SWR is short for Standing Wave Ratio. SWR is important for all feedlines and antennas.

This is where radios and music have similarities – all waves behave the same. Imagine the strings of a grand piano. Each is a different length to create a different note. The different notes are different frequencies. Sometimes these strings lose tension and need to be tuned.

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

HF is

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

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