12.5. Antenna Reception On Board
Today radio receivers belong to standard equipment on board
all sea-going vessels.
Too little attention is paid to the fact, that a receiver functions
only as good as the quality of the antenna being used.
What should one take into account when choosing the
antenna?
• The frequency range of the antenna must have the same
range as the receiver, at least have the same range that is
going to be used.
• The output impedance of the antenna must match the
input impedance of the receiver. In other words, the antenna
and the receiver must conform.
• The antenna and the cables leading to the receiver
should transmit the least possible interference to the receiver.
This entails proper grounding of the receiver and the antenna.
Receivers used in the maritime sector are usually constructed to
function within a range of 100kHz to 30MHz i.e. from the lowest
LW range to the top of the SW range. This corresponds to a
wavelength of 3000 metres to 10 metres, which is a vast range.
Since the length of an antenna is in a fixed ratio to a
wavelength, it is understandable that it only matches one
frequency properly. When using a long-wire antenna e.g.
insulated back-stay, a suitable matching transformer should be
fitted between the antenna and the lead to the receiver. There
are such adapters, e.g. a marine balun (MLB)
(balanced/unbalanced), which match the antenna via a
shielded cable to the receiver quite satisfactorily. The antenna
should be installed as high as possible away from any disturbing
factors. Practically every electric or electronic device send out
interfering signals, which should not reach the receiver. Modern
devices carrying a CE approval are more or less disturbance
free. In the case of older devices, it is necessary to take steps to
decrease the interference which they cause. The lead from the
antenna to the receiver must be non-dissipative, well shielded
coaxial cable with the same impedance as the receiver input.
Usually it is laid parallel to other disturbing cables and electric
devices, A badly shielded cable will pick up local interference
and pass it on to the receiver. The receiver itself should be
connected to a grounding plate at the least possible distance.
