Small common mode choke for analyser antenna measurements using 2843000202 (BN43-202)

The project is design, implementation and test of a small common mode choke for use with an analyser for antenna measurements.

The choke must have medium to high Zcm from 1 to 30MHz. It is intended to be used with analysers supporting SOL calibration, so effectively any impedance transformation within the fixture is compensated and the reference plane is the load side terminals of the device.

The candidate core is a low cost #43 binocular ferrite core that is fairly easy to obtain.

Above is a first pass check of the likely Zcm at 1.8MHz using a Fair-rite 2843000202 (BN43-202) binocular core. These chokes have relatively low self resonance frequency so a value for Cs is supplied that delivers self resonance at around 5MHz. Zcm at 1.8MHz needs 8-9t, 8.5t will be used (ie the twisted pair enters one end of the binocular and leaves the other end for convenient layout). (8.5t is not strictly correct, but it is a close approximation in this case.)

Above is a model for the size of circles within a circle, we can accommodate a twisted pair having overall diameter 1.05mm. That will be a very tight fit, so 0.25mm ECW will be used (about 0.3mm OD).

Above is the completed prototype. The core is secured to the prototype board with double sided foam tape, then clear heatshrink overall.

Above is a sweep of Zcm of the prototype.

Above is a theoretical model calibrated to the observed self resonant frequency.

Above is the Short and Load parts of a SOL calibration set. The Load comprises two 1206 0.1% 100R soldered in parallel (they are not 1000Ω, 1000 is the code for 100Ω).