SWR explained: why a perfect match reads 1:1

Waves that bounce back

Waves that roll up a beach just fade away. Waves that hit a seawall bounce straight back. Where the bounced waves meet the ones still coming in, they add up in some spots and cancel in others, so the water heaves up and down in place. That’s a standing wave.

Your signal does the same inside the coax. The cable is built for 50 ohms. An antenna that matches it takes all the power, like the beach. An antenna cut to the wrong length is the seawall: part of the power bounces back down the cable.

What the ratio means

Standing wave ratio compares the biggest swing on the cable with the smallest. It’s a measure of how well the load, your antenna, is matched to the feed line.

With a perfect match nothing comes back, every spot on the cable swings the same, and the ratio is 1:1. A higher number means a worse match: at 4:1, more than a third of your power is coming back. That’s an impedance mismatch, not 4 dB of anything. Most radios are happy below about 2:1.

Why your radio turns itself down

The power that bounces back heads for your radio. So when SWR climbs past a certain level, most solid-state radios reduce their own output to protect their output amplifier transistors.

If your power output looks low, check the SWR.

How to measure it

Use a directional wattmeter. Put it in the feed line, between the radio and the antenna. One reading shows the power going up the cable, the other the power coming back, and together they give you the SWR.

• Pick a meter made for your frequency and your power level. One built for HF may not read right on 2 meters.
• If the reading jumps around, look for a loose connection in the antenna or the coax.
• To see where your antenna is resonant, use an antenna analyzer. It sweeps across the band and draws the SWR, and the bottom of the dip is where the antenna is resonant. A dip too low in frequency means the antenna is too long: trim it and the dip moves up.
• Testing without going on the air? Use a dummy load: a 50-ohm non-inductive resistor mounted on a heat sink. It’s a perfect match, so your power turns into heat and stays off the air.

Three things to remember

1. One to one is a perfect match.
2. High SWR means power is bouncing back, and your radio turns itself down.
3. Measure it in the feed line, with a directional wattmeter.

Every question this covers

12 questions from the 2026–2030 Technician pool, word for word. On the exam the choices may come in a different order.

1. T4A02 Which of the following should be considered when selecting an accessory SWR meter?

   1. A The frequency and power level at which the measurements will be made Answer
   2. B The distance that the meter will be located from the antenna
   3. C The maximum SWR anticipated on the transmission line
   4. D The ability of the meter to compensate for a poor transmission line match to the antenna

2. T4A05 Where should an RF power meter be installed?

   1. A In the feed line, between the transmitter and antenna Answer
   2. B At the power supply output
   3. C In parallel with the push-to-talk line and the antenna
   4. D In the power supply cable, as close as possible to the radio

3. T7B04 Which of the following might be the cause of low RF power output from a solid-state transceiver?

   1. A Poor amplifier noise figure
   2. B Poor amplifier linearity
   3. C Low SWR
   4. D High SWR Answer

4. T7C01 What is the primary purpose of a dummy load?

   1. A To prevent transmitting signals over the air when making tests Answer
   2. B To prevent over-modulation of a transmitter
   3. C To improve the efficiency of an antenna
   4. D To improve the signal-to-noise ratio of a receiver

5. T7C02 Which of the following is used to determine if an antenna is resonant at the desired operating frequency?

   1. A A VTVM
   2. B An antenna analyzer Answer
   3. C A Q meter
   4. D A frequency counter

6. T7C03 What does a typical RF dummy load consist of?

   1. A A low-voltage power supply and an AC relay
   2. B A 50-ohm non-inductive resistor mounted on a heat sink Answer
   3. C A low-voltage power supply and a DC relay
   4. D A 50-ohm inductive reactance mounted in a shielded enclosure

7. T7C04 What reading on an SWR meter indicates a perfect impedance match between the antenna and the feed line?

   1. A 50:50
   2. B Zero
   3. C 1:1 Answer
   4. D Full Scale

8. T7C05 Why do most solid-state transmitters reduce output power as SWR increases beyond a certain level?

   1. A To protect the RF output amplifier transistors Answer
   2. B To comply with FCC rules on spectral purity
   3. C Because power supplies cannot supply enough current at high SWR
   4. D To lower the SWR on the transmission line

9. T7C06 What does an SWR reading of 4:1 indicate?

   1. A Loss of -4 dB
   2. B Good impedance match
   3. C Gain of +4 dB
   4. D Impedance mismatch Answer

10. T7C08 Which instrument can be used to determine SWR?

    1. A Voltmeter
    2. B Ohmmeter
    3. C Iambic pentameter
    4. D Directional wattmeter Answer

11. T9B09 What can cause erratic changes in SWR?

    1. A Local thunderstorm
    2. B Loose connection in the antenna or feed line Answer
    3. C Over-modulation
    4. D Overload from a strong local station

12. T9B12 What is standing wave ratio (SWR)?

    1. A A measure of how well a load is matched to a transmission line Answer
    2. B The ratio of amplifier power output to input
    3. C The transmitter efficiency ratio
    4. D An indication of the quality of your station’s ground connection

Keep going

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Questions from the NCVEC 2026–2030 Technician question pool, with the errata of February 19, 2026.