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Author Topic: Balun help needed..  (Read 34394 times)
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K1JJ
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« Reply #25 on: January 10, 2011, 12:50:29 PM »

There is nothing magical about a 4:1 balun.

This is a job for Super Balun!  It's time to tell them about Taylor hybrids, Steve.   Wink


T
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« Reply #26 on: January 10, 2011, 01:07:48 PM »

Yep, you all got it right. If the load is well out of balance or quite reactive a balun won't help the problem. But for Dave's application, feeding an unbalanced point on an OFC antenna, how would you suggest he match the alledged 200 ohm (reactive) impedance and prevent unbalanced currents on his feedline? 

I have issues with the whole idea that the Windom is a good solution as multiband antenna, but besides that, my suggestion of using the 4:1 Guanella balun with separate cores seems to be the least evil.

Any concensus here?
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The Slab Bacon
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« Reply #27 on: January 10, 2011, 01:43:47 PM »

Until you try to put 20 R.F. amperes through it.
[

Been there, done that  Grin  Grin Have fried PL-259s right off of the coass and sent chassis mount SO-239s to the happy hunting ground!!  (usually on 160)

But, you know what, it beats the hell out of not being on the air, or being on and being piss-weak.  Grin  Grin
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« Reply #28 on: January 10, 2011, 06:05:55 PM »

There's no correct way to feed a "Windom" fed with balanced line.

I'd either give up and make it a center fed dipole or run a single wire feed against some ground radials.

 
Yep, you all got it right. If the load is well out of balance or quite reactive a balun won't help the problem. But for Dave's application, feeding an unbalanced point on an OFC antenna, how would you suggest he match the alledged 200 ohm (reactive) impedance and prevent unbalanced currents on his feedline?  

I have issues with the whole idea that the Windom is a good solution as multiband antenna, but besides that, my suggestion of using the 4:1 Guanella balun with separate cores seems to be the least evil.

Any concensus here?
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K1JJ
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« Reply #29 on: January 10, 2011, 06:26:14 PM »

The single wire fed Windom is kinda like an end fed inverted L or quasi vertical T, in my book. It makes sense that a good radial field would enhance performance.

In the old days maybe Hiram would have called it a "bird cage without feathers."     Cheesy

T
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« Reply #30 on: January 10, 2011, 06:56:59 PM »

 I'm not surprised that you have trouble with the W2AU 4-1 balun with the ferrite rod as the core. However, I am somewhat surprised that you all appear to be unaware of the W2DU 1-1 balun, comprised simply of a 10.5" length of RG303 coax loaded with 50 #73 ferrite beads. Operates at ALL frequencies, and introduces more than 1000 ohms to the current that would otherwise flow on the outside of the shield of the feedline coax. You can either make one yourself, or obtain either a completed unit or one in kit form from the Wireman.

 I introduced this balun in a QST article in March 1983, and hundreds of them have been used ever since.

Incidentally, a balun by itself is simply a BalUn, which means balanced to unbalanced. If a balun such as the W2AU also changes the impedance, it's the impedance transformer that is part of the design of W2AU's balun that changes the impedance, not the balun by itself.

You can download a copy of the QST article from my web page at www.w2du.com. Click on 'Read Chapters from Reflections 2', and then click on Chapter 21.  I guarantee you'll find that chapter of interest,

Walt
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« Reply #31 on: January 10, 2011, 08:42:10 PM »

Walt I have two in the package if I ever need one. Nothing beats a coaxial transformer. I'm surprised you used such a high permability bead. I'm a real fan ot type 61 for low loss.
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« Reply #32 on: January 10, 2011, 09:25:19 PM »

There's no correct way to feed a "Windom" fed with balanced line.


an off-center fed wire antenna will give a 50 ohm match if you make it 1/4 wave on one leg and 3/4 wave on the other leg. This will give a 50 ohm match, but I dont think that is considered a "Windom", it also makes for a long antenna and is frequency specific. We used to put them up for the higher bands (shorter ants) for field day. They worked pretty well. But we fed them with coax. But I think the load looks "balanced" to the feeders.
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« Reply #33 on: January 10, 2011, 09:32:44 PM »

Read Witt's articles on coax feed for a Windom. Good stuff.
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« Reply #34 on: January 10, 2011, 10:01:25 PM »

Frank, #61 is relatively new--wasn't around when I developed the balun. At that time #73 was the best available then for 160 thru 40m. At that time #43 was the best available for HF freqs above 40m.

