What is an antenna

Let us recall the definition of a wave (a disturbance that propagates through space) and suppose we have a machine that produces the disturbance. Now the question is: how do we make this disturbance propagate through space? In other words: how is a disturbance turned into a wave? The answer is: "thanks to an antenna" (a transmitting one).

An antenna is a device able to radiate into space a disturbance produced inside it.

Conversely: how do we channel the energy of a wave, usually spread over a large region of space, into a small area where we can manipulate it at will (amplify it, filter it, convert its energy into energy of another kind, etc.)? Again with an antenna (a receiving one).

An antenna is a device able to pick up a disturbance occurring in the surrounding space and turn it into a disturbance inside it.

• The physical reason behind these observations can be summarised as follows: for a disturbance produced by a source to be able to propagate through the medium, the energy generated by the source must couple efficiently to the oscillating "medium".
• The antenna is therefore a mediator between the properties of the source and the properties of the medium, aimed at optimising the transmission of energy from the source to the medium (or vice versa).

Some examples

Transmission of sound

Wind instruments

• In wind instruments the medium of the waves is air, and the player directly sets the air column inside the instrument into vibration. The instrument itself must act as an antenna able to transmit the vibration from its inside to its outside.
• In this case only one measure is really necessary: the size of the instrument must match the wavelengths it is meant to radiate (indeed, instruments with a low register are generally larger than instruments with a high register).
• Other features (such as the conical bells at the end of many brass instruments) improve the performance of the instrument by further easing the transmission of sound to the outside.

String instruments

• The case of string instruments is slightly more difficult: the oscillating string is already an antenna. Indeed, unlike a pendulum or a mass-spring system, in which the oscillating mass is concentrated at a precise point, the mass of the string extends over a length equal (in the fundamental mode) to half the wavelength of the standing wave with which it vibrates (see normal modes of a string).
• However, this kind of antenna is not enough. The reason is simple: the string is an antenna made of steel (or similar), while we want to transmit the oscillation to the air. Since the speed of sound in air is much lower than in steel, for the same oscillation frequency the wave in air has a much shorter wavelength than in steel.
• A further element is therefore needed, able to transfer the oscillation to a larger number of air molecules. This is achieved with a sound box, which forms the hollow body (usually made of wood) of the instrument.

Radio transmissions

• The laws of electromagnetism, completed by J. C. Maxwell in the second half of the 19th century, state that every oscillation of the electric field is associated with an oscillation of the magnetic field, and vice versa, and that these oscillations can travel through vacuum at a definite speed (the speed of light).
• This means that it is possible to build special circuits able to sustain the oscillation of the electromagnetic field, and it opens the way to transmitting this oscillation from one point to another: from a transmitting circuit to a receiving one.
• In practice, however, a "lumped-element" oscillating circuit, in which each component occupies a small region of space (like those found inside modern electronic devices), although able to oscillate, cannot transmit or receive effectively.
• Heinrich Hertz was the first, in 1886, to build a circuit suited to the purpose: he had built an antenna (later called the Hertzian dipole). The invention had been devised solely to demonstrate the existence of the electromagnetic waves predicted theoretically by Maxwell some twenty years earlier. Asked whether his device had any possible applications, Hertz replied: "It is of no use whatsoever. It is just an experiment that shows that Maxwell was right."^[1]

Virtual laboratory: interference experiments

If you are interested in visualising interference phenomena, connect to our virtual wave laboratory. Below we suggest a series of guided experiments, but you are free to try out new configurations and new experiments. The app is very intuitive; to understand correctly what you observe, just keep in mind that the coloured areas represent the amplitude of the oscillation. The brighter the colour, the larger the amplitude of the oscillations (green for crests, red for troughs). Of course, the dark areas represent the absence of oscillation caused by destructive interference.

Suggested experiments:

• Interference between two or more point sources of the same frequency
• Interference between two point sources of different frequencies
• Interference between waves of different frequencies generated by a single source (beats)
• Reflection from a parabolic mirror

References

Further reading and links

• For information on interference in general see interference and mathematical aspects of interference.
• The page antennas and interference shows how directional antennas can be obtained by exploiting the phenomenon of interference.