SWR is Standing Wave Ratio. Its a ratio of transmitted power to reflected power.
You want all the power your radio transmits to launch off the antenna. High reflected power due to antenna mistuning or poor installiation subtracts from the transmitted power. Its not quite I am transmtting 10 watts and have 5 reflected back from a mistuned antenna so I am geting only 5 watts out but its close. Here is a copy from
"The most common case for measuring and examining SWR is when installing and tuning transmitting antennas
. When a transmitter is connected to an antenna by a feed line
, the impedance
of the antenna and feed line must match exactly for maximum energy transfer from the feed line to the antenna to be possible. The impedance of the antenna varies based on many factors including: the antenna's natural resonance
at the frequency
being transmitted, the antenna's height above the ground, and the size of the conductors used to construct the antenna.
When an antenna and feedline do not have matching impedances, some of the electrical energy cannot be transferred from the feedline to the antenna.
Energy not transferred to the antenna is reflected back towards the transmitter.
It is the interaction of these reflected waves with forward waves which causes standing wave patterns.
Reflected power has three main implications in radio transmitters: Radio Frequency (RF) energy losses increase, distortion on transmitter due to reflected power from load
and damage to the transmitter can occur.
A tramsitter and even a receiver likes to see a properly tuned antenna most operate at 50 ohms impedance which is an AC measurement not an ohmeter measurement. The SWR meter allow you to tune the antenna for best performance. Antennas change length according to frequency. They get shorter as frequency goes up. Some amateur radio operating frequencies the antenna size is Huge, others a paper clip will suffice. Hope this helps
Gary Fiber K8IZ