You might find it interesting how I got the idea for W2DU balun, as quoting a portion of Chapter 21 from Reflections:


Sec 21.11 The Labor Pains and Birth of the W2DU Ferrite-Bead Current Balun

    It is interesting to know that the original design of the W2DU ferrite bead balun is a spin-off from the development of antenna systems designed for spacecraft built by RCA’s Astro-Electronic Division, specifically the antenna system developed by the author for the World’s first weather spacecraft, TIROS 1, and its successors.
     Because the radiation patterns of the antenna system are vital to the successful operation of the spacecraft, many measurements of the patterns were required during the development stage to discover the correct physical and electrical properties of the antenna that would produce a satisfactory pattern while attached to the spacecraft.
     To obtain realistic radiation patterns of the antenna it must be mounted on the spacecraft during the measurements, because the spacecraft is in the radiation field of the antenna, and therefore distorts, or modifies the shape of the field that would be radiated from the antenna if it were operating in free space, i.e., without being mounted on the spacecraft.
     Consequently, to measure the patterns from all angles around the entire radiation sphere of the antenna, the spacecraft was mounted on a revolving pedestal having a freedom of motion that allows the spacecraft to be rotated on two separate quadrature-related axes, i.e., 90° between the axes.
     In the measurement setup used at RCA’s antenna test range, the entire spacecraft with its antenna was immersed in an electromagnetic field radiated from a ground-mounted log periodic antenna aimed at the spacecraft. The field comprised an RF signal modulated by a 1 KHz audio tone received by the spacecraft antenna. The received audio signal tone was conducted down the pedestal through a coaxial downlead cable and routed to the control room where it was recorded during the rotation of the spacecraft, recording the RF signal level.
     However, the vertically-oriented coax downlead distorted the radiation pattern when measuring either the vertical or circular polarization of the spacecraft antenna, but not when measuring with horizontal polarization. It was thus obvious that because the downlead was also immersed in the RF field, re-radiation from the downlead as a second source of RF, was distorting the field illuminating the spacecraft antenna and thus distorting its radiation pattern. Proof that the downlead was the culprit was obtained when manually moving the downlead three or four inches in any direction caused a variation in pattern level greater than 3 dB in any antenna orientation where the pattern level was a few dB below the maximum. Unacceptable.
     Consequently, to obtain true radiation patterns from the spacecraft antenna it was necessary to eliminate radiation from the downlead. I knew that a conductor of length /4 or less could not sustain an RF current, but at the VHF/UHF frequencies involved in the measurements the downlead was several wavelengths long. One way to solve the problem would be to break the downlead into individual lengths, /4 or shorter, and connect them with resistors that would effectively impede the RF, but allow the audio to travel. But the mechanical construction for this solution seemed impractical.
     During this era a new method of restricting flow of RF current on conductors was coming into vogue with the use of ferrites. Ferrite beads placed around a conductor allowed DC to flow, but restricted the flow of RF. I contemplated what would happen if I were to place an appropriate bead around the downlead at every /4 point along the coax at the measurement frequency. This arrangement would effectively break the coax into /4 RF sections electrically, but leave it intact physically. So I experimented with No. 43 bead material placed around the RG-58 cable at /4 intervals, and voila’—no more radiation from the downlead and thus no more distortion of the radiation pattern from movement of the downlead. Accurate radiation patterns from the spacecraft antennas at last!
     Several months later, while listening to on-the-air discussions of problems that occurred when using wire-wound voltage baluns constructed around a ferrite core, a light bulb turned on in my mind. My immediate thought was, if beads impeded current flow on the downlead in the radiation pattern measurement setup, why wouldn’t it also impede common mode current flow on the outside of the coax feed line resulting from the balanced input of the antenna terminating the unbalanced coax? Why not indeed! I knew current was flowing on the outside of my feedline, because when measuring the impedance at the input terminals of the line using the General Radio GR-1606-A RF impedance bridge, the indicating null would disappear while running my fingers along the line. I knew from those symptoms that the common mode current on the feedline was also destroying the accuracy of the impedance measurements.
     I then reasoned that a bead resistance of at least ten times the impedance looking into one half of the dipole should reduce the current flow on the outside of the coax shield to one-hundredth of the power delivered to the dipole half connected to the shield, an insignificant amount.
     After researching the various terminal impedances that would be encountered with dipoles throughout the HF bands the worst case situation appeared to occur on the 75-80 m band, when operating at the low end of the band at 3.50 MHz with the antenna resonant at 3.75 MHz. At 3.778 MHz the terminal impedance of my nearly resonant 125-ft dipole was 64.83 + j0.18 ohms, for a 1.28 SWR. However, at 3.50 MHz the terminal impedance was 53.17 – j144.35 ohms, for a 9.6 SWR. (These values can be seen in Table 15-5, Page 15-14, and from Fig 15-1 on Page 15-15.) The magnitude of this impedance is 153.8 ohms at 69.8°. However, this is the total input terminal impedance of the dipole, while the dipole half fed by the shield, or outer conductor of the coax is only one half of this value, or 76.9 ohms. Thus a bead resistance of approximately 800 ohms should provide adequate reduction of common mode current on the outside of the feedline in this worst-case situation.
     After studying the specifications of several ferrite beads I ordered 300 No. 73 beads from The Wireman for experimentation. A brief report of some of the experimental data that led to the design of the commercial version of the W2DU balun are shown in Sec 21.6, Page 21-7. Additional data can be seen in Fig 21-3, Page 21-8, which shows the impedance, resistance, and reactance of 50 No. 73 beads versus frequency. Observe that the bead impedance at 3.50 MHz is slightly greater than 1000 ohms, amply sufficient to reduce the common mode current on the feedline of the 80-m dipole to insignificance.
     In addition to showing the impedance plot of my 80-m dipole from 3.45 to 4.075 MHz, the graph of Fig 15-1 also provides evidence that the common mode current on two different coaxial feedlines is insignificant when the 50-bead W2DU balun is inserted between the feedline and the antenna. As mentioned earlier, common mode current on the outer surface of the coax will destroy the accuracy of any measurement of impedance at the input of the coax. When measuring the terminal impedance of an antenna by measuring the impedance at the input of its calibrated feedline, the same impedance reading will be obtained regardless of the length of the feedline as long as its calibration is accurate. However, if a common mode current exists on the outer surface of the coax, different input terminal impedances will result if the impedance is measured using coax of different lengths, even though both are accurately calibrated. The greatest difference will prevail if the difference in lengths is /4.
     Observe that in Fig 15-1 the solid lines represent measurements of the resistance, reactance, and SWR of my 80-m dipole made with a /4 length feedline, while the dashed lines represent measurements made with a /2 length feedline. Observe also that the difference between the solid and dashed lines is almost non-existent, indicating insignificant errors in measurement, and showing negligible common mode current flowing on the lines, thus proving the effectiveness of the W2DU balun in eliminating the common mode current. Note that the dipole data measured with both the /4 and /2 length feedlines plotted in Fig 15-1 appears in Tables 15-4 and 15-5.
     Thus endeth the story of the evolution of the W2DU ferrite bead balun.
 

