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Balun help needed..




 
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k4kyv
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Don
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« Reply #50 on: January 12, 2011, 10:05:24 PM »

Don,
   You certainly are correct in pointing out the differences between the OCF Dipole and the “true” Windom.  Unfortunately, this is not how these dipoles are being described and “marketed” to the amateur community.  


Same thing with "tuned feeders."   The term seems to have lost its original meaning.   If you go back and look in 1930s Handbooks a true tuned feeder is a parallel wire feed of guage, spacing and length so that it matches the feedpoint Z of its load (usually a center fed dipole of some length) to the balanced ouput network of the transmitter.   Now most hams say they have "tuned feeders" and it turns out they mean they just have a balanced tuner like a Matchbox and open wire feedline.    

Rob

"Tuned feeders" means just that - an open wire feedline terminated into an unmatched load, like the midpoint of a dipole or the end of a halfwave long piece of wire (aka end-fed Zepp). By definition, tuned feeders have standing waves. The length of the feeders determines the impedance that the tuner works into. The OWL is usually fed by a balanced tuner; parallel tuned circuit to work into a high-Z load, series tuned circuit to work into a low-Z load, or tapped down on the coil for intermediate Z.

Parallel wire feeders of gauge and spacing so that the line matches the feedpoint Z of its load form an "untuned" or "flat" feed line. It has uniform rf voltage and current along its entire length, and no standing waves. IOW, the SWR is 1:1.  The match is completely independent of the length of the feeders.

Open wire feeders can be run as a flat or as a tuned line.  Coax is normally run as a flat line, since operating it as a tuned line usually results in excessive loss.
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Don, K4KYV                                       AMI#5
Licensed since 1959 and not happy to be back on AM...    Never got off AM in the first place.

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This message was typed using the DVORAK keyboard layout.
http://www.mwbrooks.com/dvorak
W2VW
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« Reply #51 on: January 13, 2011, 04:36:08 AM »

Yea, 20 Amps, until it melts.   Tongue

One in the same. Loss is independent of the amount of current. And the difference is neglible with the loads specified - even with your 20 Amp example.

I'd expect plenty of delta T with 20 amps.
And warm copper has more loss than cold copper.
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KA2QFX
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Mark


« Reply #52 on: January 13, 2011, 12:53:22 PM »


Yea, 20 Amps, until it melts.   Tongue

One in the same. Loss is independent of the amount of current. And the difference is neglible with the loads specified - even with your 20 Amp example.

I'd expect plenty of delta T with 20 amps.
And warm copper has more loss than cold copper.
If you use oxygen-free copper much of the losses can be offset to your wallet. Smiley
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The Slab Bacon
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« Reply #53 on: January 13, 2011, 01:58:29 PM »

If you use oxygen-free copper much of the losses can be offset to you wallet. Smiley

Geeze, dont you know anything?? It has to be vacuum encapsulated, cryogenically treated,
oxygen-free copper to work properly!!    Shocked  Shocked  Wink  Wink  Grin  Grin  Grin  Grin  Grin  Grin  Grin  Grin
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"No is not an answer and failure is not an option!"
Steve - K4HX
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« Reply #54 on: January 13, 2011, 04:31:18 PM »

How much more in an eight foot section?

Yea, 20 Amps, until it melts.   Tongue

One in the same. Loss is independent of the amount of current. And the difference is neglible with the loads specified - even with your 20 Amp example.

I'd expect plenty of delta T with 20 amps.
And warm copper has more loss than cold copper.
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W2VW
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« Reply #55 on: January 14, 2011, 04:55:33 AM »

How much more in an eight foot section?

Yea, 20 Amps, until it melts.   Tongue

One in the same. Loss is independent of the amount of current. And the difference is neglible with the loads specified - even with your 20 Amp example.


I'd expect plenty of delta T with 20 amps.
And warm copper has more loss than cold copper.

Good question. How much does it take to make the center conductor hot enough to migrate through the insulation and unsolder connectors?
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k4kyv
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Don
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« Reply #56 on: January 14, 2011, 10:27:58 AM »

If you use oxygen-free copper much of the losses can be offset to you wallet. Smiley

Geeze, dont you know anything?? It has to be vacuum encapsulated, cryogenically treated,
oxygen-free copper to work properly!!    Shocked  Shocked  Wink  Wink  Grin  Grin  Grin  Grin  Grin  Grin  Grin  Grin

I can just see it now...

"High end" low-loss coax @ $550/ft, 6" ceramic OWL insulators @ $175 each and oxygen-free cryogenically treated #12 copper wire @ $150/ft.  Of course, the coax and OWL will have to be broken in before the full benefits are realised, but following the break-in period, the improvement in signal strength and modulation quality will be nothing less than astounding.

But wait.  For only $3100 you can buy an instant rf "breaker-inner" that will allow you to avoid the long break-in period.  It works by generating up to 20 amps average rf current, with a proprietary combination of phase/amplitude modulation of a proprietary waveform, successfully circumventing the classic laws of physics to allow pulses at over 50 kw peak power to pass through the line without endangering the dielectric. For best results, it is recommended that each cable or line be treated for three continuous hours before attachment to the antenna.

