What an SWR measurement tells you
SWR describes the impedance match at the measurement plane. In a 50-ohm system, an ideal 50-ohm resistive load gives SWR 1:1. A low value means little reflection there; it does not prove that an antenna radiates efficiently. A dummy load can have excellent SWR while turning nearly all the power into heat.
Set up the measurement
- Disconnect the antenna system from the transmitter. Connect only the passive antenna or feed line to the NanoVNA source/reflection port, normally CH0 or PORT1.
- Choose a frequency span covering the band of interest. For a 2-metre antenna, for example, start with a sweep around the intended operating band, then narrow it to inspect the dip.
- Perform an open, short and load calibration at the chosen reference plane.
- Select S11 and the SWR format. Place a marker at your intended operating frequency and note the SWR, not just the lowest value anywhere on the screen.
- Add a Smith trace or impedance readout. Record resistance R and reactance X at the same frequency.
At the feed point or in the shack?
Calibrating at the antenna feed point measures the antenna impedance there. Measuring from the shack with the feed line after the calibration plane measures the combined feed line and antenna. Both are useful, but answer different questions. Cable loss reduces the reflected signal returning to the instrument and can make the measured SWR look better than the match at the antenna itself.
Keep the antenna in its intended position with its usual ground plane or counterpoise. Nearby metal, the feed line and your body can change the result. If moving your hand or the cable causes a large shift, investigate the installation and common-mode currents before cutting the antenna.
Resonance and lowest SWR are different
At resonance, the measured reactance is zero. That does not guarantee that the resistance is 50 ohms. A purely resistive 25-ohm load is resonant in this sense but has SWR 2:1 in a 50-ohm system. Conversely, the minimum SWR of a real antenna system need not occur exactly where X crosses zero. Use both readouts when deciding what to adjust.
For a passive load, SWR = (1 + |Γ|) / (1 − |Γ|), where Γ is the reflection coefficient. SWR 2:1 corresponds to |Γ| = 1/3 and about 11% reflected power at that plane. This figure alone does not tell you the radiated power, because feed-line and antenna losses also matter.
Make one change at a time
Save a baseline, make a small adjustment and repeat the sweep without changing the cable arrangement. Define a useful bandwidth using the SWR limit appropriate to your equipment, rather than assuming one limit fits every system. If the dip is narrow, increase sweep resolution or reduce the span so it is not missed between frequency points.
Read the Smith chart guide to understand the impedance behind the SWR number. For trace selection on the original firmware, see the NanoVNA measurement controls.