Hello Jim N2EY, here are some comments by me about posting #32 for us to talk about....
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.
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.
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.
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
), I feel I don't have the smarts to figure this out.
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.
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.
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.
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.
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.
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