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Author Topic: Balun for OCF Dipole  (Read 4620 times)
K1HH
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Posts: 7




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« on: June 11, 2010, 10:53:48 AM »

I've been building a 4 to 1 Guanella type balun on a single FT-140-61 core.  I hope to use this balun with a 40 meter OCF dipole for Field Day.

I've built two models, one per Sevick, using #20 solid wire and seven turns on each half of the toroid, one clockwise, the other CCW. The second balun uses an identical number of turns but uses some 18 ga. speaker/zip cord. Primary side windings are paralled; secondary side is in series.

Running an MFJ-249 as the primary side signal source and terminating the secondary side with a 200 ohm resistor I do NOT get particulary low SWR readings at 3.75 MHZ. It runs about 2.3 to 1.  Forty meter readings are about 1.6, 20 meters 1.2 to 1.4, 15 meters 1.2, 10 meters 1.1 to 1.5, and 6 meters 1.3 or so.
The variants on a given band above are from test of each separate balun.

Are these readings in the ballpark? What can be done to improve low frequency response? I have larger core materials (FT-200-2), but was looking to use a smaller, lighter weight unit for FD.

Respectfully submitted,

Rodger
K1HH



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WX7G
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« Reply #1 on: June 11, 2010, 11:33:39 AM »

To improve 80 meters use 10 turns rather than 7.
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K1HH
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Posts: 7




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« Reply #2 on: June 11, 2010, 01:24:18 PM »

Thanks Dave. 

I'll give that a try. I actually had enough lead length on the 20 ga wire model to make one more turn, but at 8 I didn't see much change. I'll probably have to use smaller wire to get 10 turns on each half of the core.

R.
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G3TXQ
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Posts: 1453




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« Reply #3 on: June 11, 2010, 01:34:17 PM »

Rodger,

A few things to be aware of with a 4:1 Guanella balun:

1) If you want it to act as a current balun you MUST wind the 2 chokes on separate magnetic circuits - not on a single core. Wound on a single core it acts more like a voltage balun.

2) Unlike a 1:1 current balun, the transmit signal at the input to a 4:1 current balun ALWAYS appears as a common-mode voltage across the choke windings, whatever the load configuration. That might mean you need a larger core, depending on the power you are running.

3) #61 material is a poor choice for your application; even 16 turns on a FT140-61 core has too little choking impedance on 40m:
http://www.karinya.net/g3txq/chokes/

73,
Steve G3TXQ
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K1HH
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Posts: 7




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« Reply #4 on: June 12, 2010, 11:37:36 AM »

I wound a 10 turn unit today, and the frequency response has improved greatly on 75M.
The unit runs from 7 to 30 mhz between 1.3 and 1.4 swr; 75M at about 1.55 and 6M at 1.6. I want to use this as a balun for a 40M OCF dipole for field day.

I spent quite a bit of time last night on the internet reviewing different author's work with baluns. There are lots of varying opinions from some well known hams.

I do not have Guanella's original work, and I don't know if he envisioned a 4 to 1 balun on a single core, but Sevick did and other authors have built such units and placed articles of their construction  on the internet.

My question now is, if this device is not a Guanella current balun, what is it and does it work?

Thanks to the group.

Rodger
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G3TXQ
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« Reply #5 on: June 12, 2010, 12:36:12 PM »

My question now is, if this device is not a Guanella current balun, what is it and does it work?

Rodger,

Take a look at Fig 1 here:

http://www.karinya.net/g3txq/temp/4%20to%201%20current%20balun%207.jpg

Notice that the common-mode voltages across the two chokes must always equal the input voltage. That is:

Vcm1 + Vcm2 = Vin

Also notice that the voltage at load terminals A & B must be:
VA = Vin + Vcm1
VB = Vin - Vcm2

If we now constrain Choke 1 and Choke 2 to share the same magnetic circuit, we force their common-mode voltages to be identical, so:

Vcm1 = Vcm2 = Vin/2

and no matter what the load configuration the balun will try to force:

VA = Vin + Vcm1 = Vin + Vin/2 = 1.5 Vin
VB = Vin - Vcm2 = Vin - Vin/2 = -0.5 Vin

What we have created is a "Voltage UnBalun" which will try to force the voltages at the load terminals to 1.5 Vin and 0.5 Vin with respect to ground.

Steve G3TXQ
« Last Edit: June 13, 2010, 04:31:03 AM by Steve Hunt » Logged
K1HH
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Posts: 7




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« Reply #6 on: June 12, 2010, 03:37:47 PM »

Steve:

I have read and enjoy the math. I've read yours, and Rauch's, and Trask's and others.
Its interesting to note that Sevick talks about how a Ruthroff (voltage) balun acts like a Guanella (current) at times but doesn't mention the reciprocal. He also notes that there has been a lack of standardization in the literature. That certainly is true. As an example 1 to 4 or 4 to 1. Which is the primary....the radio or the antenna side? While I own a large 4 to 1 Balun-Designs and a large 1 to 1 DX Engineering balun, I thought this little device would be an easy device to accomplish impedance transformation and current balancing for a Field Day low power (100W) OCF dipole. Since I have the cores, I can certainly wind a dual core model too.

