Transmission media (A-level only) — AQA A-Level Physics

Transmission media is the A-level topic that explains how information physically travels from a transmitter to a receiver. You study three main categories: metal conductors such as copper twisted pair and coaxial cable, optical fibres that guide infrared or visible light by total internal reflection, and wireless media such as radio waves, microwaves and satellites that use free space. For each medium you must be able to describe the mechanism of propagation and compare attenuation, dispersion, bandwidth, noise immunity, security and cost.

This topic matters because every communication system, from a home broadband link to a transatlantic fibre cable, is designed around the trade-offs between these media. It builds directly on waves, refraction and total internal reflection from AS, and it feeds into later work on signal regeneration, multiplexing and the electromagnetic spectrum. Exam questions often ask you to choose a medium for a given application and justify your choice with quantitative or comparative reasoning.

• →Attenuation is the loss of signal amplitude or power as it travels, measured in decibels (dB) or dB km-1; it increases with distance and is caused by absorption and scattering in the medium.
• →Dispersion is the spreading of a signal pulse in time. In optical fibres, modal dispersion arises from different ray paths and chromatic dispersion from different wavelengths travelling at different speeds; both limit the maximum bit rate.
• →Optical fibres guide light by total internal reflection at the core-cladding boundary, which requires the core refractive index to be greater than the cladding and the angle of incidence to exceed the critical angle.
• →Copper cables carry electrical signals and suffer from resistive heating, skin effect and electromagnetic interference, but they are cheap, easy to terminate and can carry power.
• →Wireless media use free-space propagation of electromagnetic waves; their range and data rate depend on frequency, atmospheric absorption, diffraction and the need for line-of-sight at microwave frequencies.

• Identify metal wire, optic fibre and free-space electromagnetic waves as transmission media and state that each guides or carries a signal between transmitter and receiver.
• Describe metal wire as guiding an electrical signal, with advantages such as low cost and simple connection, and disadvantages such as attenuation and susceptibility to electromagnetic interference.
• Describe optic fibre as guiding light by total internal reflection, with advantages such as low attenuation, high bandwidth and immunity to electromagnetic interference.
• Describe radio waves as free-space electromagnetic radiation that can diffract around obstacles and travel long distances, and microwaves as higher-frequency radiation suited to line-of-sight, high-bandwidth links.
• Compare media using criteria such as attenuation, bandwidth, interference, security, cost and installation requirements.
• Ground waves propagate along or near the Earth's surface and can follow its curvature because long-wavelength radiation diffracts significantly around the Earth.
• Diffraction is greater when wavelength is comparable to, or larger than, the size of the obstacle or aperture; the Earth itself acts as the obstacle for ground waves.
• Sky waves travel into the atmosphere and are gradually refracted by the ionosphere, where free electrons make the refractive index vary with height and frequency.
• Reflection at an ionospheric layer is a simplified model of the continuous refraction that can return a sky wave to the ground.
• The ionosphere's ionisation varies with time of day, season and solar activity, so sky-wave propagation is variable and frequency-dependent.
• Shorter wavelengths diffract less around the Earth, so ground-wave communication is more effective at long wavelengths.
• Multiple hops between the ionosphere and the Earth's surface can allow sky waves to travel very long distances.
• Satellite links use microwaves because these frequencies pass through the ionosphere and atmosphere with relatively little attenuation.
• An uplink transmits from a ground station to the satellite; the satellite amplifies and retransmits the signal as a downlink, often using a different frequency to prevent interference.
• Geostationary satellites have a 24-hour orbital period above the equator, so ground dishes can remain fixed; this suits broadcast and continuous communication.
• Low-Earth-orbit satellites travel faster and closer, giving shorter signal delay but requiring tracking or a constellation of satellites for continuous coverage.
• Typical satellite transmission frequencies lie in the microwave region, approximately 1–30 GHz, with bands such as C, Ku and Ka allocated to different services.
• Higher microwave frequencies allow greater bandwidth but suffer more atmospheric attenuation, especially in heavy rain.
• States that up-link and down-link use different carrier frequencies.
• Explains that using the same frequency would cause the receiver to pick up its own transmitted signal.
• Describes de-sensing as the strong local signal overloading or desensitising the receiver, reducing its ability to detect the weak incoming signal.
• Explains that a frequency offset (duplex spacing) allows filtering so the receiver is not de-sensed.
• Applies the idea to a satellite or duplex radio system with a numerical example of frequency separation.
• Identifies at least two transmission media, such as copper cable, optical fibre, and wireless.
• States an advantage of a medium in terms of data transmission rate, cost, or security.
• States a disadvantage of a medium in terms of data transmission rate, cost, or security.
• Compares media, for example fibre has higher data rate than copper but higher installation cost.
• Relates the choice of medium to a specific context, such as security needs or budget.

