12.13 Reception Antennæ onboard Vessels
Radio receivers are today standard equipment on board sea-going
vessels.
In most cases it is seldom taken into account, that every receiver, no
matter how expensive it is, can only function as good as its antenna.
What should one take into consideration when choosing an antenna?
x 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.
x The output impedance of the antenna must match the input
impedance of the receiver. In other words, the antenna and the
receiver must conform.
x 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 100 kHz to 30 MHz, i.e.
are designed for use in the lowest LW ranges to the highest SW range.
This corresponds to a wave range of approx. 3000 to 10 metres, an
extremely vast range. Since the ratio of the length of an adapted
antenna is always in fixed proportion to the wave-length, it is
understandable that a long-wire antenna of a certain length can only
be correctly adapted to one single frequency. Therefore, when using a
long-wire antenna such as an insulated back-stay, it is necessary to fit
between the antenna and the receiver cable, a suitable adapter. Such
adapters exist (e.g. MLB marine longwire balun), which solve the problem
as adapter with a large frequency range between the longwire and the
shielded lead cable quite satisfactorily.
The antenna itself should be as high as possible and installed as far away
as possible from local disturbances.
Practically every electric and electronic piece of equipment transmit
interference signals which should not reach the unit via the antenna.
New equipment with corresponding approvals or CE seals have been
designed and constructed, free of disturbance as far as is possible.
Measurements must betaken to make older units free of interference.
The lead from the antenna to the receiver must be a non-dissapative
well shielded coaxial cable with the same impedance as the receiver
input. These are usually laid parallel to other (disturbing) cables and
near other electrical devices. A badly shielded cable picks up local
interference and transmits them to the receiver. The receiver itself should
be connected at the shortest possible distance to a grounding sponge.
