An in-line SWR and power meter (a directional wattmeter) sits in the feed line and reports how much power goes toward the antenna and how much comes back. The ratio between the two gives the SWR, the quickest check that a transmitter, line and antenna are working together. See SWR and reflections for what SWR means.
How it works
The meter takes a tiny sample of each wave and never absorbs real power. SWR comes from the ratio of the two.
A short section of line runs beside the main line and picks up a tiny fraction of each passing wave. Because of its geometry, one end responds only to a wave heading toward the antenna and the other only to a wave coming back. Each end feeds a diode detector and a meter. The meter never absorbs real power; it only samples.
Power absorbed = forward power − reflected power.
Load absorbs75 W
The power the antenna actually receives is forward minus reflected. Here are common values (all from Γ = (SWR − 1) ÷ (SWR + 1); reflected power = Γ²):
SWR
Reflected power
Return loss
Mismatch loss
1.0
0%
infinite
0 dB
1.5
4%
14.0 dB
0.18 dB
2.0
11.1%
9.5 dB
0.51 dB
3.0
25%
6.0 dB
1.25 dB
5.0
44.4%
3.5 dB
2.55 dB
Even 3:1 costs only about 1.25 dB, a quarter of the power. Reflections matter less to the signal than to the equipment: many transmitters fold their power back as SWR rises, to protect their output transistors.
Peak and average power
Most simple meters respond to average power. A key-down CW or FM carrier is steady, so the reading is the true power. But a voice SSB signal rises and falls, so an average meter reads far below the peak envelope power (PEP) your radio is rated for. For SSB you need a meter with a peak-reading mode.
Pitfalls
Frequency and power range. A meter is accurate only over the band and power it was built for, and may need a minimum forward power to show SWR.
It reads at its own location. Cable loss between the meter and the antenna makes the reading look better than the antenna's own SWR:
Loss makes the input SWR look better than the antenna's. A tuner makes the radio happy but cannot change the SWR on the line.
SWR measured at line input2 : 1
Before or after the tuner. A meter between rig and tuner shows what the radio sees; a meter between tuner and antenna shows the line's real SWR. See Antenna tuners.
Low SWR is not a good antenna. A dummy load reads 1:1 and radiates nothing. SWR says only that the match is good.
Built-in meters are fine for tuning but are rough indications, not calibrated instruments.