SWR is short for Standing Wave Ratio. SWR is important for all feedlines and antennas. SWR is a very complex and deep topic. This page will be a general overview. You can do more in depth reading from the article linked below.

SWR is the measurement of how efficiently your antenna

SWR is expressed as a ratio. You will see 2 numbers, like 1:1 or 3:1.

The optimal SWR is 1:1. 1:2 is acceptable, and 1:3 and higher are problematic.

What do these numbers mean?
Todays amateur radios expect to see 50 ohms of impedance when transmitting. Whether this is a tuned antenna or 50 ohm dummy load, the result is the same – 1:1 SWR.

What if an antenna is properly tuned?
Let’s say you have an antenna that is a close match, but not quite there. Let’s say there is 100 ohms of impedance. Some quick math:

100 Ω ÷ 50 Ω = 2
Therefore, the SWR is 2:1.

Because of their small size, most VHF/UHF antennas don’t need tuners to adjust for SWR. Most commercial VHF/UHF antennas have some physical adjustment to get the antenna tuned.

Every amateur radio station that uses HF should have an antenna tuner available. Not all antennas need them, but it is a good idea to have a tuner and power meter to monitor the output and reflected power. If a multiband antenna is used, a tuner may be necessary to ensure a correct match. Read more about antenna tuners here: ANTENNA TUNERS

If you see SWR change as you are transmitting, you may have some parts affected by arcing or heat, or you may have a loose connection.

You can read more about SWR from the ARRL here: https://www.arrl.org/files/file/Technology/tis/info/pdf/q1106037.pdf

Here is a fantastic video of visual SWR from AT&T/Bell Labs: