Producing radio waves
Definition
Oscillation
A repeated movement back and forth about a fixed central position.
Definition
Alternating current
A current that repeatedly reverses its direction of flow around a circuit.
Definition
Aerial
A metal rod or wire that emits or receives radio waves.
- A radio transmitter produces an alternating current that changes direction millions of times every second and feeds it into a metal aerial.
- The alternating current makes the free electrons in the aerial oscillate, moving rapidly backwards and forwards along it.
- Moving charges produce electric and magnetic fields, so oscillating electrons produce fields that oscillate too.
- These oscillating fields spread out from the aerial as an electromagnetic wave.
- The wave has the same frequency as the alternating current in the aerial.
- When this frequency is in the radio range, roughly 1×104 Hz1\times10^{4}\ \text{Hz}1×104 Hz to 1×109 Hz1\times10^{9}\ \text{Hz}1×109 Hz, the wave produced is a radio wave.
- Radio waves are therefore produced by oscillations in electrical circuits.
Receiving radio waves
- When a radio wave reaches the metal aerial of a receiver, its oscillating fields push the free electrons in the aerial backwards and forwards.
- The wave therefore induces an oscillation of the electrons, which is a small alternating current in the receiving circuit.
- The induced alternating current has the same frequency as the radio wave that produced it.
- As the aerial absorbs the wave, energy is transferred from the wave to the receiving circuit.
- The induced current is very small, so the receiver amplifies it before picking out the information it carries, such as speech or music.
A two-way link
- Radio communication depends on both effects working together.
- At the transmitter, electrical oscillations produce radio waves.
- At the receiver, radio waves induce electrical oscillations.
- No electrons travel from the transmitting aerial to the receiving aerial, because only the wave and its energy cross the gap.
- The frequency is the same at every stage, so the current in the receiving aerial alternates at the same frequency as the current in the transmitting aerial.
Example
Frequency of an induced current
- A long-wave radio station transmits radio waves with a wavelength of 1500 m1500\ \text{m}1500 m.
- The current induced in a receiving aerial has the same frequency as the wave, so find the frequency of the wave:
- The current in the transmitting aerial and the current induced in the receiving aerial both change direction back and forth 200 000200\,000200000 times every second.
Common Mistake
- Do not say that electrons travel through the air from the transmitter to the receiver, because the electrons in each aerial only oscillate in place.
- Do not say that a steady direct current in an aerial produces radio waves, because the charges must oscillate to produce a wave.
- Do not say that the induced current has a different frequency from the radio wave, because the two frequencies are always the same.
Exam technique
Two-way recall questions
- Give both halves of the idea: oscillations in a circuit can produce radio waves, and radio waves can induce oscillations in a circuit.
- Describe the current as oscillating or alternating, because "a current flows" does not show that it keeps changing direction.
- If a question is about communication, say where each effect happens: production at the transmitter and induction at the receiver.
Self review
- What kind of current in an aerial produces radio waves?
- What happens to the free electrons in a receiving aerial when a radio wave reaches it?
- How does the frequency of an induced current compare with the frequency of the radio wave that induced it?
- Do any electrons travel between a transmitting aerial and a receiving aerial?
- Calculate the frequency of the current induced by radio waves with a wavelength of 3.0 m3.0\ \text{m}3.0 m.
