AQA A-Level Physics Paper 3 (3BE), June 2025: Question 5

Mark scheme

Show the mark scheme

$Mark scheme table detailing levels of response from 1 to 6 marks based on the discussion of ground waves, sky waves, and space waves. Indicative content outlines operating conditions such as frequencies and propagation mechanisms (ground wave: 150–300 kHz via surface diffraction; sky wave: 3–30 MHz via ionospheric refraction/reflection; space wave: 2–100 GHz via line-of-sight to satellite), along with corresponding advantages and disadvantages regarding range, bandwidth, atmospheric attenuation, and capital cost.$

Question Answers Additional comments/Guidance Mark AO

05 The mark scheme gives some guidance as to Ground waves (surface waves) 6 AO1

what statements are expected to be seen in a • use long waves (LW) 150 kHz to 300 kHz (wavelength 1 km to 2

1- or 2-mark (L1), 3- or 4-mark (L2) and 5- or km)

6-mark (L3) answer. Guidance provided in • waves are diffracted by Earth’s surface – travel along curvature.

section 3.10 of the Mark Scheme Instructions

document should be used to assist in Advantages

marking this question. • large area of coverage for given power

• absorption of E-M wave is less at low frequency

Mark Criteria Disadvantages

• may still be receivers in geographical shadows

6 All three pathways covered in • attenuation with distance

some detail. • LW - only able to carry limited information due to restricted

6 marks can be awarded even if bandwidth

there is an error and /or parts of Accept

one aspect missing. medium wave (MW) 300 kHz to 3 MHz (0.1 to 1 km)

5 All three pathways covered, at Disadvantage

least two in detail. • attenuation in this band increases with increase in frequency

Whilst there will be gaps, there • diffraction may be less prominent hence more shadows in the

should only be an occasional higher end of this band

error.

How to answer it

Radio Wave Propagation: Ground, Sky, and Space Waves

📌 What This Question Tests

This 6-mark extended response question assesses your knowledge of electromagnetic wave transmission pathways around the Earth's curvature where line-of-sight is blocked:

Physical propagation mechanism and frequency / wavelength criteria for ground waves, sky waves, and space waves.
Trade-offs of each transmission mode (bandwidth, range, atmospheric interference, and economic factors).
Structuring comparison responses to hit all level-of-response criteria systematically without omission.

Examiner Marking Structure: 6-Mark Extended Response

How marks are distributed across levels (AO1)

Level	Marks	Qualitative Criteria
Level 3	5 – 6	All three pathways covered in detail. Full explanation of operating conditions, plus at least one advantage and one disadvantage for each. To achieve 6 marks, minimal or no gaps should be present.
Level 2	3 – 4	Two pathways discussed successfully, or one in full detail and two partially. Sound understanding with minor omissions.
Level 1	1 – 2	Only one pathway discussed or partial attempts across two pathways. Answers display isolated knowledge fragments.

🧠 Exam Technique: Subheadings Are Your Best Friend

In 6-mark comparative questions, students lose marks primarily by forgetting a component (e.g., giving advantages but forgetting disadvantages). Divide your lined paper with clear headings: Ground Wave, Sky Wave, and Space Wave, and explicitly address Condition, Advantage, and Disadvantage under each.

Pathway 1: Ground Wave (Surface Wave)

Diffraction around the curvature of the Earth

✅ Model Answer Components

Condition: Requires low frequencies / long wavelengths: Long Wave (LW: 150 kHz – 300 kHz , λ = 1 km – 2 km ) or Medium Wave (MW: 300 kHz – 3 MHz , λ = 100 m – 1 km ). Long wavelengths allow waves to diffract around the curvature of the Earth.
Advantage: Provides large area coverage for a given transmitter power; low frequency signals suffer relatively little atmospheric absorption.
Disadvantage: Limited bandwidth due to low carrier frequencies (cannot transmit high data rates/video); signal attenuates with distance over land; geographical shadows behind large mountains.

💡 Physics Mechanism

For diffraction to be significant, the obstacle or aperture size must be of comparable order to the wavelength (λ ≈ d). Because the curvature of the Earth and hills are massive, only electromagnetic radiation with kilometer-scale wavelengths can diffract continuously along the ground.

Pathway 2: Sky Wave

Ionospheric refraction/reflection back to Earth

✅ Model Answer Components

Condition: Requires Short Wave (SW) frequencies typically between 3 MHz and 30 MHz ( λ = 10 m – 100 m ). The wave travels into the ionosphere where ionized gas layers refract the wave downwards back to Earth (effectively acting like total internal reflection).
Advantage: Very long-distance / intercontinental transmission without expensive satellites; lower attenuation compared to ground waves at the same high frequencies.
Disadvantage: Unstable and prone to fading; highly susceptible to changes in solar activity, day/night ionization cycles, and geomagnetic storms.

💡 Critical Frequency Insight

If the frequency is below 0.5 MHz , the wave is absorbed by the lower ionosphere (D-layer). If the frequency exceeds roughly 30 MHz , the refractive index change is insufficient to bend the wave back, and it penetrates into deep space.

Pathway 3: Space Wave (Satellite Link)

Line-of-sight microwave transmission through the ionosphere

✅ Model Answer Components

Condition: Requires high-frequency microwaves ( 2 GHz – 100 GHz , λ = 3 mm – 150 mm ). High frequencies penetrate through the ionosphere without being reflected or absorbed. Requires direct line-of-sight between ground station and satellite (uplink and downlink use separate frequencies to prevent feedback).
Advantage: Very high bandwidth (supports broadband, digital TV, high data streams); enormous global coverage footprint from a geostationary satellite; free from ionospheric fading.
Disadvantage: High initial deployment cost (building and launching satellites); microwaves are attenuated by heavy rainfall / moisture in the atmosphere (rain fade); slight latency/delay over long orbital distances.

📐 Useful Reference: c = f λ

To convert between given frequencies and wavelengths:

Ground (LW): f = 300 kHz = 3 × 10⁵ Hz → λ = (3 × 10⁸) / (3 × 10⁵) = 1000 m
Sky (SW): f = 30 MHz = 3 × 10⁷ Hz → λ = (3 × 10⁸) / (3 × 10⁷) = 10 m
Space (Microwave): f = 30 GHz = 3 × 10¹⁰ Hz → λ = (3 × 10⁸) / (3 × 10¹⁰) = 0.01 m = 1 cm

Common Examiner Traps & Misconceptions

❌ What Lost Students Marks

Confusing the physics mechanism: Saying ground waves "reflect off the ground" (incorrect – they diffract around Earth's surface) or that sky waves "bounce off clouds" (incorrect – they are refracted by ionized plasma in the ionosphere).
Vague frequency terms: Writing just "high frequency" or "low frequency" without naming the band (LW, SW, Microwaves) or citing approximate numerical ranges.
Ignoring "Conditions": Stating only advantages and disadvantages while failing to state the essential condition (e.g. line-of-sight and ionospheric penetration for space waves).
Bandwidth vs Range confusion: Claiming ground waves carry the most information. In reality, bandwidth is directly proportional to carrier frequency, meaning microwaves carry vastly more information than long waves.

Topics

Optional topics · 3.13 Electronics (A-level only)