I have corresponded with many people trying to make valid measurements of a Guanella 1:1 balun, also known as a common mode choke.
In common mode, the device looks like a ferrite cored inductor. That might sound simple, but it is anything but, and it can be a challenge to measure.
There are lots of measures quoted, most of them IMHO are bogus or incomplete, usually specious… which accounts for their popularity.
This article focusses on making a valid measurement of the R and X components of the choke’s common mode impedance by the simplest means that is likely to give direct results.
Let’s start by proving the measurement instrument
I have taken an ordinary 1% metal film resistor and measured it with an accurate ohmmeter to be 221.4Ω. That is good, it reconciles with its markings. Now this is not a pure resistance at RF, it has some self inductance so we must expect to see that when measured with the VNA.
The wires are trimmed short. The bare end can be safely inserted into the centre pin of a SMA(F) jack.
One wire is poked into the Port 1 centre pin and the plastic clothes peg secures the other wire to the outside male threads of the SMA jack. Alternatively you could use a zip tie to secure the other wire to the outside male threads of the SMA jack.
Above, the test configuration.
Above, a screenshot. Focusing on the marker at 1MHz so that self inductance effects are small, we measure Z=222.1+j0.303. That reconciles well.
Now let’s move on to an unknown inductor
To simulate a small common mode choke, I have wound a Fair-rite 2843000202 (BN43-202) binocular core with 6t of single conductor stripped from an 4pr LAN cable. Note the specific core part, non-genuine parts may have different responses. The wires are trimmed short and 8mm of insulation stripped.
The bare end can be safely inserted into the centre pin of a SMA(F) jack.
Above is the test setup. One wire is poked into the Port 1 centre pin and the plastic clothes peg secures the other wire to the outside male threads of the SMA jack. Alternatively you could use a zip tie to secure the other wire to the outside male threads of the SMA jack.
The key thing is very short connections, they are less than 0.6° over the frequency range measured.
Here is a screenshot. The inductor is self resonant around 18.7MHz. The R and X curves are very similar to what you would get for a practical Guanella 1:1 balun though commonly the resonance would be somewhat lower, below 10MHz often. Narrower peaks will be observed with lower loss ferrite, eg #61 mix.
For the enquiring mind
If you have your own technique, fixture, test jig, cables etc try measuring a resistor like this and a choke like this (preferably exactly like this) by the method I have shown and your own technique and compare them.
Read widely, question everything.