High end phosphor-bronze antenna wire, only $300/ft.

Low-loss ceramic antenna insulators from Germany, $475 each.

D-X antenna grease, $135/oz.
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Don, K4KYV                                       AMI#5
Licensed since 1959 and not happy to be back on AM...    Never got off AM in the first place.

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http://www.mwbrooks.com/dvorak
Jim KF2SY
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« Reply #57 on: January 14, 2011, 01:05:40 PM »

Don,
   You certainly are correct in pointing out the differences between the OCF Dipole and the “true” Windom.  Unfortunately, this is not how these dipoles are being described and “marketed” to the amateur community.  Not unlike the junque merchants blurring the difference between a diplexer and a duplexer.  But I digress…

   Perhaps the more important result of your distinction is that the true Windom presents a reactive load of reasonable magnitude which a Pi network can match;  having thus tuned the system to resonance with the accompanying benefits.

   Whereas with the OCF dipole the desire is to force the feed end to approximate 50 ohms resistive and pretty much ignore the reactance at the feedpoint of the antenna.  Compounding that, the antenna doesn’t resonate exactly on the desired harmonics, so  achieving any effective match through the balun is highly unlikely.  IMHO 

   The published results for these OFC dipoles leads me to believe that their broad-banded  response is more the result of system loss than signal production.



Mark,
Can't the broadbanded nature of OCF antenna be because of the lower Q of the antenna?
(Perhaps instead of losses as you said.)

The OCF's dipoles have much higher feedpoint impedances versus a center fed dipole.  Thus when you change frequency, the corresponding change in feedpoint impedance will be a much smaller change in percentage of feedpoint impedance versus freq,/feedpoint changes in lower Q center-fed antennas. Huh

Maybe these ratios are the same, but I doubt it.   Wonder what with EZNEC modeling, the differences would reveal.   


Disclosure:
Using a homemade OCF dipole fed with ladder line since late 2005.
Couple the transmitters to a HB Link Coupled Tuner via a coax 1:1 current choke balun. 
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KA2QFX
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Mark


« Reply #58 on: January 14, 2011, 10:12:25 PM »

Hey Jim,

Lower Q,wider apparent bandwidth, correct. Lower Q = lower ratio of reactance to  resistance, generally, that is coincident with higher loss.   IOW, it takes a wider frequency excursion before the X of the antenna = the R of the system, where that 45 degrees phase shift occurs and response drops 3dB.
 
From the EZNEC modeling I've done using published dimensional data for several "Windom" recipes I see no evidence that this is a "broad-band" antenna.  Being fed off-center (at neither a node or anti-node) does give it the ability to develop resonant modes for both odd and even harmonics of it's half-wave fundamental.  However, those harmonics do not exhibit significantly broader characteristics than would be expected using any other feed.   

By the way, you mention you are using a 1:1 balun, feeding a link coupled tuner? Why? The Link coupled tuner IS a balun as well as having the ability to provide the reactive match to the OCF feedpoint reactance.  I'd lose the balun and and just go with the tuner alone. You'll probably little see a sharper tuning and the whole system will work a little better. IMO.

Overall, except for the inherent imbalance of an antenna with different length arms, your tuner and OWL feed is significantly better than the balun at the antenna technique being discussed in this thread.   But Don is right in the fact that the original single wire feed Windom will likely produce the most efficient match using a single ended tuner worked against ground.

Mark
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Tom WA3KLR
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« Reply #59 on: January 15, 2011, 05:36:21 PM »

To put comments from this thread other relevant threads of the past together in one concise suggestion:

The off-center antenna you stated is 266 feet overall, half-wave resonant at 160 meters.  Skip the OCF design and have it center-fed instead, with ladder line, no balun at the feed point, and the tuner in the shack be a balanced design or as Don said an unbalanced tuner with a 1:1 balun between it and the transmitter. 

The chassis of an unbalanced tuner fed with the 1:1 choke balun needs to be kept isolated from the transmitter/station ground if the antenna tuner common ties to it’s chassis (which it probably does).
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73 de Tom WA3KLR  AMI # 77   Amplitude Modulation - a force Now and for the Future!
W2VW
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« Reply #60 on: January 16, 2011, 11:25:56 AM »

Thanks Tom. Because of your post I've abandoned my off center fed bird feeder. I simply threw the bird seed on the snow.

To put comments from this thread other relevant threads of the past together in one concise suggestion:

The off-center antenna you stated is 266 feet overall, half-wave resonant at 160 meters.  Skip the OCF design and have it center-fed instead, with ladder line, no balun at the feed point, and the tuner in the shack be a balanced design or as Don said an unbalanced tuner with a 1:1 balun between it and the transmitter. 

The chassis of an unbalanced tuner fed with the 1:1 choke balun needs to be kept isolated from the transmitter/station ground if the antenna tuner common ties to it’s chassis (which it probably does).

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