Stumped: 4:1 current balun in reverse

[kj5cn]

Hi guys,

I am trying to use a 4:1 balun in reverse, to match a 50Ω line to a 13Ω load. The balun itself is a classic Guanella construction, wound on T240-52 core and shows a nearly perfect characteristic over the frequency range of interest (10 to 55 MHz) when connected in a "traditional" way, with 50Ω generator (NanoVNA) at one end and a 200Ω non-inductive resistor at the other (Guanella_1to4.png).
However, when hooked up in reverse (NanoVNA working into the Hi-Z side and a 13Ω carbon composition resistor across the Low-Z side), the results are radically different (see Guanella_4to1.png). I was expecting the balun to be completely reciprocal, so this result has me stumped. Anyone has ideas why it behaves this way?

Peter.

[shabaz]

Hi,
It's only reversible provided that the impedances are kept the same, i.e. the 4:1 balun is specifically for 200 ohm to 50 ohm conversion, or 50 ohm to 200 ohm. If the terminations are away from these values, then there's a mismatch. The reason this occurs is because the transformer is based on transmission lines, which expect to be used at their characteristic impedance.
The parallel wires may have approximately 50 ohm characteristic impedance. You ideally need 25.5 ohms char. impedance, by using the geometric mean formula (sqrt(50*13)).
From your photo, the wires look quite thick (maybe 0.8-0.9mm at a guess), and if they are twisted (instead of placed parallel), at about 8 or 9 twists per inch, you may reach about 25 ohm characteristic impedance.
Then, it should work better (hopefully!). There's a slightly longer (but hopefully easy-to-follow) explanation here: Understanding Guanella Components (Baluns). There may be errors in it though, so please don't rely on it fully, If anyone spots errors, please let me know and I'll get it updated.

[kj5cn]

Shabaz, thanks! Your explanation makes sense. I will try to rewind it with twisted line and retry.

Peter.

[RFDx]

I will try to rewind it with twisted line and retry.

Just to have an idea where to start: 0.8mm (AWG20) to 1mm (AWG18) enamelled copper wire with uniform ~3...4 [7...10] twists per cm [inch]. You can easily measure the line impedance with the nanoVNA. As you are trying to go from 50 Ohm down to a lower impedance, the stray (leakage) inductance of the transformer becomes an obstacle and you will probably also have to do some compensation. Btw., judging from the picture, the two parallel wire transmission lines for the 50:200 Ohm balun seem to be more in the vicinity of 50...60 Ohm instead of 100 Ohm.

[Andy Chee]

I am trying to use a 4:1 balun in reverse, to match a 50Ω line to a 13Ω load.

Look here for some 50:13 transformer designs

https://ia801609.us.archive.org/17/items/fe_Ham_Radio_Transmission_Line_Transformers_Handbook/Ham_Radio_Transmission_Line_Transformers_Handbook_text.pdf

[A.Z.]

a TRUE Guanella with a ratio other than 1:1 MUST be wound using multiple 1:1 CORES, the 4:1 in this case will need two cores, for further infos, please see

https://www.dj0ip.de/balun-stuff/1-vs-2-core-baluns/

as an alternative, you may consider a regular transformer (also known as "voltage balun"), in such a case given that you want a 4:1 transformation ratio, following the "square root law" the windings ratio will be sqrt(4) that is 2:1 so, for example you may have 4 winding and 2 windings (the windings count depends, amongst other factors, from the permeability of the core and the frequency range); at that point you'll also want to add a 1:1 Guanella to act as a choke between the transformer and your transmission line, the transformer will achieve voltage balance, while the choke current balance