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Author Topic: Will a Balun work "backwards?"  (Read 6981 times)
KM3K
Member

Posts: 279




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« Reply #30 on: July 12, 2010, 01:08:47 PM »

To Len L6LHA,

Thanks, Len, for the elucidating commentary in reply #28.

I want to call attention to the opening sentence in my reply #21, which I'll repeat here, "In the spirit of the hobby, I pass along what I have learned and don’t profess to know all the answers."
 
Like the TV personality, Bill O'Reily, I'm a simple man.
So, I'm still searching for the perfect book on the topic of "baluns and ununs".
To me, it seems like you are the man to put that book together.
Here could be the title, "Baluns/Ununs for Dummies".
From all accounts, the "Dummies" books are re-writes of common-knowledge stuff and yet they sell in incredible quantities.

Peace!!

73 Jerry KM3K

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K6LHA
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Posts: 349




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« Reply #31 on: July 14, 2010, 01:04:33 PM »

To Len L6LHA,

Thanks, Len, for the elucidating commentary in reply #28.

I want to call attention to the opening sentence in my reply #21, which I'll repeat here, "In the spirit of the hobby, I pass along what I have learned and don’t profess to know all the answers."

Then try not to state some facts AS IF they were facts when they are wrong.

Quote

Like the TV personality, Bill O'Reily, I'm a simple man.
O'Reilly is far from "simple" and political and egotistical to a fault.

Quote
So, I'm still searching for the perfect book on the topic of "baluns and ununs".
To me, it seems like you are the man to put that book together.
No, you are just pissed off because you didn't get the praise you thought you deserved.  Chances are you've never actually built any balun and certainly haven't tested one on the bench with instruments...after verifying the test set-up error margins.  Come back when you've actually done some work on the subject instead of mouthing-off some opinions after just looking at ads.

K6LHA
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N2EY
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Posts: 3842




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« Reply #32 on: July 14, 2010, 03:06:20 PM »

Yes

Wow, not very much negotiating room there.
Almost makes me ask myself what part of yes don't I understand. Wink

Well, I was trying to be brief...

Just a note to the cautious reader about using a balun backwards...

Dr. Sevick W2FMI (SK) has categorically written about the 1:4 Ruthroff-balun (voltage-balun):
"The Ruthroff balun only works in one direction. The high-impedance side is always the balanced side. The Guanella balun, on the other hand, is bilateral. It can work as well in either direction, depending on which terminal is grounded. Therefore, a Guanella balun can easily be designed to match a 50-ohm coaxial cable to a 12.5-ohm balanced load."

With that in mind, I still wonder though what feature of a 1:4 Ruthroff-balun (voltage-balun) suffers when using it "backwards".

I think what W2FMI was trying to say is this:

The Ruthroff design always results in the high-impedance end being the balanced end. You can't get an impedance step-down with a Ruthroff by changing where you connect the ground.

But with some Guanella designs, you can get an impedance step-up or step-down by changing the grounding point.

IOW, he wasn't talking about the flow of energy not being reversible. He was talking about the impedance ratios.

----

Now for something related but different.

ISTM that the basic difference between the Ruthroff "voltage balun" concept and the Guanella "current balun" concept, *as applied to amateur HF baluns* is this:

The Ruthroff voltage balun designs are such that a large part (or all) of the energy being transferred from the unbalanced side to the balanced side, or vice-versa, has to go through the magnetic balun core. Whether the core is air, ferrite, or powdered iron, a solenoid or a toroid doesn't matter, lots of the RF has to go through it in a Ruthroff voltage balun.

But the Guanella current balun designs are such that they don't require a large part of the energy being transferred from the unbalanced side to the balanced side, or vice-versa, to go through the magnetic balun core. The core only exists to reduce common-mode RF.

This is what I think you were trying to say about flux-linkages and such in a TLT.

For example, consider the classic 4:1 Rothroff balun that consists of a bifilar wire winding on a powdered-iron toroid core, with the windings connected series-aiding and the center-tap so produced being grounded. It's the kind of balun commonly supplied and used with amateur-radio HF Transmatches of many kinds for at least 40 years.

In such a balun, the unbalanced side is connected to ground and one ungrounded winding end, while the balanced side is connected across both ungrounded winding ends. The 4:1 ratio results from the 2:1 turns ratio squared. The only way for the desired RF energy to get from one ungrounded winding end to the other is through the balun winding and core. Otherwise there's no path.

The Rothroff balun under consideration deals with common-mode *voltages* on the balanced side by shorting them to ground. This happens because the bifilar winding impedances essentially cancel each other out in the common mode.  

Now consider the Guanella balun that consists of a coax cable winding on a powdered-iron or ferrite toroid core. It's the kind of balun you showed in the links.

In such a balun, the desired RF energy gets from one end to the other *inside*the coax. The balun core has no effect because it's outside the coax.

The Guanella balun under consideration deals with common-mode *currents* on the balanced side by presenting them with a very high impedance to ground. This happens because the common-mode currents have to deal with the impedance produced by winding the coax on the toroid core.  

Which balun to use depends on the application.

----

Some fun facts:

- Coax was in use by hams even before WW2. There was even a QST article (Feb 1938, by W2BZR) about making your own air-insulated rigid coax from copper pipe and common hand tools.

