Problem — TV aerials and polarisation

Problem

Why must television aerials be oriented in a certain way (vertical or horizontal) to receive the signal well? What does it mean for an electromagnetic wave to be “polarised”?

Solution

What polarisation is. In a plane electromagnetic wave the electric field $\vv E$ oscillates along a well-defined direction, perpendicular to the direction of propagation. Saying that the wave is polarised means precisely that $\vv E$ always oscillates along the same direction (for example vertical or horizontal). The direction of oscillation of $\vv E$ defines the direction of polarisation of the wave.

How the aerial works. A rod aerial is a straight conductor. The electric field of the arriving wave sets the free electrons of the conductor in motion along the axis of the aerial: this gives rise to an induced electromotive force, which constitutes the received signal.

• If the aerial is parallel to $\vv E$, the component of the field along the wire is maximum and the induced emf is maximum: optimal reception.
• If the aerial is perpendicular to $\vv E$, the component along the wire is zero and the induced emf is zero: no signal.

In general, if $\theta$ is the angle between the aerial and $\vv E$, the useful component of the field goes as $\cos\theta$ and the received power as $\cos^2\theta$ (the same dependence as Malus’s law).

Conclusion. The aerial must be aligned with the polarisation direction of the transmitter: if the station transmits with vertical polarisation, the aerial must be vertical; if horizontal, horizontal.

$\ev{\text{maximum reception when aerial} \parallel \vv E\text{ (transmitter polarisation), zero if} \perp}$

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Topics: Onde elettromagnetiche Concepts: Polarizzazione e legge di Malus · Onda elettromagnetica