Transmission Media: Wires, Fibres & Radio Revision Notes | A Level AQA Physics

1. Why Transmission Media Matter

Every time you stream a video, make a phone call, or connect to Wi-Fi, a signal is travelling through some kind of transmission medium. The choice of medium determines how fast data travels, how far it reaches, how secure it is, and how much it costs to install. Getting this choice right is an engineering decision with real consequences.

AQA expects you to compare three broad categories: metal wire, optic fibre, and electromagnetic propagation (radio and microwave). Each has a distinct physical mechanism, and each comes with trade-offs. This lesson builds the conceptual framework you need to analyse and evaluate those trade-offs in an exam context.

2. Metal Wire: The Familiar Conductor

Metal wire — typically copper — carries electrical signals as oscillating voltages or currents. Picture the signal as a pressure wave pushing electrons back and forth along the conductor. The wire is cheap to manufacture, straightforward to connect, and the technology is mature and well understood.

The key limitation is attenuation. As the signal travels, energy is lost to the resistance of the wire and to electromagnetic radiation leaking outward. This means signals must be amplified (boosted) at regular intervals along long cable runs, adding cost and complexity. Copper wire also acts as an antenna — it can pick up external interference and, critically, it radiates the signal outward, making it relatively straightforward to intercept.

Data transmission rate through copper wire is limited by its bandwidth. Higher-frequency signals carry more data per second, but high-frequency signals attenuate more rapidly in copper. This is why traditional telephone copper pairs (twisted-pair cables) struggle to deliver the multi-gigabit speeds that modern broadband demands over long distances.

3. Optic Fibre: Light as the Signal Carrier

Optic fibre replaces electrical signals with pulses of light — typically infrared laser light — travelling through an extremely pure glass or plastic core. The core is surrounded by a cladding layer with a lower refractive index, and this difference is what keeps the light trapped inside through a process called total internal reflection.

Think of it like a very long, thin mirror tube. The light bounces along the fibre at angles that always exceed the critical angle at the core-cladding boundary, so essentially none of it escapes. Because light travels at $3.0 \times 10^8$ m/s in a vacuum (and only slightly slower in glass), and because the frequency of light is enormously higher than any electrical signal in copper, the potential bandwidth is vastly greater.

Attenuation in optic fibre is dramatically lower than in copper wire. Modern single-mode fibres can carry signals over tens of kilometres before needing regeneration, compared with a few kilometres for copper. The signal is also immune to electromagnetic interference because light does not interact with external electric or magnetic fields.

Security is a significant advantage. Because the signal is light confined inside a solid glass core, tapping the fibre without detection is extremely difficult — any physical intrusion causes measurable signal loss that can be detected. This makes optic fibre the preferred medium for high-security and high-volume data links such as undersea intercontinental cables and financial trading networks.

4. Electromagnetic Propagation: Radio and Microwave

When no physical cable is practical — across oceans, to moving vehicles, or to remote locations — electromagnetic waves carry the signal through space or the atmosphere. The frequency of the wave determines how it propagates, and AQA requires you to understand three distinct mechanisms: ground waves, sky waves, and diffraction of long-wavelength radiation.

4a. Ground Waves

Ground waves are radio waves that travel along the surface of the Earth, effectively hugging the curvature of the ground. They are associated with low frequencies — roughly in the range of a few kHz up to about 3 MHz (the LF and MF bands). At these frequencies, the wavelength is long enough that the wave diffracts around the curvature of the Earth rather than shooting off in a straight line into space.

The mechanism is diffraction: when a wave encounters an obstacle or a curved surface whose size is comparable to or smaller than the wavelength, the wave bends around it significantly. The Earth's surface acts as that obstacle. Long-wavelength (low-frequency) waves, whose wavelengths can be kilometres in scale, are much larger than typical surface features and diffract significantly around the Earth's curvature, whereas short-wavelength (high-frequency) waves do not bend enough and travel in straight lines, quickly leaving the surface.

Ground waves are attenuated by the conductivity of the ground — they induce currents in the Earth's surface, losing energy as they travel. This limits their practical range to a few hundred to a few thousand kilometres depending on frequency and ground conductivity. AM radio broadcasting in the medium-wave band uses ground-wave propagation for local and regional coverage.

