NanoVNA setup for shunt match tasks

NanoVNA setup for common antenna system measurement tasks offered an example NanoVNA configuration well suited to the most common antenna system tuning / adjustment tasks.

This article looks at a different case, a configuration to support measurement, design, and implementation / tuning of a shunt match.

A shunt match scheme is one where the antenna with low feed point R at resonance is detuned to add some capacitive or inductive reactance, which is then offset with a shunt reactive element of the opposite sign, for the outcome of a load impedance of 50j0Ω.

VNA Calibration

The VNA is OSL calibrated at its Port 1 jack.

Measurements in this example will be made through a 50Ω coax tail of about 1m, so we need to adjust the reference plane to the feed point. In this example, the native reference plane is the NanoVNA jack, and e-delay is used to approximately offset the reference plane. It is a good approximation in this case.

You could instead calibrate the fixture to include the coax tail, but you will need appropriate cal parts… and if they are poor, the previous method may be more accurate.

Above, a measurement is made of the coax tail with an open circuit (OC) at the far end, and e-delay iteratively adjusted so that the Smith chart plot is a dot at R=infinity+j0, the right hand end of the Z=0 axis above.

Matching

The matching scheme involves:

  1. adjust the antenna so that its feedpoint admittance Y=1/50+JB (where B is +ve); then
  2. add a shunt susceptance -B to offset B; then
  3. calculate the required lossless inductance; then
  4. design a suitable coil.

The NanoVNA is configured with traces 2 and 3 being admittance components G and B, with scale factors appropriate to this task.

Above is step 1, the antenna adjusted for Y=0.02+jB, B is 0.0252S in this case.

The same scenario with the Smith chart scaled in admittance. The antenna is tuned until marker 1 (at the desired frequency) lies on the G=0.02S arc.

Above is step 2, a shunt susceptance of -0.0252S is added to take Y or Z to the chart centre (Z=50+j0Ω, Y=0.02+j0S), follow the blue arrow (which follows the G=0.02S arc). The chart above is a Z scaled Smith chart, it can be used, just watch the value of G at top of the display to adjust the antenna for target G (20mS in this example).

Step 3 is the calculate the required lossless inductance.

Required susceptance -25.2mS, \(L=\frac1{2 \pi f B}=\frac1{2 \pi 3640000 \cdot 0.0252}=1.74 \text{$\mu$H}\).

Step 4 is to design a prototype coil.

Because a real inductor brings some loss to the solution, and in any event, you cannot make an inductor to exact value, it will be necessary to iteratively adjust the antenna tuning and inductor (eg by squeezing its length) for a perfect match at the test frequency.

The example used here is a loaded 80m vertical, but the technique is equally applicable to a 144MHz Yagi with hairpin match.