How does a receiving antenna pick up a radio signal?

The oscillating electric field of the wave pushes electrons back and forth in the receiving antenna. A tiny alternating current is induced at the same frequency as the wave.

Why should a receiving dipole be lined up with the transmitting dipole?

Radio waves from a dipole are polarised, with the electric field parallel to the rods. A receiving dipole at right angles to the field picks up almost no signal.

How is information carried by a radio wave and decoded?

Information is added by modulating a carrier wave, changing its amplitude, frequency or phase. The receiver separates the information from the carrier, a process called demodulation.

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Sound and Light

Dipole antennas, wireless networks and radar

3 things to know
A2 4

Overview

Transmitting and receiving dipoles, half-wave dipole length with a worked value, carrying information by radio, wi-fi and Bluetooth, radar pulses and echo timing, and microwave frequencies.

What you need to know

Answer each one before you reveal it.

Your recallNot started

Numbers

Calculate the length of a half-wave dipole for a 2.4 GHz signal.

Key term

How does a dipole antenna transmit radio waves?

Key term

How long is a half-wave dipole antenna?

Questions from this subtopic

Each one with the model answer underneath.

1
How does a dipole antenna transmit radio waves?
Model answer
A dipole antenna is two metal rods fed with an alternating current. The oscillating charges produce changing electric and magnetic fields that travel outwards as radio waves.
2
How long is a half-wave dipole antenna?
Model answer
A half-wave dipole measures half the wavelength of the signal from end to end. The antenna resonates at that frequency, so it transmits and receives most efficiently.
3
Calculate the length of a half-wave dipole for a 2.4 GHz signal.
Model answer
The half-wave dipole is 6.25 cm long. The working is λ = 3.00 × 10⁸ ÷ 2.4 × 10⁹ = 0.125 m, and half of 0.125 m is 0.0625 m.
4
How does a receiving antenna pick up a radio signal?
Model answer
The oscillating electric field of the wave pushes electrons back and forth in the receiving antenna. A tiny alternating current is induced at the same frequency as the wave.
5
Why should a receiving dipole be lined up with the transmitting dipole?
Model answer
Radio waves from a dipole are polarised, with the electric field parallel to the rods. A receiving dipole at right angles to the field picks up almost no signal.
6
How is information carried by a radio wave and decoded?
Model answer
Information is added by modulating a carrier wave, changing its amplitude, frequency or phase. The receiver separates the information from the carrier, a process called demodulation.

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