NanoVNA for the radio amateur: how to measure SWR and tune an antenna

Ten years ago, an instrument that shows the SWR and complex impedance of an antenna across an entire band at once cost as much as a used car and lived in a lab. Today the NanoVNA is a pocket vector network analyzer the size of a cigarette pack for a couple of thousand rubles, and it does almost everything the same. For a radio amateur it's the most useful measuring tool after the multimeter: with it you can see in a minute at what frequency your antenna resonates, how well it's matched, whether the cable is broken and exactly where. This article — no academic jargon — covers how to use it in practice: calibrate it, measure SWR, find resonance and trim the antenna in the right direction.

What the NanoVNA is and how it works

VNA stands for Vector Network Analyzer. "Vector" — because it measures not only how much signal was reflected, but also the phase, i.e. the full complex picture. From this data the instrument itself computes SWR, return loss, impedance, draws the Smith chart and a time-domain trace. Physically it's a small box with a screen (2.8″ on the NanoVNA-H, 4″ on the H4), a wheel or touch control, USB-C for power and a computer link, and two SMA RF connectors:

• Port 1 (CH0) — the reflection port. This is where you connect the antenna or cable. The instrument measures the S11 parameter — what came back. This is the working port for 95% of an amateur's tasks: SWR, resonance, cable length.
• Port 2 (CH1) — the transmission port. It measures S21 — how much signal passed through. Needed for filters, duplexers, attenuators, cables measured "through."

The kit comes with three calibration standards: Open, Short, Load (a 50 ohm load) — usually three small SMA caps, plus a short SMA jumper cable. They're not for decoration; without them the instrument is useless — more on that below.

Calibration — without it everything is a lie

This is the most common beginner question and the most common reason for "the instrument shows nonsense." The NanoVNA measures at the end of its port, but between the port and the antenna there's always an adapter, a jumper cable, connectors — and each one introduces its own loss and phase shift. Calibration "zeroes" the instrument exactly at the point where you'll connect the antenna, and removes the influence of everything up to that point.

The standard SOLT procedure is used (Short — Open — Load — Thru). The order is:

1. First set the range. In the STIMULUS menu set START and STOP around the operating frequency. For 2 m (in Russia that's 144–146 MHz) — for example START 143 MHz, STOP 147 MHz. The calibration is valid only for this range.
2. Screw the jumper cable (the one supplied in the kit) onto Port 1. You must calibrate at the far end of the jumper — where the antenna connector will later go. This is the key point: the standards are screwed onto the end of the cable, not directly onto the instrument.
3. CAL → RESET (clear the old calibration) → CALIBRATE.
4. Screw OPEN onto the end of the cable → press OPEN.
5. Switch to SHORT → press SHORT.
6. Switch to LOAD (50 ohm) → press LOAD. (ISOLN and THRU can be skipped when measuring a single antenna — they're needed for S21.)
7. DONE → save to a slot (SAVE → for example slot 0). Done.

Measuring an antenna's SWR and S11

Calibration is saved — now the antenna itself. Unscrew the standard, screw the antenna's connector (or its feedline) onto the end of the jumper cable. In the DISPLAY → FORMAT menu choose SWR for trace 1 — the instrument will plot SWR vs. frequency. It's useful to set the second trace to SMITH or LOGMAG (S11 in dB, return loss), so you can see both the match and the reactance.

A V-shaped dip — a "checkmark" — will appear on the screen. Its lowest point is the resonance: the frequency where the antenna is best matched and the SWR is minimal. Put a marker (MARKER) at the bottom of the dip — the instrument will show the frequency and value, for example 145.500MHz  1.15.

How to read the SWR figure:

If instead of a "checkmark" you see a flat line near SWR 1.0 across the whole range — that's almost always not a "perfect antenna" but a forgotten calibration with the Load still attached or a break. A real antenna always gives a characteristic dip.

Resonance: which way to trim the element

Suppose the SWR dip of your homemade 2 m antenna turned out to be not at 145 MHz but at 138 MHz — that is, below the desired frequency. The basic rule for whip/wire antennas (and resonators in general):

• Resonance below target → the antenna is too long → shorten the element. Less length = higher resonant frequency.
• Resonance above target → the antenna is too short → lengthen it (or add a loading coil). More length = lower frequency.

Work in small steps and write down the result — it's better to undercut than to overcut (growing a piece of metal back is harder). A rough guide: to raise the resonance of a quarter-wave 2 meter whip by ~1 MHz, you change the length by literally a few millimeters. Trim → re-measure → trim. For typical designs — a quarter-wave GP, balcony variants — see our separate write-ups (links below): they cover counterpoise lengths and the effect of the ground/balcony on resonance.

Cable length and finding a break (TDR)

The NanoVNA can work as a time-domain reflectometer (TDR): it sends a "pulse" down the cable and watches how long it takes the reflection to return. From the delay it calculates the distance to the end of the cable — or to the point of damage. This is a lifesaver when the feedline is buried in a wall/mast and you can't tell whether it's intact.

What you need to know:

1. The cable's velocity factor (VF). For RG-58 ~0.66, for foamed polyethylene/LMR — about 0.82–0.85. Without it the instrument computes the "electrical" length, not the physical one.
2. It's measured from the S11 port (CH0) via the transform function (TRANSFORM → TDR / Low Pass submode), or by exporting S1P data to a PC program (NanoVNA-Saver, processing scripts).
3. The top scan frequency (STOP) is chosen to match the expected length: the longer the cable — the lower the STOP. The physics is simple: the maximum observable "range" is set not by the STOP frequency itself but by the frequency step Δf = STOP/(number of points−1) — the maximum reflection delay equals 1/Δf. So for a long feedline you narrow STOP, for a short one you raise it. A practical guide: a STOP of about 130–150 MHz lets you confidently "see" cables tens of meters long; the exact value is easier to find by trial (if the peak runs into the right edge of the trace — lower the STOP), and the velocity factor VF is entered as a separate parameter, it doesn't affect the choice of STOP.

How to read the result: an open end gives a reflection spike upward (open), a short circuit — downward (short). If the spike arrives earlier than the full cable length was expected — there's a break, a soaked connector or a crushed section there. Poorly soldered/oxidized connectors also show up on the time-domain trace as extra spikes — something that isn't always visible by eye on the connector (see connector repair).

Common beginner mistakes

• Didn't calibrate / calibrated on a different range. The most common cause of "crazy" numbers. Changed START/STOP — repeat SOLT.
• Calibrated right at the instrument but measured through a cable. Then the whole cable "entered" the measurement. Calibrate at the end where the antenna plugs in.
• Connected a powered-on radio / pressed TX. The number-one way to send a NanoVNA to the trash. Passive measurements only.
• Didn't discharge the static from a long outdoor feedline before connecting.
• Scans too wide a range — the dip is "smeared" across a couple of points, the resonance isn't visible. Narrow the band.
• Loose/cheap SMA adapters. A bad adapter = a measurement error and a ruined jack. Don't over-tighten SMA, the torque is small.
• Trusts a "perfectly flat 1.0 line." That's a symptom of an error (Load attached / break), not a perfect antenna.
• Measures a radio's SWR at full power with a regular SWR meter "while at it." That's no longer about the NanoVNA — but if you mix up the tools and feed power into the analyzer, the consequences are described in the red box at the top.