Guanella’s 1:1 balun and his explanation – Zcm

Guanella’s 1:1 balun and his explanation gave Guanella’s equivalent circuit and analysis of an example air cored choke of the type shown by Guanella.

The analysis was presented in an LTSPICE model of a ‘test bench’ implementation of the balun, and it showed that on a slightly asymmetric load, common balance was only good in a small region around the choke’s self resonant frequency of 41MHz.

One metric that is useful in indicating the effectiveness of a Guanella 1:1 balun in achieving current balance or reducing common mode current is the choking or common mode impedance Zcm of the stand alone balun.

Modern thinking and experience is that |Zcm| needs to be 1000Ω or higher for effective common mode reduction on many HF wire antennas, and considerably higher for some highly asymmetric antennas.

Zcm of the example air cored solenoid balun

Above is Zcm for the example balun. It is very low at low frequencies and rises to 133+j914Ω at 30MHz.

The quite low Zcm below 30MHz explains why it was a dismal failure at reduction of common mode current. Adding more turns would improve Zcm, but exacerbate the already poor impedance transformation.

This balun was not at all suitable for HF, and use of a magnetic core was mentioned as a possible solution.

Zcm of a ferrite cored toroidal balun

Above is Zcm of a 11t balun wound on a FT240-43 toroid. The ferrite core acts on the common mode choke element and has negligible effect on the differential transmission line mode. The distinct advantages  are that:

  • it uses 25% of the wire length so TL electrical length at 30MHz is 36°; and
  • Zcm is much higher mid band and |Zcm| is not less than 2000Ω from 3.5 to 30MHz.

The resistive element of Zcm, Rcm represents the core loss. If there is sufficient Zcm, common mode current Icm is sufficiently low that little power is lost. For example if at some transmitting frequency Rcm=2000Ω and Icm=0.01A, then P=I^2R=0.01^2*2000=0.2W.

Conclusions

A ferrite core can substantially increase Zcm whilst reducing TL length.

Power lost in the ferrite core can be very low in a well designed system.

References

  • Duffy, O. Dec 2010. Baluns in antenna systems. https://owenduffy.net/balun/concept/cm/index.htm (accessed 31/05/19).
  • ———. May 2011. Measuring common mode current. https://owenduffy.net/measurement/icm/index.htm (accessed 31/05/19).
  • Guanella, G. Sep 1944. New methods of impedance matching in radio frequency circuits. The Brown Boveri Review.