In addition to what Bob has just said, another place where a 4:1 balun is
good is in the Off-Center-Fed Dipole.
Most people have learned that the OCFD (sometimes called Windom) has a
feedpoint impedance of 300 Ohms.
This is wrong.
The feedpoint impedance of the antenna will vary from band to band and
especially by height, but we have to choose to feed it with what is commonly
available. A 6:1 balun will match 50 Ohms to 300 Ohms, and a 4:1 balun
matches it to 200 Ohms.
According to Buck, who has probably sold more Windom/ODFD antennas than all
other manufacturers combined, the Windom will have about 235 Ohms in typical
installations (under 50 feet of height), and only above 70 ft. does it rise
to about 280 Ohms. It never hits 300 Ohms. AND of course local obstacles
will influence these values.
But Buck only used one feedpoint, roughly 1/3 - 2/3 split.
In other words, he feeds it 1/3 of the way in from one of the ends.
There is no rule mandating you choose this point.
I have been experimenting with many other split ratios - that's why I have
built 12 of these antennas in the past 2 years.
Moving the feedpoint also changes the optimum "best impedance".
Computers are great for modeling this.
So in general, an OCFD should be fed with a 4:1 Current Balun, with the two
transformers of the balun each on separate cores. So you always need a balun
with two toroids inside. If you run high power, you will need 4 cores (1
per transformer). If you don't do that, you will get common mode current
problems. Unfortunately under certain circumstances, one very good US
manufacturer of baluns is recommending using a balun with both transformers
on the same stacked core if you have these heating problems. The real
solution is to use 4 cores. I'm sure the guy know very well what he is
doing, but nobody would buy it if it was twice as big and cost 50% more.
The problem with 4:1 baluns (and 6:1 baluns) is, although they will continue
to transform at their design ratio, their ability to "BALUN" (impede CMC)
goes to hates in a hand basket when the SWR is high. The 1:1 balun
continues to perform its duties, despite higher SWR. You just have to be
sure the ferrite is hefty enough to withstand the heat.
For this reason, we are now seeing ODFD antennas being fed with a 4:1 balun
AND a 1:1 choke attached directly to the balun to suppress the CMC.
73
Rick, DJ0IP
-----Original Message-----
From: TenTec [mailto:tentec-bounces@contesting.com] On Behalf Of Bob McGraw
- K4TAX
Sent: Thursday, July 11, 2013 4:40 AM
To: rmcgraw@blomand.net; Discussion of Ten-Tec Equipment;
n4py3@earthlink.net
Subject: Re: [TenTec] OCF antennas evolution
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
>>> _______________________________________________
>>> TenTec mailing list
>>> TenTec@contesting.com
>>> http://lists.contesting.com/mailman/listinfo/tentec
>>>
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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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