Mf/Hf Fundamental Info - Sailor System 5000 MF/HF User Manual

1 MF/HF Fundamental info

Propagation of MF and HF Radio Waves.
MF/HF radiocommunications provide a medium and long range service. The 1.6-4 MHz
marine band is intended primarily for coastal operation beyond normal VHF communication
range. A reliable range of more than 150 nautical miles can be expected in most areas in the
daytime, more in the nighttime. Propagation of the radio waves in this band is mainly by
ground waves i.e. the waves from the transmitter aerial follow the earth's curvature to the
receiver aerial. The high frequency range 4 - 30 MHz can provide communication for
hundreds or even thousands of nautical miles. The long range is achieved by sky waves
reflected from the ionosphere. Propagation of the radio waves depends on a number of
factors such as frequency, time of day, time of year, and solar activity. The channels
allocated to the maritime mobile service in the HF range are divided into a number of bands:
4, 6, 8, 12, 16, 18, 22, 25 MHz to allow a suitable frequency band to be selected for commu-
nication dependent on distance and time of day.
Radiotelephony
The mode of emission used for telephony transmissions in the marine bands is SSB (single-
sideband, J3E). On the international distress frequency 2182 kHz compatible AM (amplitude
modulation, H3E) may be used in addition for communication with non-GMDSS ships. AM
mode is used also when receiving broadcasting. The frequencies for radiotelephone distress
and safety traffic in the HF bands are 4125 kHz, 6215 kHz, 8291 kHz, 12290 kHz, and 16420
kHz. Working frequencies for public correspondence with coast stations are arranged in pairs
for duplex/semi-duplex operation. For the HF bands these channels are allocated numbers
by ITU on an international basis. In addition a number of simplex frequencies are available in
each band for ship-to-ship communication.
Radiotelex
Marine telex is also referred to as (NBDP) 'Narrow Band Direct Printing'. Due to the narrow
bandwidth of the transmissions, a longer range may be expected compared to radiote-
lephony. The frequencies for radiotelex distress and safety traffic are 2174.5 kHz, 4177.5
kHz, 6268 kHz, 8376.5 kHz, 12520 kHz, and 16695 kHz. Working frequencies for public
correspondence with coast stations are arranged in pairs. For the HF bands these channels
are allocated numbers by ITU on an international basis. In addition a number of simplex
frequencies are available in each band for ship-to-ship communication.
DSC
DSC (Digital Selective Calling) is an automatic calling system which allows a specific station
to be contacted and made aware that a station wishes to communicate with it. In addition to
calls to specific stations the system can also be used to call groups of ships and this is of
significance for its use for DSC distress alerting. DSC is an alerting signal only and the
communication which follows the call is made on an appropriate frequency band using
radiotelephony or radiotelex. The frequencies for DSC distress and safety calling are 2187.5
kHz, 4207.5 kHz, 6312 kHz, 8414.5 kHz, 12577 kHz, and 16804.5 kHz. Calling frequencies
for public correspondence with coast stations are arranged in pairs, both international and
national frequencies are assigned. In addition the frequency 2177 kHz may be used for ship-
to-ship calling.
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