- In the 1950s B&W sold air-core balun coils for HF amateur use. (QST, June 1957, article by W6EBY shows a typical application.) Heath made a kit (B-1) based on them. They consisted of a pair of solenoid coils with bifilar windings, which could be connected for 1:1 or 4:1 ratio. By the 1960s hams were using "iron core" (mostly powdered iron) baluns for such applications.

73 de Jim, N2EY
« Last Edit: July 14, 2010, 03:17:09 PM by James Miccolis » Logged
KM3K
Member

Posts: 279




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« Reply #33 on: July 22, 2010, 12:31:49 PM »

Hello Jim N2EY, here are some comments by me about posting #32 for us to talk about....

Quote
But with some Guanella designs, you can get an impedance step-up or step-down by changing the grounding point.
KM3K: replace “some” with “ratios other than 1:1”.  This is true for a floating-load.

Quote
IOW, he wasn't talking about the flow of energy not being reversible.
He was talking about the impedance ratios.
KM3K: I think you may be right; someday I’m gonna get to wind a Ruthroff-balun and measure it just to see how much the ratio is affected.

Quote
Now for something related but different.
ISTM that the basic difference between the Ruthroff "voltage balun" concept and the Guanella "current balun" concept, *as applied to amateur HF baluns* is this:
The Ruthroff voltage balun designs are such that a large part (or all) of the energy being transferred from the unbalanced side to the balanced side, or vice-versa, has to go through the magnetic balun core.
Whether the core is air, ferrite, or powdered iron, a solenoid or a toroid doesn't matter, lots of the RF has to go through it in a Ruthroff voltage balun.
KM3K: This is true when the coil’s inductive-reactance is too low to prevent the undesired and potentially harmful conventional-transformer-current from flowing.
We can expect, at low-frequency, to have conventional-transformer-current in both style baluns (more so with the Ruthroff-balun) and there is little that can be done to prevent it at low-frequencies.
The coil’s inductive-reactance can be increased by adding more turns and/or by using a higher permeability and/or by increasing the core’s cross-section area (eg. stacking cores).
It is desired for both the Guanella (current) and Ruthroff (voltage) baluns to transfer energy by transmission-line mode.

Quote
But the Guanella current balun designs are such that they don't require a large part of the energy being transferred from the unbalanced side to the balanced side, or vice-versa, to go through the magnetic balun core.
The core only exists to reduce common-mode RF.
KM3K: IMHO the word “only” should be removed.
Sevick has a plot showing a core greatly improves the overall frequency-response but inexplicably he does not explain the mechanism to account for this improvement.
I'd like to know why this improvement happens.
Although I've had a course in "EM-fields" (and passed it Smiley), I feel I don't have the smarts to figure this out.

Quote
This is what I think you were trying to say about flux-linkages and such in a TLT.
For example, consider the classic 4:1 Rothroff balun that consists of a bifilar wire winding on a powdered-iron toroid core, with the windings connected series-aiding and the center-tap so produced being grounded.
It's the kind of balun commonly supplied and used with amateur-radio HF Transmatches of many kinds for at least 40 years.
KM3K: OK; I have no experience with this. 

Quote
In such a balun, the unbalanced side is connected to ground and one ungrounded winding end, while the balanced side is connected across both ungrounded winding ends.
The 4:1 ratio results from the 2:1 turns ratio squared.
The only way for the desired RF energy to get from one ungrounded winding end to the other is through the balun winding and core. Otherwise there's no path.
KM3K: This is a less efficient way (means it loses power) because it uses flux-linkages.
A better way is to use the transmission-line mode.

Quote
The Rothroff balun under consideration deals with common-mode *voltages* on the balanced side by shorting them to ground. This happens because the bifilar winding impedances essentially cancel each other out in the common mode.
KM3K: Actually, the common-mode-current is reduced by the winding’s inductive-reactance.
Now about the 'cancel each other'....not so. 

Quote
Now consider the Guanella balun that consists of a coax cable winding on a powdered-iron or ferrite toroid core.
It's the kind of balun you showed in the links.
In such a balun, the desired RF energy gets from one end to the other *inside*the coax.
The balun core has no effect because it's outside the coax.
KM3K: As noted above earlier, the core, for reason(s) not explained by W2FMI (SK), is needed to enhance the balun’s efficiency over a greater frequency range.

Quote
The Guanella balun under consideration deals with common-mode *currents* on the balanced side by presenting them with a very high impedance to ground.
This happens because the common-mode currents have to deal with the impedance produced by winding the coax on the toroid core.   
KM3K: strike-out “on the balanced side” and IMHO you’ve got it.

Quote
Which balun to use depends on the application.
KM3K: As I’ve written in another posting elsewhere, for a one-time application, unless limited by size, cost, or weight, my choice is a Guanella-balun.
To me, it seems like a Philadelphia lawyer is needed to sort thru the many restrictions on when to use a Ruthroff-balun.
Sure, there may be a Ruthroff-balun that’ll work but I ask myself if it is worth the hassle.
If I can afford it, I’d go with the safety of the Guanella-balun and move on.

73 de Jerry KM3K
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