(How) does this balun work?

I was sent a pic of a balun and asked to explain how it works.

With no other detail than the pic, it is difficult to supply a complete answer.

Nevertheless, an analysis of what is presented follows.

There are two transmission line sections of approximately equal length formed from parallel brown and white wires. These sections are wound on the core in the same sense magnetically (ie the direction in which they pass through the inside of the toroid), and are paralleled at both ends.

Without dimensions, it is a guess that characteristic impedance Zo of each line section is probably between 100 and 150Ω if they were uniformly space just touching each other. The latter is hard to achieve, so perhaps to the high end of the range mentioned… lets assume 140Ω for discussion.

Since the two line sections appear in parallel, Zo of the combination is half that of each section, so lets assume around 70Ω. (This analysis was confirmed by measurement.)

So one view of what we have is an ordinary Guanella 1:1 balun with common mode impedance due to 12t of 70Ω transmission line.

The configuration would lead to lower InsertionVSWR wt 50Ω than using a single line section of that type. Nevertheless, it is unlikely to quite achieve the InsertionVSWR when the choke is wound with good 50Ω coax as is usually done for a low InsertionVSWR balun.

Does it have lower loss?

Most loss in these types of baluns is due to magnetising losses in the ferrite core material, and that will be unchanged. Transmission line loss might appear to be lower, but if poor insulation like PVC was used, it might well be poorer in that respect than using 50Ω coax.

The transmission line configuration might well lead to a lower self resonance frequency which can be good or bad, depending on the application.

Does it work?

I have explained how I think it works, and with a suitable core material for the application, it might well work well. I would like to have seen a plot of Zcm vs freq… but balun ‘designers’ rarely publish such, and probably don’t measure the parameter.

It seems Youtube channel TRX Bench may have been my reader’s source of the pic.