Current baluns

Recently I made two current baluns, I used 0.8 mm transformer wire, copper transformer wire and 4c65 ferrite cores that I got from dx-wire in Germany. On the internet I found two different designs that either come with one or with two cores, the question is now which design performs best. In order to measure the performance we terminate each balun with a 200 Ohm resistor. At the input we expect to see an impedance of 50 Ohm with a SWR of 1 so that no signal is returned. The schematic of both baluns is as follows:

Schematic of the 4:1 current balun

With a single core design we use one toroid were core 1 is wound on the right half and core 2 is on the left half of the ring. With the dual core design two 4c65 toroids are used.

Single core 4:1 balun

4:1 current balun on one core

This is a 4:1 balun made from 4c65 material, a better description of how it is made can be found on A miniVNA was used to measure the SWR and the resistance |Z| between 1 and 50 MHz:

VNA 1: SWR and |Z|

Dual core 4:1 current balun

4:1 current balun on two cores

This is also a 4:1 current balun, the schematic follows from The observed SWR and |Z| obtained by the miniVNA are:

VNA 1: SWR and |Z|

A verification of both baluns was performed with another antenna analyzer measuring the SWR between 1 and 35 MHz. The miniVNA returns more information than the antenna analyzer, in fact the miniVNA returns both the in-phase and the quadrature component of the resistance, the antenna analyzer reports the scalar resistance, although not fully correct we call the antenna analyzer VNA2 in the sequel.

VNA2 The SWR is observer, the blue graph is for a one core balun, the green one is for the dual core balun, and the red curve is a 1:4 voltage balun made by PH2CV Okko. This balun is made from a different ferrite material, namely a purple toroid whereas our is grey white.


The intended use for current baluns is to match the impedance of Delta loops, folded dipoles, or dipoles with loading coals, etc, to 50 Ohm coax cable in the HF domain, so up to 30 MHz. Current baluns are filters for common mode signals, the problem is relevant for frequencies below 30 MHz where it is increasingly difficult to construct antennas that fit near the house. We confirm that the dual core 4:1 current balun is preferred over the single core version.

Ideally the observed resistance |Z| measured by the VNA should stay close around 50 Ohm when the balun is terminated with a 200 Ohm resistor. The Guanella 4:1 current balun wound on a single core shows high SWRs below 5 MHz and this is expected to be a problem for 160m and 80m band antennas. Both VNA measurements confirm this finding. Between 5 and 30 MHz it does not really matter which balun we use. Beyond 30 MHz the ferromagnetic properties of the 4c65 material will change. Also, the baluns were tested for normal temperatures. Whenever a lot of power is dissipated in the balun it will become hot. When the Curie temperature of the ferrite material is reached the efficiency will change.

VNA1 was a miniVNA, you can find it in the Wimo webstore,  VNA2 is a different one. PE5STE Stefano helped me with the miniVNA measurements, VNA2 is owned by PH2CV Okko. For both VNAs you need a laptop and VNA1 must first be calibrated before you can use it.

Last update: 20-Jan-2017


3 thoughts on “Current baluns

  1. Hoi.
    Zoals ik op scherm gsm zie lijken ze idd wat mintgroen net als die van mij.
    Heb ook nog een paarse met doorsnede 23mm x 7.5mm
    Hoor het graag
    73 de pd1acj

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