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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
communication 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). AM mode is used when receiving broadcast transmissions. The international
distress frequency for radiotelephony in the MF band is 2182 kHz. 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. For the MF band ITU
number are allocated for Region I only. 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 ‘all ships’ and 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 is made on an appropriate working frequency using
radiotelephony. 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.
