Why are TV aerials that shape

asks MScott:
A. Your TV aerial - also known as a Yagi-Uda array� (it was named after two Japanese electrical engineers: Hidetsugu Yagi, who invented it in 1928, and Shintaro Uda, who perfected it for television reception in 1954) - is not one thing. It has many elements.

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Q What are they
A. At one end, it will have a metal grid which acts as a reflector, focusing television signals on to the aerial - it's the same as having a silver-coloured reflector in a torch to focus all the light from the bulb into a beam.

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Then there are a pair of spines which sit immediately in front of the reflector. They are the dipole (two oppositely magnetised poles). The dipole is the only part of the aerial which is connected to the receiver (your TV) through a coaxial cable. The dipole is about half a wavelength long, and the reflector is a quarter of a wavelength behind the dipole.

Further along are the 'director elements' - which look like rows of spines. They are quarter of a wavelength apart and half a wavelength long. They increase the gain of the aerial - the weaker your signal, the more of them you'll need. And the larger the aerial, the more precisely it has to be focused at the signal from the transmitter.

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Q. Why are some aerials placed in an upright position, while other are horizontal

A. It depends whether the local transmitter is using horizontal polarisation (which means the director elements have to be on the horizontal plane) or a vertically polarised signal (where your aerial would have to be vertically aligned.

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Q. Why the difference

A. This helps to avoid interference from transmitters which are sited close together.

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Q. Why hasn't there been anything to replace this aerial shape - such as the sort of dishes satellite TV uses
A. It's simple - it works very well for its size. The Yagi-Uda array has proved very successful as a VHF and UHF television receiving aerial. If you were to use a parabolic aerial (satellite dish-type) to get the same performance, it would have to be very much larger.

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By Sheena Miller

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