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Steve - K4HX
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« Reply #35 on: January 11, 2011, 12:34:30 AM »

Nah, no difference. 20 amps is 20 amps, regardless of the impedance. The coax doesn't know the difference, unless you exceed the dielectric breakdown!


Until you try to put 20 R.F. amperes through it.
[

Been there, done that  Grin  Grin Have fried PL-259s right off of the coass and sent chassis mount SO-239s to the happy hunting ground!!  (usually on 160)

But, you know what, it beats the hell out of not being on the air, or being on and being piss-weak.  Grin  Grin
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Steve - K4HX
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« Reply #36 on: January 11, 2011, 12:59:56 AM »

Some good info on Windoms here.
  
http://n9wn.com/data%20files/OCF%20Dipole.pdf
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« Reply #37 on: January 11, 2011, 03:40:03 AM »

The heck it doesn't.

Nah, no difference. 20 amps is 20 amps, regardless of the impedance. The coax doesn't know the difference, unless you exceed the dielectric breakdown!


Until you try to put 20 R.F. amperes through it.
[

Been there, done that  Grin  Grin Have fried PL-259s right off of the coass and sent chassis mount SO-239s to the happy hunting ground!!  (usually on 160)

But, you know what, it beats the hell out of not being on the air, or being on and being piss-weak.  Grin  Grin
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KA2QFX
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Mark


« Reply #38 on: January 11, 2011, 11:33:48 AM »

There's no correct way to feed a "Windom" fed with balanced line.

I'd either give up and make it a center fed dipole or run a single wire feed against some ground radials.


I concur.

Hmmm, from Steve’s referenced article http://n9wn.com/data%20files/OCF%20Dipole.pdf. 5th page, Fig. 8a, a pair of 100 ohm transmission lines, wound through separate cores, connected to transform 50 ohms to 200 ohm.  Where have I seen that before?  
BTW: Walt is credited for that current balun design in the article.

I can’t see Witt’s stuff, it all seems to be “ARRL Members Only” access.  Homey don't do dat no mo'.

