An antenna tuner, transmatch,
antenna coupler or antenna tuning unit (ATU) matches a transceiver with
a fixed
impedance (typically 50
ohms for
modern transceivers) to a load (feed line and
antenna) impedance which is unknown, complex or otherwise does not
match. This mismatch is usually caused when using a non-resonant
antenna (one that is not the correct electrical length as compared to
the
wavelength of the signal). An ATU allows the use of one antenna for
a broad range of frequencies. A tuned antenna is never as good as a
naturally resonant antenna due to additional induced losses on the feed
line due to the
SWR (multiple reflections), and losses in the ATU itself.
Strictly speaking the 'ATU' is only an antenna matching unit, as it is
unable to change the resonance frequency of the aerial.
Further, most marine SSB-HF installation require a minimum of 100 square
feet of ferrous ground plane, irrespective of the antenna tuner
selected. Basically, the more the ground plane, the better the
operability of the SSB radio system.
