Your First S21 Measurement

S21 measures the signal transmitted from Port 1 to Port 2. This is essential for characterizing filters, amplifiers, cables, and attenuators.

What You Will Learn

Connect Port 1 (CH0) to DUT input

This is the source port. The NanoVNA generates a test signal here.
Connect Port 2 (CH1) to DUT output

This is the receiver port. The NanoVNA measures what comes through.
Power on the NanoVNA

The sweep starts automatically. You should see activity on both S11 and S21 traces if both are enabled.

Enable an S21 trace

Go to DISPLAY > TRACE > TRACE 2 to enable a second trace.
Set the trace to S21 LOGMAG

Go to DISPLAY > FORMAT S21 (THRU) > LOGMAG

This shows transmission in dB:
- 0 dB = no loss (full transmission)
- -3 dB = half power transmitted
- -10 dB = 10% power transmitted
- -20 dB = 1% power transmitted
Set the channel to S21

Go to DISPLAY > CHANNEL and select S21 (CH1).

Access these via DISPLAY > FORMAT S21 (THRU).

Format	What It Shows	Good For
LOGMAG	Transmission gain/loss in dB	Filter passband, insertion loss
PHASE	Transmission phase in degrees	Phase response, group delay
DELAY	Group delay in seconds	Filter linearity
LINEAR	Linear magnitude (0-1)	Viewing transmission coefficient
SMITH	S21 on Smith chart	Matching network analysis

For more detailed component analysis:

Set the frequency range

Set a range that covers the filter’s passband plus some stopband on each side.

For a 10.7 MHz IF filter:
- START: 10M
- STOP: 11.5M
Calibrate with THRU

For accurate measurements, perform at least a THRU calibration:
- Connect Port 1 cable directly to Port 2 cable (bypass the DUT)
- Go to CAL > CALIBRATE > THRU
- Wait for sweep to complete
- Select DONE
Connect the filter

Insert the filter between the two cables.
Read the passband

The flat region at the top of the trace is the passband. Use markers to measure:
- Center frequency
- Insertion loss (value at passband center)
- -3 dB bandwidth (use marker search)
Check stopband rejection

The regions outside the passband show how well the filter blocks unwanted signals.

Set a wide frequency range

Attenuators should be flat across frequency:
- START: 50k (50 kHz)
- STOP: 900M (900 MHz)
Calibrate with THRU

Connect cables together and perform THRU calibration.
Insert the attenuator

Connect the attenuator between the two ports.
Read the attenuation

The trace should be flat. The value (in negative dB) is the attenuation.

Example: A -6 dB trace means a 6 dB attenuator.

If you use an external attenuator to protect the NanoVNA from high-level signals, you can compensate for it.

Go to S21 Offset setting

DISPLAY > SCALE > S21 OFFSET
Enter the offset value

Enter the attenuation of your external attenuator in dB. For a 20 dB attenuator, enter 20.
Readings are now corrected

The S21 display now shows the true transmission value with the attenuator loss removed.

Short the far end of the cable

Put a short circuit at the end of the cable you want to measure.
Connect the near end to Port 1

Measure S11, not S21 for this method.
Alternative: Two-port method

Connect one end to Port 1, other end to Port 2. The S21 reading shows one-way loss.

Add a phase trace

Enable another trace and set it to DISPLAY > FORMAT S21 (THRU) > PHASE
Interpret the phase
- A linear phase slope indicates constant group delay
- Phase jumps of 180 degrees occur at sharp nulls
- Use DELAY format to see group delay directly
Set electrical delay

To remove the linear phase slope from a cable:
- Go to DISPLAY > SCALE > E-DELAY
- Enter the cable delay in seconds (or use marker to set)

Setup
- Frequency range: 1 MHz to 500 MHz
- Traces: S21 LOGMAG and S11 LOGMAG
Calibrate
- Perform OPEN, SHORT, LOAD on Port 1
- Perform THRU between ports
- Select DONE
Connect the filter
Expected results for 100 MHz low-pass filter
- S21 near 0 dB below 100 MHz (passband)
- S21 drops sharply above 100 MHz (stopband)
- S11 shows good match (low return loss) in passband
Find the cutoff frequency
- Use marker search to find -3 dB point on S21
- This is the filter’s cutoff frequency