Naked Science Forum
General Science => General Science => Topic started by: paul.fr on 15/02/2008 23:17:20
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see this tower:
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It does not look directional, does it? So why do i have to have my tv ariel pointing the correct way?
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You have to point your EYES in the right direction to see the tower so you will also have to point your antenna to get a signal from it.
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That's one hellova tower !!
I don't suppose they get a good mobile phone signal beneath that monster !!
That thing ain't standing now is it ?
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A radio aerial needs to intercept as much signal as possible for the receiver. The only way that you can do this is to accept that you will not be able to receive strong signals from all directions at once. The radio waves are travelling in straight lines from the mast to you and at a long distance these are essentially parallel so you need to intercept as large an area of a plane perpendicular to the direction of the aerial. but it's not quite as simple as that you have to make sure that the waves arrive at the pickup point all in phase.
For most radio waves from long waves to near microwaves one of the best ways of picking up signals is called a half wave dipole which consits of a pair of wires about a quarter of a wavelength long connected to a balanced cable. but as the frequency gets higher the waves get shorter the aerial gets smaller and cannot pick up as much of the signal so techniques have to be used to concentrate the signal near the aerial.
Most normal TV aerials conform to the Yagi design this has a receiving dipole that is about half a wavelength long. Behind it is a reflector to reflect the signal back to the receiving dipole in phase so you get twice as much signal. In front of the dipole are a series of director elements that are a bit shorther that the receiving dipole. These act like an artificial dielectric and tend to focus the signal from a larger area than the receiving dipole towards the dipole like a lens and again increase the signal at the receiving dipole.
Another way is to place a large parabolic plate or mesh behind the receiving dipole to reflect and focus the signal onto the receiving dipole this is what is happening with satellite dishes
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I used to work on TV transmitters and gave up trying to explain to folks how they got their TV. They were surprised that there were buildings at the bottom full of gubbins. Most think there is just a mast it is 're-focuses' the signal coming in from somewhere by some magic.
Most masts are just support structures. The transmitting (VHF - UHF - TV/FM) aerials are at or near the top. Some have white fibre glass tubes (like huge cigarettes) with the aerial 'stack' inside. These 'stacks' are directional and usually consist of many simple aerials lined up one above another in order to send most of the signal towards the horizon or just below it (no point in wasting power by transmitting at steep angles up into the sky..or much at low angles) and usually towards all points of the compass but sometimes more maybe be directed in one broad direction.
http://tx.mb21.co.uk/gallery/fenton.php
Sometimes at low frequencies, LW and MW AM.... The mast itself is a simple radiator and is insulated from earth by large ceramic insulators. More usually there are two masts as support structures and the aerial is a wire in the shape of a letter 'T' (e.g Droitwich..Radio 4 198 LW and other services).
I have worked with a bloke who was at Emley Moor on the day its mast collapsed.
http://en.wikipedia.org/wiki/Emley_Moor
The old Emley mast is like that of Belmont or Winter Hill.. Steel tube.
Belmont (Lincs) is the tallest structure in the EU I think.
http://en.wikipedia.org/wiki/Belmont_transmitting_station
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A radio aerial needs to intercept as much signal as possible for the receiver.
You forgot the very important function of rejecting unwanted signals! Service planning relies on this when choosing transmitter sites and frequencies for a network of transmitters.