Re: bisectional winding of two Guanella transformers tied in parallel
Some people like that technique when using wires with PTFE or FEP jacket for the winding. That makes sense, because the characteristic impedance of such wire pair (placed tightly in contact, best twisted gently) is about 100 to 120 ohm. Having two 1:1 transformers in parallel make the whole thing look like a 50 ohm transmission line. However, thick enameled wires, when run together, typically have about 30 to 40 ohm characteristic impedance. Having two of them in parallel would make the whole system more like 20 ohm. A single winding would make the characteristic impedance closer to 50. I donāt think that matters much in HF but you might want to reconsider the winding choice.
Re: measurement on common mode choke or 1:1 current balun
Iām amazed. Many websites and youtube videos are measuring common mode choke performance in terms of dB by simply inserting one winding between port 1 and port 2. That measurement technique or the measured numbers wonāt really mean much, since common mode current source is not 50 ohm. The impedance of the common mode source is more likely a lot higherā¦ thatās why we are trying to achieve high impedance in the choke windings. I would just ignore those and focus on people who correctly understand the nature of the common mode current caused by the antenna system.
Well spotted. The impedance of my double-core setup isnāt good, and itās messing up the SWR when placed on the transmission line between the VNA and the 49:1 transformer. So, I will definitely reconsider my winding choice.
Thatās why I focused on attenuation measurements in my first post. You really have to search for specialist blogs to find out what itās really about.
I debated for a while whether I should bother posting another RF engineering post on SOTA forum but Iām glad I did, if it saved your troubleshooting effort a bit.
If you want to stick with thick enameled wires, and if you want to make it very close to 50 ohm, it is best to put a thin tape (Kapton tape- the yellow tape often used to tape on the bare lithium pauch cells - which has low dielectric loss, heat resistant, and comes in right thickness) on one of the wires to make the insulation a bit thicker. The distance between the two wire surfaces becomes the enamel thickness x2 + the tapeās thickness. That brings the characteristic impedance of the very lightly twisted pair close to 50 ohm. Then if you just do single core Guanella 1:1 construction, I think the isolation and SWR should be ok. If you want to use two cores, you can simply stack the cores. But even without the Kapton tape trick, it should be close enough.
Yes, it seems like the single core bifilar has better impedance and the SWR is not affected so much. I measured it at 3k ohm anyway, which should be more than enough, according to what you said before.
It doesnāt work so well. I tried. R is not as linear as it is with the two cores in series. As you can see on that screenshot:
Thatās most likely due to the capacitance between adjacent turns. You could cascade two cores like you did, or somehow increase the separation of the winding turns.