Thank you for your explanations and advice.

Rodger
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N4CR
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« Reply #7 on: June 12, 2010, 07:37:13 PM »

We had an extensive discussion of why two cores are needed here:

http://www.eham.net/ehamforum/smf/index.php/topic,41385.0.html

I was a little thick, but eventually the light came on.
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73 de N4CR, Phil

We are Coulomb of Borg. Resistance is futile. Voltage, on the other hand, has potential.
G3TXQ
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Posts: 1453




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« Reply #8 on: June 13, 2010, 02:06:48 AM »

Rodger,

A Voltage Balun will drive equal currents into the two legs of a load if the load is "floating" with respect to ground and there is no common-mode path. It has to, because the current that goes into one leg of the load must flow out of the other leg - there's nowhere else for it to go!

Therefore Trask is correct when he says that a 4:1 Guanella balun on a single core drives balanced currents into a floating load, but so does a 4:1 voltage balun, and *it* requires only one bifilar winding on the single core; what's more, the 4:1 Voltage Balun drives balanced voltages wrt ground which the 4:1 Guanella on a single core does not.

I never understood Trask's strange position on this, particularly given this statement from him in his paper on Transmission Line Transformers:

"In general, the Guanella 1:4 impedance transformer requires that each of the transformer sections be constructed on separate cores as the voltages are dissimilar even though the currents are the same."

How true!!

73,
Steve G3TXQ
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K1HH
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Posts: 7




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« Reply #9 on: June 13, 2010, 03:02:12 PM »

Thanks Phil for the link. A lot of good reading and its good to know I'm not the only person with these questions.

Steve, I also reread Sevick's book and now find his comments about two windings on a single core working only in certain instances depending on whether the load is floating, grounded at center, or grounded at bottom (Unun).

Whatever the device is...two windings on a common toroid...it continues to be propogated in the literature and sold by various online vendors as a current balun.

I'll whip up a two toroid version tomorrow, plenty of time for Field Day.

Thanks and 73s,

Rodger
K1HH
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NR4C
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Posts: 306




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« Reply #10 on: June 14, 2010, 06:09:06 AM »

For a bit more data, check out the balun design by N1LO at:  http://k4rc.net/Newsletter.htm/ and check out the link under 2008 labeled "BALUN Plan".  This is a two core, switchable 4:1/1:1 balun.  It is very simple to make, and works well with my OCF 80 meter antenna.  It is sugested to use 10 turns rather than 9 as the nine turn version has a little 'blip' in the 17 meter band, 10 turns moves it below the 17 meter band.

...bc   nr4c
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G3TXQ
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Posts: 1453




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« Reply #11 on: June 14, 2010, 07:43:19 AM »

For a bit more data, check out the balun design by N1LO at:  http://k4rc.net/Newsletter.htm/ and check out the link under 2008 labeled "BALUN Plan".  This is a two core, switchable 4:1/1:1 balun.  It is very simple to make, and works well with my OCF 80 meter antenna.  It is sugested to use 10 turns rather than 9 as the nine turn version has a little 'blip' in the 17 meter band, 10 turns moves it below the 17 meter band.

...bc   nr4c

I never understood that arrangement - why you would switch the two chokes in parallel for a 1:1 balun, rather than just use one? We go to all the trouble of trying to wind a choke with a high common-mode impedance, and then throw half of it away by connecting two in parallel Smiley

I could understand it if it was important to maintain a winding characteristic impedance of 50 ohms, but the article stays the intended application is a multiband antenna fed with parallel line, in which case the winding impedance isn't at all critical.

I believe that Elecraft do the same in their B2 balun kit.

What is it these guys know that I'm missing?

73,
Steve G3TXQ
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K1HH
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Posts: 7




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« Reply #12 on: June 14, 2010, 12:44:28 PM »

Thanks Ron for the links. Its a nice Newsletter too.  I can while away time reading each issue.

Steve.  Elecraft and even MFJ (I think) have made switchable baluns. It may be for an alleged convenience for the customer or a marketing reason, and does not imply that they know any more than you. Remember, other people do things for their reasons, not yours.

I believe the Elecraft even uses a binocular core for their balun.

I have found some internet vendors selling the "two-windings on one FT-140-61 core and capable of full legal limit power."

Methinks there's some smoke being blown, and most assuredly some smoke might be let out.

Rodger
K1HH
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G3TXQ
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« Reply #13 on: June 14, 2010, 01:01:58 PM »

I believe the Elecraft even uses a binocular core for their balun.

Rodger,

The idea of using a binocular core is sound. Unlike the toroid it's able to support different flux levels in the two chokes because the magnetic path is not common.

73,
Steve G3TXQ
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