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A model of a Guanella 1:1 choke balun using a Jaycar LO1238 core

This design is a variation of an Order-1 Transmission Line Transformer described by Guanella in 1944. This type of balun is also known as a current balun as it tends drive equal but opposite currents on the wires of the balanced port.

This article applies the lossy transmission line model described at A model of a practical Guanella 1:1 balun to such a balun made from a Jaycar LO1238 35mmx21mx13mm cores with eight turns of RG-174/U coaxial cable. The LO1238 cores are readily available in Australia, and at the time of writing retailed for A$4.50 for a pack of 2 cores.

Fig 1: Modelled choke impedance

Fig 1 shows the common mode choke impedance for calculated from measured core characteristics. Quantity Zcm is the magnitude of Rcm+jXcm.  The model includes 2.5pF of shunt capacitance to calibrate the model to a measured balun. The winding was a conventional configuration rather than the crossover configuration often used with these baluns. The crossover configuration might be convenient, but in this case the resonance was about 15% lower in frequency indicating higher stray capacitance.

Fig 2: modelled VSWR and loss with 50Ω balanced load

Fig 2 shows the VSWR and loss for the isolated balun terminated in a 50Ω load with the centre of the load grounded.

Note that losses for other loads may be quite different, and this model does not attempt to include the effects of common mode currents coupled from an antenna radiator. See A model of a practical Guanella 1:1 balun for more discussion of this topic.

Table 1: Choke impedance
Frequency (MHz) R (Ω) X (Ω)

1.8

315

846

3.6

882

1234

7.1

2035

1701

10.1

3160

1451

14.2

4157

64

18.1

3862

-1378

21.2

3195

-2077

24.9

2301

-2421

28.5

1642

-2382

Table 1 sets out the choke impedance in the HF amateur bands for input to NEC models or the like.

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Changes

Version Date Description
1.01 09/02/2007 Initial.
1.02 07/03/2008 Revised with stray capacitance.
1.03    
1.04    
1.05    

Disclaimer

Use at your own risk, not warranted for any purpose. Do not depend on any results without independent verification.


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