Science Questions

How does a Yagi aerial work? Andrew, Cambridge

Dave - A Yagi aerial is the sort of thing you see, TV aerials, basically. The ones where you’ve got a long bit pointing towards the transmitter and lots of cross bars along it.

Kat - So a typical TV aerial.

Chris - Dave, why are they that shape?

Dave - What’s going on here is the idea of an aerial like this is to make the aerial very directional. It’ll only pick up signals from the direction of the transmitter and not pick up all the interference from all the other directions. Basically, radio waves are a form of electromagnetic wave. If they hit metal they’ll make and electric current flow backwards and forwards along it.  The bars on the aerial, when the radio waves hit it electricity sloshed backwards and forwards along it. That electricity sloshing backwards and forwards starts to transmit itself. It makes its own radio waves. You’ve got a whole series of these all absorbing radio waves and then re-emitting them again and again. The only direction whereby all the radio waves are re emitted by these little segments – the only direction where they all add up and increase the direction of the signal is in the direction along the bar, towards the tv transmitter. The signal gets bigger and bigger and you get a little detector at the back which detects that signal and sends it down to your tv. Quite often at the back they’ve got a little reflector so it reflects any signal that doesn’t get absorbed by the sensor at the back.

Chris - Why is it called a Yagi aerial?

Dave - I think it was invented by a Japanese guy called Yagi.

November 2008