Category: NanoVNA

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. Continue reading NanoVNA setup for shunt match tasks

Fazed by s11 phase magic? mentioned the effect of phase wrapping on s11 phase plots, and the apparent discontinuity that is actually an artifact of the wrapping process.

Phase wrapping is the presentation of phase values to always appear in the range typically -180° to 180° (or sometimes 0° to 360°).

F1AMM published an example .s1p file of an antenna system measurement. Let’s use that as a real world example to demonstrate the effect of phase wrapping.

Above is a Smith chart presentation of the data. You might interpret the curve near the marker to show a sudden flip from -ve phase to +ve phase… but is phase discontinuous? Continue reading s11 and phase wrapping

NanoVNA setup for common antenna system measurement tasks showed a display configuration better suited to those tasks.

It is tedious to set the display up using the device menu, and setups may vary with different NanoVNA hardware and firmware.

The firmware I used was NanoVNA.H.v1.2.20 which allows some setup using serial port commands. This article describes the technique.

The command used is the trace command

trace {0|1|2|3|all} [logmag|phase|delay|smith|polar|linear|swr|real|imag|r|x|z|zp|g|b|y|rp|xp|sc|sl|pc|pl|q|rser|xser|zser|rsh|xsh|zsh|q21] [src]
trace {0|1|2|3} [lin|log|ri|rx|rlc|gb|glc|rpxp|rplc|rxsh|rxser]
trace {0|1|2|3} {scale|refpos} {value}

For this exercise, I used Teraterm5 which allows setting delays after each character and after each line so that the NanoVNA is not overrun.

Above, the Teraterm serial port setup. Continue reading NanoVNA setup for common antenna system measurement tasks – scripting the setup

A common task is an overall assessment of an antenna system, this article looks at NanoVNA display configuration that will often suit stand alone:

1. measurement at any point on the feed line; and
2. measurement with the reference plane, either by direct connection, fixture calibration or approximate calibration using e-delay.

Caution

When measuring an antenna system with the NanoVNA:

• drain any static charge at the coax connector before offering the connector up to the NanoVNA; and
• do not leave the instrument attached any longer than necessary to make the measurements.

Case 1: measurement at any point on the feed line

Since the phase relationship of the reflected wave at the point of observation relative to that at the feed point is unknown the only meaningful statistics are those based on the magnitude of s11 (|s11|), |s11|, ReturnLoss, and VSWR.

My NanovVNA does not offer a ReturnLoss plot natively, you could use |s11|_dB remembering to multiply all values by -1 (ie ReturnLoss_dB=-|s11|_dB.

Otherwise, the VSWR plot is most useful.

A Smith chart plot of s11 is sort of useful, but there is an unknown rotation from the feed point.

Since they are of no real value, you could disable traces 1, 2 and 3 to make the display less cluttered.

Case 2: measurement with the reference plane at the feed point, either by direct connection, fixture calibration or approximate calibration using e-delay

In this case, the phase of s11 is meaningful which means:

• the Smith chart plot is properly presented wrt the chosen reference plane; and
• R and X components of impedance can be properly calculated and presented.

An example

Let’s look at an example antenna sweep where the NanoVNA measurements are wrt the feed point (e-delay has been used as an approximate correction for a short feed line tail). The examples are from NanoVNA.H.v1.2.20 firmware.

Above is a screen capture, the colours are inverted for printing. Continue reading NanoVNA setup for common antenna system measurement tasks

The widespread takeup of the NanoVNA has given new life to the resonance myth. Heard on air some years ago was this enlightenment:

anyone who has blown across the top of an empty milk bottle and observed resonance knows that you really need a resonant antenna to fairly suck the power out of the transmitter.

Phase of s11

Let’s divert to the new pitch that phase of s11 equal to 0° is a key optimisation target.

Adapted to the NanoVNA is this capture from an instructional video:

The voice over is explaining that the (load) voltage and current are in phase at the cursor in this phase of s11 chart (check the axis title). The discussion asserts that phase=0° is goodness. Continue reading Fazed by s11 phase magic?

RF compensation of power relays referred to a video I have recently posted RF compensation of power relays.

Above, the example relay.

So, does this technique work for bigger relays?

Firstly, small is beautiful… it is easier to get good compensation of smaller relays over a wider frequency range.

Above is an example relay by K5UJ for discussion. I do not have measurement data for this relay box, but experience tells me that at HF, the compensation technique discussed above is likely to give good results for its intended purpose as a HF T/R relay. Continue reading RF compensation of power relays – bigger relays

I have recently posted a video on RF compensation of power relays.

Above, the example relay. Continue reading RF compensation of power relays