VHF and MF/HF Radiotelephony - Operation, Channels and Range
What the exam tests here
Practical operating knowledge: which channel or frequency to use for what purpose, transmitter power, propagation and range, and the calling and reply conventions. Questions frequently contrast VHF line-of-sight behaviour with MF ground wave and HF sky wave.
Channels you must know
- Ch16 156.800 MHz - distress, safety and calling.
- Ch70 156.525 MHz - DSC only, no voice.
- Ch13 156.650 MHz - bridge-to-bridge navigation safety, 1 W.
- Ch06 156.300 MHz - ship-to-ship.
- Ch09 156.450 MHz - alternate calling (marinas, port).
- Ch72 156.625 MHz - ship-to-ship.
- Ch15/17 - low power (1 W) on-board.
- Ch87B/88B (161.975/162.025 MHz) - AIS 1 and AIS 2; do not transmit voice there.
MF/HF distress frequencies
2182 kHz radiotelephony, 2187.5 kHz DSC, 2174.5 kHz NBDP. HF pairs include 4125/4207.5/4177.5, 6215/6312/6268, 8291/8414.5/8376.5, 12290/12577/12520 and 16420/16804.5/16695 kHz.
Range and propagation
VHF is essentially line of sight: about 20-30 nm typical, improved by antenna height. MF ground wave gives roughly 100-150 nm by day, less reliable beyond that; at night MF sky wave can extend range but with fading. HF works by sky wave over thousands of nautical miles, and you choose the band by time of day: lower frequencies (4/6 MHz) at night, higher (8/12/16 MHz) by day.
Worked example
At 02:00 a vessel 800 nm away does not answer on 8 MHz. Reason: the daytime-suitable band is above the critical frequency at night. The operator should shift to 4 or 6 MHz, where the lower frequency is reflected rather than escaping through the ionosphere.
Traps
- Using Ch13 at high power; it must be 1 W to keep the bridge-to-bridge range short.
- Using Ch16 for routine chat instead of a working channel.
- Staying on a day HF band at night and concluding the radio is broken.