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I have gotten some great input about this, but it remains a problem that such outputs as “real” and “imaginary” come with no documentation as to what they are. I have written a 1 pager that some moderator may want to review, edit, and add to the wiki. I don’t even know how to attach things to this group in a way that they carry through, so I will cut and paste it:

NanoVNA Output Variables



LOGMAG – For S11 measurements this is the Return Loss and is measured in dB. For passive networks it will always be negative. For S21 measurements this is the INSERTION LOSS in dB.

SWR – The Standing Wave Ratio, often also called Voltage Standing Wave Ratio (VSWR), is often used for measuring antennas. For S11 measurements an SWR of 1 is a perfect match (zero reflected power). The figure is normally quoted as a ratio (e.g. 2:1). An antenna SWR of 3:1 would normally be considered good, while lower than 2:1 would be excellent. Use of SWR for S21 measurements may be meaningless.

LINEAR – Same as LOGMAG, but expressed as a ratio. For S11; 1 would be 100% power reflected, 0 would be 0% power reflected.

PHASE – The relative phase difference or delay between the signal source voltage and received signal voltage.

SMITH – This is a chart of resistance and reactance. The (real) resistance is shown on the horizontal center line, with 0 ohms (a short circuit) at the 9 o’clock position and infinite ohms (an open circuit) at the 3 o’clock position. The center point indicates whatever normalized resistance is being used, but for the nanoVNA it is always 50 ohms. Any point above the horizontal center line indicates positive (i.e. inductive) reactance and any point below indicates negative or capacitive reactance. This output reports the capacitance (C) or inductance (L) at the marker position. Note that a transmission Smith chart, on CH1, may be meaningless.

POLAR – Same trace as the SMITH chart, but provides the voltage reflection coefficient as a complex number, as shown on marker readings. On this chart, the 9 o’clock position represents -1+j0, the 3 o’clock position is 1+j0, and the 12 and 6 o’clock positions are 0+j1 and 0-j1 respectively.

DELAY – ???

REAL – This is simply the real part of the reflection coefficient gamma (i.e. the number on the Polar chart).

IMAGINARY – This is the imaginary part of the reflection coeff. gamma (i.e. the number on the Polar chart).

RESISTANCE – This is the resistance encountered in the DUT, as shown on the Smith chart.

REACTANCE – This is the reactance of the DUT, as shown on the Smith chart.

DELAY has to be understood here as "Group Delay". It highlights the transit time of a signal through a DUT (Device Under Test) versus frequency. It is usually currently related to transmission measurement (s21).

Its calculation is the mathematical derivative of the phase with respect to frequency. If there is PHASE distortion, i.e. a breakout on its monotonous variation according to frequency, the group delay remains no more at a constant value.

Group Delay = -(1/360)*(phi2-phi1)/(f2-f1)

For the NanoVNA, (f2-f1) is the elementary frequency step (Hz) between two measurement points, which depends of FSTART, FSTOP and choosen number of measurements points (101 ... 401). It means that (phi2-phi1) is the phase value difference (Degrees), between two neighboring measurements points.

Group Delay is a relevant parameter, when for example you consider a non-sinusoidal signal passing through a filter. If Group Delay is not constant, the shape of the signal will be altered at the output of the device, due to a possible time shift on its harmonic components.

A last word about Group Delay when related to reflection measurement (s11). It's less intuitive, but we can consider this example with a DUT which is an opened coaxial cable (length 2 meters), one of my favorite educative DUT ! Here the Group Delay (around 18,2 ns, see snapshot) highlights the total transit time at Port 0 level, i.e. a round trip of the signal through the coaxial cable. From it you can calculate the electrical length of the coaxial cable, and if you know the velocity factor, its physical length.

Round trip : 300.000.000 (m/s) * 18,2 E-09 = 5,46 m
Electrical length : 5,46 / 2 = 2,73 m
Physical length : 2,73 * 0,73 = 1,99 m 0,73 is the velocity factor value for RG58A/U cable type

As advanced firmware for NanoVNA now includes automatic calculation for coaxial length (thanks to DiSlord) based on Smith chart, the previous method is not necessary the most accurate or relevant. It is just for an explanation of Group Delay in s11 measurement context.

Jean-Roger / F6EGK