Depending on your installation, all three components may be inside
a single case; or the transceiver may be a single unit with the DSC
Controller and CH70 watch receiver as a separate, combined unit; or
may comprise three separate units using a DSC Controller which can be
interfaced with many different makes of VHF and SSB radio.
The VHF transceiver, with the main telephone handset, performs
the same functions as pre-GMDSS equipment - sending and receiving
voice calls and messages. Remote handsets may also be connected. The
DSC Controller, with it's keypad {which may be on the main transceiver
unit; on the handset; or on a separate DSC Controller}, allows you to
compose Digital Alerts for transmitting via the VHF transceiver to other
vessel(s} or shore stations - to let them know that you want to talk, by
voice, on a particular working channel. The Channel 70 Watch Receiver
receives Digital Alerts from other vessels and from Coast Stations,
decodes the alerts and displays them on the controller screen.
Power supply........
Equipment will normally use a Direct Current (DC 12 Volt or 24
Volt) Power Supply - direct from the radio-station batteries or emergency
generator when mains power has been lost; or through the mains rectifier/
battery-charging system, during normal operation.
VHF antennas and propagation
The VHF radio antenna is designed to release your transmissions
efficiently out into the earth's atmosphere - and to receive transmissions
from other VHF radio users, and pass those received signals on to your
DSC Controller and/or the loudspeaker/handset.
The way that radio waves pass through the atmosphere (and
beyond into space, in some cases) is called propagation. VHF radio waves,
in normal circumstances, take a direct path from transmitting antenna to
receiving antenna. This 'Direct Wave' propagation limits VHF
communication to line-of-sight which, for ships at sea, means about 10%
further than the visual horizon (about the same range that you can see a
light over the horizon).
VHF radio waves can travel for great distances into space, using
direct, line-of-sight propagation (see figure 6.3).
To ensure best range between your vessel and any other vessel!
Coast Station - your antenna should be fitted as high as possible; clear
of obstructions; and vertical.MARINE COMMUNICATIONS 129