This is a project to design and build a Guanella 1:1 (current) balun for use on HF with wire antennas and an ATU.
For use with a tuner, the most important design criteria are:
- high voltage withstand;
- high common mode impedance;
- generous power handling given our 400W PEP limit, possibly 1kW in the future.
Insertion VSWR is NOT a critical parameter for this application nor this design, any impedance transformation caused by the balun is incidental to that of the antenna system and dealt with by the ATU. If you need low Insertion VSWR, do not build this design.
High voltage withstand
I have procured some high temperature, high voltage, low loss 1mm (#18) stranded wire for the purpose.
Today I tested the wire on a HiPot tester. The maximum voltage available is 5kV RMS AC, or 7kVpk. I tested one insulated conductor to a knife-edge electrode at 7kVpk for 2 minutes and there was no sign of breakdown. This test was repeated at five locations on the wire. Whilst I expect from material characteristics that the wire will withstand more than this, a wire to wire withstand of 14kV is more than adequate for the application… something else will break down first (like a coax connector, caps in the ATU etc).
High common mode impedance
Above, the prototype wound for measurement. The medium µ ferrite core is available from Neosid Australia, and is better suited to the task than the smaller FT240 form toroids from the US (China). Initial tests are that common mode impedance Zcm is consistent with the design model, more tests to be made of Zo of the line formed (though that is not a priority parameter for a tuner balun).
Power handling is achieved by:
- designing for transmission losses;
- achieving high common mode impedance to deliver low common mode current; and
- using a large core to provide surface area for heat dissipation.
Next installment… Design / build project: Guanella 1:1 “tuner balun’ for HF – #2