Using SPICE on antenna baluns

Guanella’s 1:1 balun and his explanation gave a LTSPICE model of Guanella’s 1:1 balun.

The LTSPICE model was of a ‘test bench’ implementation of the balun which comprised an air cored solenoid of two wire transmission line, with a slightly asymmetric lumped load.

This article discusses limitations of SPICE in modelling practical baluns.

Guanella’s 1:1 balun and his explanation – Zcm gave the characteristics of a example ferrite cored 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 key characteristics are:

  • TL Zo=100Ω and electrical length at 3.6MHz is 0.43°; and
  • Zcm at 3.6MHz is 1007+j1862.

Let us create a model for a narrow band of frequencies around 3.6MHz where will will consider Zcm approximately constant.

Above is the LTSPICE schematic.

Above is the response. We will focus on the results at 3.6MHz (where Zcm is most accurate).

The cursor 2 value is total common mode current and cursor 1 is differential current. It can be seen that common mode current is very low, 35dB less than differential current so despite the asymmetric load, there is very little common mode current. The balun is effective is reducing common mode current (much more so than the air cored example given that reduce common mode current by less than 4dB at 3.6MHz).

The green plot is Zin, and it is quite close to 50Ω by virtue of the electrically short TL section at 3.6MHz.

Modeling an antenna scenario

The load formed by R2 and R3 could be replace by the frequency dependent three component model as discussed at Equivalent circuit of an antenna system, and the frequency specific balun Zcm used (as above), and a solution obtained for a single frequency.

Essentially, whilst it is possible to insert these single frequency values to drive the computation engine, it is not a good tool for the job when one really wants to capture the frequency dependent characteristic of the balun and the complexity of antenna and feed line coupled conductors, radiation etc.

To some extend, it suffers the problems discussed at Using Ohms law on antenna baluns.

Conclusions

SPICE is a quite capable modelling tool, however:

  1. SPICE does not really have convenient tools for modeling the frequency dependent complex impedance of ferrite cores often used in HF baluns; and
  2. SPICE does not really allow for entry of the coupled antenna and feed line conductors (ie modeling the antenna 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.