An interesting case study of measurement of a balun’s Insertion VSWR

Jeff, 2E0CIT, sent me a Rigexpert AA-170 measurement file of his test of Insertion VSWR of a commercial balun.

Insertion VSWR is the VSWR looking into the balun with a matched load (termination) on its output, it is a measure of imperfection of the balun. It ought to be a specification item for low Insertion VSWR baluns, but it rarely given.

A broadband low Insertion VSWR balun must be wound with a transmission line of the nominal impedance, 50Ω in this case, and in the case of 50Ω , it is most likely to be coax.

Above is the initial VSWR plot received. The VSWR response is poorer than one might want in a low Insertion VSWR balun… but to drill down on the reasons, the Smith chart view of the data gives insight.

Above, the Smith chart speaks to us. The spiral hints that the termination is not 50+j0Ω and that there is a substantial length of transmission line between the Rigexpert and the termination.

The most common issue with a termination attached to a balun with screw terminals is that the pigtails of the termination resistor, and the pigtails inside the balun where the coax breaks out to the screw terminals. The pigtails are not a continuation of the 50Ω and their distributed inductance and capacitance give rise to reflections.

Lets assume that in this case:

the departure from ideal at the termination end of the balun can be approximated as a frequency independent resistance in series with a frequency independent inductance; and

the “substantial length of transmission line” I mentioned is 50Ω.

Let's use Antscope's add/subtract cable feature to subtract a length of line of RG213 type to obtain a Smith chart plot where R=50 and X is positive varies with frequency.

Above is the outcome, it is an estimate of Z at the load, measured at the system input and adjusted for a length of RG213. The cable parameters were adjusted while watching the Smith chart to obtain the desired plot.

Above are the cable parameters. Initially they were RG213 characteristics from the database, and length was adjusted to get R approximately 50Ω, but it was a bit wavy and it was found that a small adjustment to Zo improved the fit… hinting that the line might not be exactly 50Ω.

So, it appears that whilst we might expect the balun to contain less than a metre of 50Ω coax, it looks like there was 6+m between the Rigexpert and the balun. This was later confirmed to be the case.

The Smith chart plot shows a load end impedance of 50Ω in series with an inductance of around 330nH. A further test with shorter leads got this value down to around 100nH.

So, the measured Insertion VSWR of around 2.7 at 30MHz:

  • is an underestimate as it is measured through 6+m of cable; and
  • is probably mainly attributable to series inductance in the internal and external pigtails.

A targeted rework of the termination and a shorter coax patch cable yielded better results, Insertion VSWR at 30MHz of 1.5, some of which remains due to the termination… it is a challenge to make a 70mm long 50Ω with insignificant self inductance.