out of this
antenna without all of the negative side effects.
I have managed it while running 500w. I've not tried it with 1500.
I have an RF Ammeter. I can prove that my CMC is negligible (until I remove
my chokes).
My 80m OCFD works 80/40/30/20/15/10m WITHOUT A MATCHBOX (except of course I
need a matchbox for tuning the high end of 80m).
(that was NOT a typo; mine also works great on 15m - but few people know how
to do this)
I feed it with one single coax, well hidden from the neighbors.
If I had permission to switch to openwire (which is visually more
intrusive), I would do it in a heartbeat, but I can't do it.
So my recommendation is do NOT use the OCFD unless you have no other choice,
but if you must, learn how do deploy it right.
If you get it right, you probably will not have the problems you have been
reading about here.
73
Rick, DJ0IP
-----Original Message-----
From: TenTec [mailto:tentec-bounces@contesting.com] On Behalf Of Robert
Mcgraw
Sent: Wednesday, July 10, 2013 10:45 PM
To: n4py3@earthlink.net; Discussion of Ten-Tec Equipment
Subject: Re: [TenTec] OCF antennas evolution
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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--
Disclosure:
I am a Tentec Ambassador and compensated according to the Tentec Ambassador
plan. I serve as a volunteer beta test person for the Omni
VII, Eagle and Argonaut VI products. Otherwise, I hold no business or
employment interest with Tentec.
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