An antenna that isn't an appropriate fraction of the transmitted frequency's wavelength eg 1/4, 1/2 etc, reflects a proportion of the energy back down the wiring into the transmitter's circuits. Some antennas can have their length adjusted prior to use to suit the intended frequency, or to be 'close enough' for a limited range of frequencies. Some trailing wire types can be like this, and I used to use a wire dipole antenna when I was an army radio operator that would be unwound to the correct length prior to being strung.
The Antenna Matching Unit is a form of variable resistor that removes the need to change the length of the antenna. Instead it electrically simulates the additional impedance that the fixed length antenna lacks, allowing a shorter antenna than would otherwise be needed. Some types of AMUs require a short transmit burst to set themselves, others have an automatic preset related to the frequency selected.
It's not free though. The antenna will not not be as efficient as a physically correct antenna length.
Last edited by Tinstaafl; 21st Aug 2005 at 14:31.