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A model of the W2DU 1:1 choke balun using Jaycar LF1258 cores

Walt Maxwell described and recommended in his book "Reflections II", a choke balun made by placing several ferrite sleeves around a coaxial cable. The 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 18 Jaycar LF1258 12mmx5.6mmx20mm cores threaded over RG-58C/U coaxial cable. The LF1258 cores are readily available in Australia, and at the time of writing retailed for A$12.75 for three packs of 6 cores.

Fig 1: Modelled choke impedance

Fig 3 shows the common mode choke impedance for calculated from measured core characteristics. Quantity Zcm is the magnitude of Rcm+jXcm.

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

Fig 4 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

67

444

3.6

262

577

7.1

566

781

10.1

762

897

14.2

1008

1006

18.1

1199

1053

21.2

1320

1086

24.9

1473

1127

28.5

1534

1137

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

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Version Date Description
1.01 08/02/2007 Initial.
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