A model of the W2DU 1:1 choke balun using Jaycar LF1260 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 12 Jaycar LF1260 16mmx8mmx29mm cores threaded over RG-58C/U coaxial cable. The LF1260 cores are readily available in Australia, and at the time of writing retailed for A$9.00 for two packs of 6 cores.

Fig 1: Modelled choke impedance

Fig 3 shows the common mode choke impedance for calculated from measured core characteristics.

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	105	447
3.6	475	677
7.1	804	745
10.1	954	783
14.2	1155	811
18.1	1303	831
21.2	1381	846
24.9	1422	872
28.5	1493	877

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

Changes

Version	Date	Description
1.01	08/02/2007	Initial.
1.02
1.03
1.04
1.05

V1.01. Last changed: 06 March 2008 14:06

A model of the W2DU 1:1 choke balun using Jaycar LF1260 cores

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