The article describes a current balun intended for measurement use with low power instrumentation. It is lightweight and can be made with materials readily available in Australia (LF1260 cores are a little over $1 each in packs of six). The target application is for a 600mm square small loop for field strength measurement below 15MHz.
The design is an implementation of (Duffy 2007) which used RG174 coax for the choke to give low Insertion VSWR.
The balun is intended for low power measurements and will withstand dissipation of a few watts.
The LF1260 cores are made from a medium µ ferrite and have an ID of 7.8mm.
Above, the cores will accommodate four round conductors of diameter 3.2mm, so they will comfortable accommodate the four passes of RG174 (2.5mm dia). (For the mathematically minded, the minimum enclosing circle diameter for four equal circles is 1+√2 times the diameter of the smaller circles.)
Above, the balun cores are housed in a small Jiffy box with a BNC-F flange mount connector at one end and a pair of M4 screw terminals at the other. Small brass tabs were made as non-rotating terminal tags and the M4 brass screws soldered to them. The cores are attached to each other with a piece of double-sided foam tape to prevent them shattering, and two pieces of the same used to secure the cores to the box. A packing between the cores and lid helps to hold them in place.
As discussed at Baluns – show me the numbers, implementation details without quantitative models or measurement are of little value.
Above is the measured common mode impedance.
Above is a plot of Matched Line Loss of the 375mm length of RG174 within the balun. The coax in this case uses silver plated copper centre conductor which has good RF resistance at these frequencies, be very cautious with any coax that uses silver clad steel (SCS) inner conductor strands, the silver cladding is often inadequate for these frequencies.
Above, application to the small untuned square loop for field strength measurement.
The LF1260 are sold by Jaycar in Australia, and are a different material to that in the mainstream US supplier products.
A pair of Fairrite 2631625102 (#31) cores should have similar characteristics.
Above is a prediction of the balun using 2631625102 with a guess that the equivalent self capacitance is around 5pF.
The 2631625102 cores are about A$4 each from Element14, I may purchase a couple on a future order and make and measure a balun. Perhaps Australians should value the LF1260, A$7.50/6 at the time of writing.
Another option is Fair-rite part 2643625102 (~$4 each from Element14) is of similar size and also medium µ, but slightly different characteristic to the LF1260.
Above is a model prediction of the common mode impedance of a similar choke using the Fair-rite 2643625102 cores.
I have not constructed a balun using these cores, and it does have slightly different characteristics, but somewhat similar performance can be expected.
- Duffy, O. Feb 2007. A model of a Guanella 1:1 choke balun using two Jaycar LF1260 cores. VK1OD.net (offline).
- ———. Mar 2012. Common mode choke design tool. VK1OD.net (offline).