Design / build project: Guanella 1:1 ‘tuner balun for HF’ – #7

Seventh part in the series documenting the design and build of a Guanella 1:1 (current) balun for use on HF with wire antennas and an ATU.

  • This article describes a measurment of common mode impedance Zcm of the packaged balun.


The prototype fits in a range of standard electrical boxes. The one featured here has a gasket seal (a PTFE membrane vent was added later).


Above, the exterior of the package with M4 brass screw terminals each side for the open wire feed line, and an N(F) connector for the coax connection. N type is chosen as it is waterproof when mated.


The interior shows the layout. The wires use XLPE high temperature, high voltage withstand, moderate RF loss insulation. Two short pieces of 25mm electrical conduit serve to position the balun core against the opposite side of the box, and a piece of resilent packing between lid and core holds the assembly in place.

The fixture

The fixture described is not perfect, and it is challenging to measure common mode impedance of the packaged balun. The fixture here tries to use the shortest possible connections. The VNA is calibrated at its SMA jack.

Above, a N short circuit plug is screwed onto the N jack on the balun, and a single 0.5mm wire is zip tied to the knurled collar for connection to that end of the balun.

0.5mm wires are connected to each of the two screw terminals, and twisted together to make the connection to the other end of the balun.

Note that each end of the balun has its conductors shorted.

The twisted pair will be clamped to the outside of the VNA Port 1 SMA jack, and the single wire inserted into that jack… all done with the shorted possible wires.

Above, the prepared conductors.

Above, the twisted pair will be clamped to the outside of the VNA Port 1 SMA jack (using a modified clothes peg), and the single wire inserted into that jack… all done with the shortest possible wires.

The fixture is not perfect, more on that later.

Above, the measurement of impedance using s11 reflection.

Of most interest to me is the frequency range over which impedance is high. The above plot has markers at the lower and upper frequencies where |Zcm|>2000Ω. It remains above 1000Ω from 1.8MHz to 30MHz, so it is somewhat effective over that range.

Above is the R+jX plot with the markers displayed.

As mentioned, the fixture is not perfect. It is likely that the intrinsic self resonant frequency measured is a little low, and the response generally is squeezed a little left. The upper 2kΩ bound may well be over 20MHz.