Gosh. In the midst of the OCF discussion one of the answers for where to use a
4:1 balun just has to be for an OCF dipole. Most people now feed them at the
200 ohm point with a 4:1 Balun. There is even a favorite:
http://www.balundesigns.com/servlet/the-75/OCF-balun-4-cln-1/Detail
I have wound a few baluns and ununs but none were any prettier than that,
cheaper maybe but not prettier.
After that a common mode choke and maybe another common mode choke somewhere
else down the line.
I've just been reading this stuff. I haven't had an OCF in at least a decade.
Jon
On Jul 10, 2013, at 7:39 PM, Bob McGraw - K4TAX wrote:
> Just got an e-mail asking "so where is it best to use a 4:1 balun?"
>
> Remember when we learned that a folded dipole antenna exhibited a typical
> feedpoint impedance of some 300 ohms? Well the 4:1 balun works just great
> transforming the 300 ohms to 75 ohms. Years ago the Pi network in our
> transmitters would match 75 ohms without a hitch plus RG-59 was very
> available and inexpensive. Now, bring the folded dipole closer to ground and
> that 300 ohm impedance will drop to something like 200 ohms. Add a 4:1 balun
> and one has a resonant antenna, the balun is operating between matched
> impedances, and your transmitter sees something like 50 ohms.
>
> Of course the folded dipole is a single band antenna so don't expect to use a
> tuner and make it work other bands. But it is one of the quietest antennas
> of any type, form or function plus likely covers the 80M band with less than
> a 1.5:1 SWR and NO TUNER.
>
> I have one cut for 3885 and it out performs the center fed dipole, the
> balanced fed dipole and the full wave loop. I just wish I had resources and
> supports to put up one for each band. I construct mine from heavy duty 300
> ohm window type line supporting the ends and center with home made strain
> relief supports that does not connect to the conductors but clamps on the
> outer jacket. They stay up through wind and winter snow and ice.
>
> Just remember that using a 4:1 balun that one is not matching the feedline
> impedance. Many think that if one uses 450 ohm line that a 4:1 balun is
> required. If that antenna is close to the ground the center feed impedance
> is most likely about 25 ohms. A 1/4 wave of open wire line transforms it to
> about 50 ohms. A 4:1 balun then drops it to about 12.5 ohms. Tests have
> shown that tuners operating at lower impedance values typically have higher
> loss. It would then make sense to use a 1:1 balun thus the tuner would
> operate at about 50 ohms. Or perhaps a tuner would not be needed at all.
>
> 73
> Bob, K4TAX
>
>
>
>
>
>
>
>
> ----- Original Message ----- From: "Robert Mcgraw" <rmcgraw@blomand.net>
> To: <n4py3@earthlink.net>; "Discussion of Ten-Tec Equipment"
> <tentec@contesting.com>
> Sent: Wednesday, July 10, 2013 3:45 PM
> 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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>>>>
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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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