Attenuation is the gradual loss of a wave's intensity as it travels. Attenuation is caused by spreading, absorption and scattering.

Path loss is the total reduction in signal power between a transmitter and a receiver. Path loss includes free space loss plus absorption, reflection and diffraction by buildings, trees and the atmosphere.

How does a radio signal change when the distance from the transmitter increases tenfold in free space?

The intensity falls to one hundredth, because intensity follows an inverse square law. The fall in signal level is 20 dB.

A transmitter gives an intensity of 4.0 × 10⁻⁶ W m⁻² at 2.0 km. What is the intensity at 8.0 km, allowing for free space loss only?

The intensity is 2.5 × 10⁻⁷ W m⁻². The working is 4.0 × 10⁻⁶ × (2.0 ÷ 8.0)² = 2.5 × 10⁻⁷ W m⁻².

Why does a wi-fi signal weaken more through walls than through open air?

Walls absorb and reflect part of the radio energy, adding to the loss from spreading. Water in people and plants also absorbs 2.4 GHz waves strongly.

CCEA A-Level Life and Health Sciences 0008

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Sound and Light

Attenuation of radio waves and the Doppler effect

3 things to know
A2 4

Overview

Attenuation, free space and path loss with worked intensities, walls and wi-fi, the Doppler effect for sirens with worked frequencies, radar speed guns and Doppler ultrasound.

What you need to know

Answer each one before you reveal it.

Your recallNot started

Key term

What is attenuation?

Key term

What is free space loss?

Key term

What is path loss?

Questions from this subtopic

Each one with the model answer underneath.

1
What is attenuation?
Model answer
Attenuation is the gradual loss of a wave's intensity as it travels. Attenuation is caused by spreading, absorption and scattering.
2
What is free space loss?
Model answer
Free space loss is the fall in signal intensity as a wave spreads over a larger area in empty space. The intensity follows an inverse square law with distance.
3
What is path loss?
Model answer
Path loss is the total reduction in signal power between a transmitter and a receiver. Path loss includes free space loss plus absorption, reflection and diffraction by buildings, trees and the atmosphere.
4
How does a radio signal change when the distance from the transmitter increases tenfold in free space?
Model answer
The intensity falls to one hundredth, because intensity follows an inverse square law. The fall in signal level is 20 dB.
5
A transmitter gives an intensity of 4.0 × 10⁻⁶ W m⁻² at 2.0 km. What is the intensity at 8.0 km, allowing for free space loss only?
Model answer
The intensity is 2.5 × 10⁻⁷ W m⁻². The working is 4.0 × 10⁻⁶ × (2.0 ÷ 8.0)² = 2.5 × 10⁻⁷ W m⁻².
6
Why does a wi-fi signal weaken more through walls than through open air?
Model answer
Walls absorb and reflect part of the radio energy, adding to the loss from spreading. Water in people and plants also absorbs 2.4 GHz waves strongly.

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