• 💡When comparing media, name the criterion first and then apply it, for example attenuation: optic fibre has lower attenuation than metal wire.
• 💡Link each medium to a realistic context, such as fibre for long-haul high-bandwidth links and radio for mobile or broadcast links.
• 💡Use correct terminology such as total internal reflection, attenuation, bandwidth and electromagnetic interference.
• 💡When explaining ground-wave range, explicitly link wavelength to the amount of diffraction around the Earth rather than saying only that long waves travel further.
• 💡For sky waves, describe the change in refractive index with height in the ionosphere and state that the wave bends progressively until it may return to Earth.
• 💡Use a labelled diagram showing the Earth's surface, the ionosphere and possible paths; annotate where diffraction, refraction and reflection occur.
• 💡Quote typical frequencies in the microwave region, such as gigahertz values, and link the choice to low atmospheric absorption.
• 💡Compare geostationary and low-Earth-orbit systems in terms of delay, coverage and tracking rather than listing only one type.
• 💡Explain why uplink and downlink frequencies differ: to avoid the strong transmitted signal interfering with the much weaker received signal.
• 💡Define de-sensing clearly in your answer, linking it to receiver overload.
• 💡Use a labelled diagram or numerical example to show the frequency separation.
• 💡Explain the consequence of not using different frequencies, not just state that they are different.
• 💡Use a table to compare media across rate, cost, and security for clarity.
• 💡Give a specific example, such as 'fibre offers data rates of 1 Gbit s⁻¹ over several kilometres'.
• 💡Link your comparison to the context given in the question, such as a hospital or a home network.
• 💡When comparing media, always use a point-evidence-application structure. For example: 'Optical fibre has lower attenuation (about 0.2 dB km-1), so repeaters can be 50-100 km apart, reducing cost on long-haul links.'
• 💡Learn typical numerical values: attenuation of copper is a few dB km-1, optical fibre is about 0.2 dB km-1, and the critical angle for a typical fibre is around 80 degrees. Quoting these earns credit in evaluation questions.
• 💡In 'explain' questions, always link cause to effect. Do not just say 'there is dispersion'; say 'different rays travel different path lengths, so they arrive at different times, so pulses broaden and overlap, limiting the bit rate'.

• Saying optic fibre carries an electrical current; correct by stating that it guides light by total internal reflection.
• Claiming radio waves cannot carry digital data; correct by stating that radio links carry modulated signals, including digital information.
• Treating microwaves as a different physical phenomenon from radio waves; correct by stating that both are electromagnetic waves and differ mainly in frequency and propagation behaviour.
• Treating ground waves as travelling in a straight line through the Earth; correct this by explaining that they follow the surface and spread around the curvature by diffraction.
• Describing sky waves as reflecting perfectly from a sharp mirror-like layer; correct this by explaining gradual refraction in a medium whose refractive index changes with height, with reflection as a limiting description.
• Assuming the ionosphere is constant; correct this by linking its changing ionisation to time of day, solar activity and frequency, which changes which frequencies return to Earth.
• Stating that satellites use sound waves or visible light; correct this by identifying microwaves as the typical transmission medium.
• Confusing uplink and downlink; correct this by defining uplink as Earth-to-satellite and downlink as satellite-to-Earth.
• Claiming all satellites are geostationary; correct this by distinguishing geostationary orbits from low-Earth orbits and their different delay and tracking requirements.
• Thinking that different frequencies are needed only to avoid interference between different users; correction: the key issue is the receiver being de-sensed by its own transmission.
• Confusing up-link and down-link; correction: up-link is Earth to satellite, down-link is satellite to Earth.
• Assuming the frequency difference must be large; correction: any sufficient offset that allows filtering and avoids de-sensing is adequate, though practical systems use specific bands.
• Claiming fibre is always cheaper; correction: fibre usually has higher installation cost than copper, though it may have lower long-term maintenance.
• Ignoring security in comparisons; correction: security is a required consideration, so discuss interception or tapping risks.
• Confusing data rate with bandwidth; correction: data rate is the actual transmission speed, while bandwidth is the range of frequencies available, though they are related.
• Students often say that digital signals are immune to noise. In fact noise is added to the signal, but because only two discrete levels must be distinguished, the signal can be regenerated at repeaters, whereas analogue noise accumulates and cannot be removed.
• Many students think total internal reflection in an optical fibre is reflection from a mirrored coating. It is not; it occurs at the boundary between core and cladding because the core has a higher refractive index, and no light is lost through the cladding.
• Students frequently confuse attenuation with dispersion. Attenuation is a loss of amplitude or power; dispersion is a spreading of the pulse in time. A signal can be strong but still unreadable if dispersion has caused pulses to overlap.

1. 1Day 1-2: Read the specification and make a comparison table for copper, optical fibre and wireless media with columns for mechanism, attenuation, dispersion, bandwidth, noise immunity, security and cost.
2. 2Day 3-4: Work through calculations on attenuation in dB and dB km-1, and practise ray diagrams for total internal reflection in a step-index fibre.
3. 3Day 5-6: Write model answers to 6-mark comparison and explanation questions, using the point-evidence-application structure and quoting typical numerical values.
4. 4Day 7-8: Complete past-paper questions on this topic, mark them against the scheme, and make a list of every mark you dropped and why.
5. 5Day 9-10: Use active recall to test yourself on definitions and mechanisms, then redo any questions you found difficult without notes.

• 📋Calculation of output power or attenuation in dB or dB km-1 for a given cable length. Advice: write the dB equation, substitute carefully and check whether the question asks for power or amplitude, since the factor is 10 for power and 20 for amplitude.
• 📋Explain why a particular medium is chosen for a given application, often 4-6 marks. Advice: give at least two advantages and one disadvantage, and link each to the specific context such as long-haul internet, LAN or satellite TV.
• 📋Describe and explain the propagation of light in an optical fibre, including a ray diagram. Advice: label core, cladding, angle of incidence and critical angle, and state that n_core > n_cladding.
• 📋Compare analogue and digital signals in terms of noise and regeneration. Advice: define both, explain how noise affects each, and state that regeneration restores a digital signal but analogue signals cannot be perfectly restored.