What you'll learn
This revision guide covers how radio waves are used for communication purposes, including television, radio broadcasting and mobile phones. You'll understand the properties of radio waves as part of the electromagnetic spectrum, how they transmit information, and why different frequencies are used for different applications.
Key terms and definitions
Radio waves — electromagnetic waves with the longest wavelengths (from about 1 mm to over 10,000 m) and lowest frequencies in the electromagnetic spectrum.
Frequency — the number of complete waves passing a point per second, measured in hertz (Hz).
Wavelength — the distance from one point on a wave to the equivalent point on the next wave, measured in metres (m).
Transmitter — a device that produces and sends out radio waves carrying information.
Receiver — a device that detects radio waves and extracts the information they carry.
Carrier wave — a radio wave of constant frequency that is modified to carry information.
Modulation — the process of adding information to a carrier wave by varying either its amplitude or frequency.
Ionosphere — a layer of the Earth's atmosphere (80-1000 km altitude) containing charged particles that can reflect certain radio waves back to Earth.
Core concepts
The electromagnetic spectrum and radio waves
Radio waves occupy the lowest frequency, longest wavelength end of the electromagnetic spectrum. All electromagnetic waves:
- Travel at the speed of light in a vacuum (3 × 10⁸ m/s)
- Transfer energy from one place to another
- Can travel through a vacuum
- Are transverse waves
The wave equation applies to all electromagnetic waves:
wave speed (m/s) = frequency (Hz) × wavelength (m)
or v = f λ
For radio waves in air or a vacuum, v = 3 × 10⁸ m/s.
Radio waves are divided into different frequency bands:
- Long wave: frequencies around 30-300 kHz, wavelengths 1-10 km
- Medium wave: frequencies around 300 kHz-3 MHz, wavelengths 100 m-1 km
- Short wave: frequencies around 3-30 MHz, wavelengths 10-100 m
- VHF (Very High Frequency): 30-300 MHz
- UHF (Ultra High Frequency): 300 MHz-3 GHz
- Microwaves: above 3 GHz (shortest radio waves, overlap with microwave region)
Radio wave propagation
Different frequency radio waves travel and behave differently:
Long and medium wave radio
- Can diffract (bend) around obstacles including hills and buildings
- Have long wavelengths suitable for diffracting around large obstacles
- Long waves reflect off the ionosphere, allowing transmission over very long distances
- Used for national and international radio broadcasting
- Can provide coverage over large areas with relatively few transmitters
Short wave radio
- Reflect off the ionosphere at certain times of day
- Can "bounce" between the ionosphere and Earth's surface multiple times
- Enable very long-distance communication (transcontinental)
- Reflection depends on time of day, season, and solar activity
- Used for international broadcasting and amateur radio
VHF and UHF radio waves
- Pass through the ionosphere rather than reflecting
- Travel in straight lines (line-of-sight transmission)
- Limited range due to Earth's curvature
- Do not diffract significantly around large obstacles
- Require transmitter and receiver to "see" each other (or use relay stations)
- Used for FM radio, television, mobile phones and emergency services
- Provide high-quality signals with less interference
Transmitting information using radio waves
Radio waves carry information from transmitter to receiver through modulation. The information (speech, music, or data) is added to a carrier wave.
Amplitude Modulation (AM)
- The amplitude (height) of the carrier wave varies according to the information signal
- The frequency of the carrier wave stays constant
- Used for long and medium wave radio broadcasting
- More susceptible to interference from electrical equipment and atmospheric conditions
- Simple technology, less expensive receivers
Frequency Modulation (FM)
- The frequency of the carrier wave varies according to the information signal
- The amplitude stays constant
- Used for VHF radio broadcasting
- Better sound quality than AM
- Less affected by interference
- Requires more complex and expensive technology
Communication systems and applications
Television broadcasting
- Uses UHF radio waves (around 470-850 MHz in the UK)
- Requires line-of-sight transmission
- Transmitters located on tall towers or high ground
- Digital television uses binary code to transmit picture and sound information
- Limited range typically 50-80 km from transmitter
Mobile phone networks
- Use UHF radio waves and microwaves (typically 800 MHz-2.6 GHz in the UK)
- Network divided into cells, each served by a base station
- Base stations relay signals to a central exchange
- Mobile phones both transmit and receive radio signals
- Same frequencies can be reused in non-adjacent cells
- 4G and 5G networks use higher frequencies for greater data capacity
Bluetooth and WiFi
- Use microwave frequencies (Bluetooth: 2.4 GHz, WiFi: 2.4 and 5 GHz)
- Very short-range communication (typically up to 10 m for Bluetooth, 50 m for WiFi)
- Lower power consumption than mobile phone signals
- Used for connecting devices wirelessly
Radio astronomy
- Radio telescopes detect radio waves from space
- Can observe the universe day and night, through clouds
- Detect objects that don't emit visible light
- Provide different information than optical telescopes
Advantages and limitations of radio communication
Advantages
- No physical connection needed between transmitter and receiver
- Can transmit over very long distances (especially long wave)
- Can reach remote locations
- Relatively inexpensive infrastructure for long wave broadcasting
- Multiple users can share the same frequency bands (with regulation)
Limitations
- Limited range for VHF/UHF (line-of-sight required)
- Signals can be blocked by large obstacles (buildings, hills)
- Interference from other radio sources
- Atmospheric conditions affect propagation
- Finite number of available frequencies must be shared
- Security concerns (signals can be intercepted)
Satellites and communication
Communication satellites orbit Earth and relay radio signals between different points on the surface.
