__ -- .-I
~ .. ---------------------
Figure 6.3
VHF 'Line of Sight' propagation. Both vessels can talk to the Coast Station but not
to each other. The SAR helo can talk to both vessels and to the Coast Station.Antenna 'polarisation': radio waves from the transmitting
antenna induce electrical energy into the receiving antenna. Maximum
energy is induced when the 'polarity' of both transmitting and receiving
antenna are the same - i.e., when both antennas are either vertical or
horizontal. Accepted practice in the marine VHF network is for vertical-
polarisation of all antennas, at sea and on shore, all over the world.Marine VHF radio - equipment operation and controls
The main controls on a Marine VHF radio, and their function/
method of operation, are:• Power on/off: may be included as a function of the volume control.
• Volume control: adjusts the volume in the loudspeaker/handset.
• Squelch control: required to suppress receiver-generated noise, not
'atmospherics'
When you switch-on for the first time, or change channel, you should adjust
the Volume and Squelch.
The best way to do this is: Volume to Zero; Squelch fully-off; turn
the volume up until the 'hissing' noise is as loud as you would
like any received voice signals to be; then turn the Squelch
carefully on until you just lose the hissing noise.
If you introduce too much Squelch, you will not receive weaker Signals.Channel Selector: may be a numerical keypad, rotary knob, or 'up-
down' push-buttons.Channel 16 Selector: a 'quick Ch16' button - to put you directly onto
Channel 16 (and on full power on most sets) in an emergency.
Overrides all other settings.130 THE NAUTICAL INSTITUTE