For the record:  If I had the room to put up the amount of wire needed for this Windom I'd still go with fan dipoles or a single center-fed driven with OWL and a link coupled tuna.  It seems from the models, and what I've read, that it ALMOST provides an efficient match and ALMOST hits the right harmonics on higher bands.  Just doesn't seem worth the bother for an ALMOST antenna.  IMHO

Mark

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WA1GFZ
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« Reply #39 on: January 11, 2011, 12:02:46 PM »

Me Too Mark
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k4kyv
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Don
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« Reply #40 on: January 11, 2011, 12:18:59 PM »

Regarding the Windom, this is not the same thing as the OCF dipole, although they work on the same principle: the feedpoint impedance of a dipole goes higher as you move away from the midpoint.  The extreme limit would be the end-fed Zepp.

The true Windom is fed with a single wire.  There is extensive discussion of  single open wire feed in the LaPort antenna book.  Yes, there is some radiation from the feeder and a good ground connection is needed, but not quite to the extent of what is needed for a vertical or inverted L or tee antenna. In the case of the Windom, the ground connection only needs to serve as the phantom "return" that is missing from single-wire feed.  Since the primary radiation is from the flat-top, just as in the case of a conventional dipole, the ground system does not have to serve as a shield to isolate the radiator from the lossy earth, as is the case of the vertical (including the ground-mounted half-wave vertical, which will resonate and take power with no ground system at all, but waste 80% of that power warming the earthworms).

The difference between a Tee / inverted L, and a Windom, is that with the former, the main radiator is the vertical "feed line", whereas with the latter, the main radiator is the flat-top.  If the Windom is adjusted properly, there should be no standing waves on the single-wire feeder, and at its resonant frequency the signal at the other end would be all but indistinguishable from that of a conventional coax or open wire fed dipole of the same height and dimensions, assuming proper tuning to resonance in each case.
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Steve - K4HX
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« Reply #41 on: January 11, 2011, 01:21:01 PM »

How so? Both conditions have 20 Amps flowing through the conductors.


The heck it doesn't.

Nah, no difference. 20 amps is 20 amps, regardless of the impedance. The coax doesn't know the difference, unless you exceed the dielectric breakdown!


Until you try to put 20 R.F. amperes through it.
[

Been there, done that  Grin  Grin Have fried PL-259s right off of the coass and sent chassis mount SO-239s to the happy hunting ground!!  (usually on 160)

But, you know what, it beats the hell out of not being on the air, or being on and being piss-weak.  Grin  Grin
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« Reply #42 on: January 11, 2011, 01:56:37 PM »

It's 20 amps throughout if the coax is terminated in it's surge impedance. If it isn't the current and voltage will vary along the cable's length.

20 amperes of M.W. or H.F. R.F. it too much to stuff through a 13 gauge piece of stranded copper enclosed in 4 tenths of an inch of insulation and braid. Try it sometime.

 
How so? Both conditions have 20 Amps flowing through the conductors.


The heck it doesn't.

Nah, no difference. 20 amps is 20 amps, regardless of the impedance. The coax doesn't know the difference, unless you exceed the dielectric breakdown!


Until you try to put 20 R.F. amperes through it.
[

Been there, done that  Grin  Grin Have fried PL-259s right off of the coass and sent chassis mount SO-239s to the happy hunting ground!!  (usually on 160)

But, you know what, it beats the hell out of not being on the air, or being on and being piss-weak.  Grin  Grin
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Steve - K4HX
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« Reply #43 on: January 11, 2011, 02:05:07 PM »

That's my point. 20 Amps is a problem, regardless the impedance. At 50 Ohms, 20 Amps is 20 kW. It should be obvious RG-213 won't take this!   Grin
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KA2QFX
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« Reply #44 on: January 11, 2011, 02:59:50 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.
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« Reply #45 on: January 12, 2011, 12:14:24 PM »

That's my point. 20 Amps is a problem, regardless the impedance. At 50 Ohms, 20 Amps is 20 kW. It should be obvious RG-213 won't take this!   Grin


Sorry, I thought this was your point:

For purely resistive loads, on RG-213, you get about 0.75 as much loss for 200 Ohm load at you do with 10 Ohm load. On an 8 foot piece of cable at 4 MHz, this difference is negligible.
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Steve - K4HX
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« Reply #46 on: January 12, 2011, 02:50:26 PM »

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.
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« Reply #47 on: January 12, 2011, 06:00:29 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
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« Reply #48 on: January 12, 2011, 07:10:43 PM »

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.
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Steve - K4HX
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« Reply #49 on: January 12, 2011, 10:00:55 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.
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