Geostationary satellites
- Orbit at 36,000 km above the equator
- Orbital period exactly 24 hours
- Remain above the same point on Earth
- Used for television broadcasting and telecommunications
- Three satellites can provide coverage to most of Earth's surface
- Signals have noticeable delay (about 0.25 seconds) due to distance
Satellites overcome limitations
- Enable communication over oceans and remote areas
- Signals not blocked by Earth's curvature
- Single satellite covers large area
- Used for satellite television, GPS, and global communications
Worked examples
Example 1: Calculating wavelength
Question: A radio station broadcasts on a frequency of 95.8 MHz. Calculate the wavelength of the radio waves. (Speed of radio waves = 3 × 10⁸ m/s)
Solution:
Step 1: Write down the wave equation v = f λ
Step 2: Rearrange for wavelength λ = v ÷ f
Step 3: Convert frequency to Hz 95.8 MHz = 95.8 × 10⁶ Hz
Step 4: Substitute values λ = (3 × 10⁸) ÷ (95.8 × 10⁶)
Step 5: Calculate λ = 3.13 m
Answer: 3.13 m (accept 3.1 m)
Marks: 3 marks — 1 for correct equation/rearrangement, 1 for correct substitution, 1 for correct answer with unit
Example 2: Explaining radio wave behaviour
Question: Explain why long wave radio signals can be received in valleys between hills, but VHF radio signals cannot. [4 marks]
Mark scheme answer:
- Long wave radio has a much longer wavelength than VHF (1 mark)
- Long waves can diffract around obstacles such as hills (1 mark)
- The wavelength is similar in size to the hills/obstacles (1 mark)
- VHF wavelengths are much shorter and do not diffract significantly around large obstacles (1 mark)
Alternative acceptable points: VHF travels in straight lines / requires line-of-sight; long wave spreads out more after passing obstacles
Example 3: Comparing transmission methods
Question: A radio station broadcasts using medium wave AM at 900 kHz and VHF FM at 96.5 MHz.
(a) Calculate the wavelength of the medium wave signal. [3 marks]
(b) State one advantage of using VHF FM instead of medium wave AM. [1 mark]
Solutions:
(a) λ = v ÷ f λ = (3 × 10⁸) ÷ (900 × 10³) λ = 333 m (or 330 m)
(b) Better sound quality / less interference / clearer signal (any one)
Total: 4 marks
Common mistakes and how to avoid them
Confusing frequency and wavelength: Remember they are inversely proportional — as frequency increases, wavelength decreases. Use the wave equation to convert between them.
Forgetting to convert units: Always convert MHz to Hz (multiply by 10⁶) or kHz to Hz (multiply by 10³) before calculations. Similarly, convert km to m when necessary.
Stating that radio waves need a medium: Radio waves are electromagnetic, not mechanical waves. They can travel through a vacuum and do not require particles to propagate.
Claiming all radio waves reflect off the ionosphere: Only certain frequencies (primarily long, medium and short wave) reflect. VHF, UHF and microwaves pass through the ionosphere.
Mixing up AM and FM: AM varies amplitude (height), FM varies frequency. Don't confuse which characteristic changes.
Not explaining diffraction properly: State that diffraction occurs when waves pass through gaps or around obstacles similar in size to the wavelength. Longer wavelengths diffract more around large obstacles.
Exam technique for "Radio waves and communication"
Command word "explain": You must give reasons why something happens, not just describe what happens. Link cause and effect clearly. For example, "VHF cannot be received because hills block the signal" needs expanding to "VHF travels in straight lines and cannot diffract around large obstacles like hills because its wavelength is much shorter than the size of the hills."
Calculation questions: Always show your working. Write the equation, substitute values with units, then calculate. You can gain method marks even if your final answer is incorrect.
Comparing different types of radio waves: Use comparative language ("longer wavelength," "higher frequency," "diffracts more") rather than absolute statements. Explicitly name both types being compared.
Extended response questions: Structure your answer logically. Use scientific terminology precisely. For 6-mark questions, aim for 5-6 distinct physics points with clear explanations.
Quick revision summary
Radio waves are electromagnetic waves with long wavelengths and low frequencies that travel at the speed of light. Long and medium waves diffract around obstacles and can reflect off the ionosphere, enabling long-distance broadcasting. VHF and UHF waves travel in straight lines and pass through the ionosphere, requiring line-of-sight for transmission. Information is added to carrier waves through amplitude modulation (AM) or frequency modulation (FM). Radio waves are used for broadcasting, mobile phones, WiFi, Bluetooth and satellite communication. Different frequencies suit different applications based on their propagation properties.