Welcome to practical physicsPracticle physics - practical activities designed for use in the classroom with 11 to 19 year olds
 

Centimetre waves

Demonstration

A microwave detector shows the standing wave pattern in the space between a microwave generator and a reflective metal barrier.

Apparatus and materials

Aluminium barrier, plane

Amplifier

Loudspeaker

Microwave transmitter

Microwave receiver

Health & Safety and Technical notes


Modern equipment using a solid-state diode transmitter is safe.  

Older equipment using a klystron tube uses hazardous voltages. The connectors on the leads between the transmitter and the power supply MUST be shielded types to minimize the risk of serious electric shock. The ventilation holes in the power supply may also give access to hazardous voltages, so its use MUST be closely supervised.

The transmitter should produce microwaves of about 3 cm.  

The microwaves can be unmodulated, in which case the receiver is connected to a meter. If they are modulated, it is possible to detect them using an amplifier and loudspeaker. A narrow probe receiver will better discriminate between nodes and antinodes than a horn receiver will. 
 
Manufacturers usually supply a full instruction book, including experiments, with their microwave kits. 

 

Procedure


Receiver


Photographs courtesy of Mike Vetterlein


a Set the microwave transmitter and receiver facing a metal barrier.  

b Move the receiver back and forth in the space between the speaker and the wall to show amplitude maxima and minima. You may need to adjust the distances carefully to produce a standing wave pattern. 

 

Teaching notes


Students find microwaves intriguing because of their invisibility. You may want to discuss how this is similar to other phenomena with which students are familiar; e.g. radio reception interference or mobile phone signal strength, and the way these result from reflections from buildings or geographical contours.  

This experiment was safety-checked in February 2006

 

Related experiments


Sound waves