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Antennas have impedance as a property. The impedance of an antenna is affected by its size and positioning as well as the wavelength of the signal it is transmitting and receiving.
The impedance of the feed point (the point where the feeder meets the antenna) should match the impedance of the feeder and transmitter.
If the impedance of the antenna does not match that of the feeder, some energy will be reflected back down the antenna to the feeder creating "standing waves".
Standing waves are formed when a wave encounters a boundary between two different mediums which allows the wave to reflect. Although one source generated this wave, it results in two travelling waves, one outgoing and one reflected. This creates an interference pattern the same as two waves overlapping from two separate sources.
A Standing Wave Ratio meter (SWR) tests whether there is a correct match between the impedance of the feeder and the antenna.
A 1:1 ratio is ideal. Anything higher that 2:1 is a mismatch which may degrade the transmitter.
In cases of mismatch, you can use an Antenna Matching Unit (AMU) (there are manual and automatic variants). This is also known as an Antenna Tuning Unit (ATU). This device reduces the SWR to ensure there is no damage to the transmitter.
An AMU is mostly required when you are working "multi-band", that is: using a single antenna from multiple frequency bands.