4b. Sky Waves: Refraction and Reflection in the Ionosphere

At higher frequencies — roughly 3 MHz to 30 MHz (the HF or short-wave band) — a different propagation mechanism becomes dominant. Waves directed upward at an angle encounter the ionosphere, a region of the upper atmosphere (roughly 60 km to 1000 km altitude) where solar radiation ionises gas molecules, creating a layer of free electrons.

These free electrons interact with radio waves in a way that reduces the effective refractive index of the ionosphere compared with the air below. As the wave travels upward into progressively more ionised layers, it is continuously refracted — bent gradually — until the angle of refraction is large enough that the wave curves back downward toward the Earth. This is analogous to how light bends when it passes from air into glass, but here the gradient of electron density causes a continuous, gradual bending rather than a sharp boundary.

At sufficiently high electron densities, the refraction is so strong that the wave is bent back to Earth. The wave then bounces between the Earth's surface and the ionosphere, travelling thousands of kilometres in a series of hops. This is why short-wave (HF) radio can achieve intercontinental communication without any physical infrastructure — the ionosphere acts as a natural reflector. Note that the correct physical mechanism is gradual refraction through a gradient of electron density, not a sharp reflection at a boundary; 'reflected' is sometimes used informally but should be avoided in exam answers where the mechanism mark requires 'refracted'.

The behaviour of sky waves depends on the ionospheric conditions, which vary with time of day, season, and the 11-year solar cycle. At night, some ionospheric layers weaken or disappear, changing which frequencies are reflected. This is why AM radio stations that use sky-wave propagation can be received at much greater distances at night than during the day — a phenomenon familiar to anyone who has picked up a distant station on a car radio after dark.

4c. Microwave Line-of-Sight and Satellite Links

At microwave frequencies (roughly above 1 GHz), wavelengths are too short to diffract around the Earth and too high in frequency to be reflected by the ionosphere. Microwave links therefore require line-of-sight paths — the transmitter and receiver must be able to 'see' each other, either directly or via a satellite relay in orbit.

Microwave links offer very high bandwidth and are used for mobile phone base-station backhaul, satellite television, and point-to-point data links between buildings. The trade-off is that they require careful alignment, are affected by heavy rainfall (which absorbs microwave energy), and satellite links introduce a one-way propagation delay of roughly 120 ms (round-trip approximately 240 ms) for a geostationary orbit — noticeable in voice calls and problematic for real-time applications.

5. Comparing Transmission Media: A Structured Evaluation

AQA requires you to compare media across three dimensions: data transmission rate, cost, and security. The table below consolidates the key points for exam use, but understanding the reasoning behind each entry is what earns marks in extended-response questions.

Notice the pattern: higher data rates generally come with higher installation costs and better security. Wireless media are cheap to deploy over large areas but offer the weakest security because the signal propagates in all directions and cannot be physically contained.

6. Exam Technique: Evaluating and Comparing Transmission Media

Extended-response questions on this topic typically use the command words 'compare', 'evaluate', or 'suggest'. Each demands a different response structure, and confusing them is one of the most common ways students lose marks.

A 'compare' question requires you to address both similarities and differences explicitly. Do not describe one medium and then the other in isolation — you must make direct comparative statements. Phrases such as 'whereas', 'in contrast to', and 'both... however...' signal to the examiner that you are genuinely comparing rather than listing.

An 'evaluate' question requires a conclusion. After presenting the evidence for and against each option, you must make a judgement. A response that lists advantages and disadvantages but never reaches a verdict will not access the highest mark band. Your conclusion should be tied to the specific context given in the question — the best medium for a rural broadcast network is not the same as the best medium for a secure financial data link.

For a 6-mark evaluate question, a strong response follows this structure: open with a direct statement of which medium is most appropriate for the given scenario; present two or three specific advantages with physical reasoning; acknowledge at least one significant limitation; then close with a justified conclusion that weighs the evidence against the scenario's requirements.

Across all three specification requirements — transmission media properties, wave propagation mechanisms, and comparative evaluation — the examiners reward responses that link physical mechanisms to observable consequences. Knowing that optic fibre uses total internal reflection is recall; explaining why that mechanism produces lower attenuation and better security than copper wire is the analysis that accesses the higher mark bands.