The basic form of radio-wave detector is the simple dipole or antenna. It consists of two vertical metal rods, one above the other and a quarter of a wavelength long. If, in the vicinity of the dipole’ there is an electromagnetic wave which has its electric field parallel to the length of the dipole then the field causes electrons in the metal rods to move. Although the rods are electrically neutral as a whole, this enforced migration of the electrons produces a small voltage difference between the ends of the rods. Because of this, a small voltage is generated across the terminals of the dipole. The voltage fluctuates in phase with the incident electromagnetic wave.

In practice the voltages generated with radio telescopes are very small (~0.1 ?V) and the quality of the system is largely set by that of the amplifier connected to the terminals of the dipole rather than by the dipole itself. All amplifiers generate some noise within them¬selves. Radio astronomers strive to reduce this noise so that the overall signal-to-noise ratio of the system is maximized. In practice there are sometimes other sources of noise which limit performance. For example, at radio wavelengths exceeding 50cm, background radiation from the Galaxy makes a significant contribution to the system noise level.

Simple dipoles are only sensitive to electromagnetic radiation with a component of their electric field parallel to the line of the rods. This means it is only sensitive to one plane of polarization. The dipole and amplifier combination is very similar to that used for television reception. The main difference is in their sensitivity to very weak signals. It is remarkable that the total amount of radio radiation collected by all the radio telescopes in the world from astronomical sources since the start of radio astronomy is about the same as the energy liberated if a small pin is dropped only one metre. This minute energy gives us some idea of the astonishing sensitivity of radio receivers.

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