VHF radio and its range
Boats mostly use VHF (very high frequency) radio, near 156 MHz. VHF waves travel in straight lines, like light. The sea curves away, so a signal can only travel as far as the horizon between the two antennas. A taller antenna sees further.
range (km) ≈ 4.12 × (√h1 + √h2)
h1 and h2 are the two antenna heights in metres. A mast of 4 m talking to a shore tower of 20 m: 4.12 × (2 + 4.47) ≈ 27 km. Raise the mast to 25 m and the range becomes about 39 km. Real range also depends on the set's power and the weather.
Morse code
Morse code sends letters as short and long signals using light, sound or radio. A dot is 1 unit long. A dash is 3 units long. Between signals there is a 1-unit gap, and a longer gap between letters.
SOS is ··· −−− ··· (three dots, three dashes, three dots). It is the world's call for help. It does not stand for any words. If one unit is 0.3 s, a dot is 0.3 s and a dash is 0.9 s. A signal lamp can send Morse when the radio is broken.
Making a distress call
A distress call is for danger to life. Use VHF channel 16 (156.8 MHz), the international calling and distress channel. Speak slowly and clearly. Say five parts in order:
- MAYDAY, MAYDAY, MAYDAY.
- THIS IS and your boat's name, three times.
- Position: where you are.
- What is wrong: fire, sinking, man overboard.
- Help needed and how many people are on board.
Then say "over" and listen. Never use Mayday for a joke or a small problem.
Radio waves: frequency and wavelength
A radio wave travels at the speed of light, 300 000 km/s (3 × 10⁸ m/s). Its frequency f is how many waves pass per second, in hertz (1 MHz = a million per second). Its wavelength λ is the length of one wave.
λ (m) = 300 ÷ f (MHz)
Marine channel 16 at 156.8 MHz has λ = 300 ÷ 156.8 ≈ 1.9 m. A higher frequency gives a shorter wave. That is why VHF antennas are short rods, about half or a quarter of a wavelength.
Try it
With a torch, send SOS to a friend across a room: three short, three long, three short. Count one-two-three for a long flash. Then slide the Morse speed in the 3D and watch the highlighted bars.
Key formulas and definitions
- VHF range (km) ≈ 4.12 × (√h1 + √h2), heights in metres
- Morse: dot = 1 unit, dash = 3 units, gap = 1 unit
- SOS = ··· −−− ··· (not an abbreviation)
- Wavelength λ (m) = 300 ÷ f (MHz)
- Distress channel: VHF 16 = 156.8 MHz
Worked examples
1. A boat's antenna is 9 m high. The shore tower is 20 m. Find the VHF range.
Range = 4.12 × (√9 + √20) = 4.12 × (3 + 4.47) = 4.12 × 7.47 ≈ 30.8 km.
2. What is the wavelength of 100 MHz radio?
λ = 300 ÷ 100 = 3 m.
3. One Morse unit is 0.3 s. Count units for SOS with a 1-unit gap between every signal and find how long it takes.
SOS has 6 dots (6 units) and 3 dashes (9 units) = 15 units of signal. Between 9 signals there are 8 gaps = 8 units. Total = 23 units = 23 × 0.3 = 6.9 s.
Common mistakes
- Thinking VHF reaches over the horizon like a mobile tower. It goes in straight lines.
- Reading SOS as "Save Our Ship". It is just a pattern that is easy to know.
- Giving the position last in a Mayday call. Give it early, in case the radio fails.
- Mixing up MHz and kHz when finding wavelength. For λ = 300 ÷ f, f must be in MHz.