radio antenae

Factual Questions
1

So my sister ran my car with a retractable, rigid antenae through the car wash today and it snapped right off.

When it got drastically shorter, of course the radio reception got bad, but it got me to thinking.

But how exactly does an antanae work? Is it surface area or height or length that matters? would a lump of metal work as good as a thin piece of wire if it was the same total mass and material?

2

hey…welcome to the SDMB :slight_smile:

i hated it when i posted my first question on the board and got no answers :slight_smile: so i’m gonna save you the trauma…even though i don’t know the answer :slight_smile:

but i think it has something to do with surface area alright…not sure though…there are a lot of electrical engineers here who might come along and tune in to your channel…so stay tuned…

3

ok… let me introduce you to our dear friend Google :slight_smile: he’s always nice to us…

quoted from :

http://rabi.phys.virginia.edu/HTW/radio.html
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I’ve heard the reason an antenna, such as the one on your car, is so long is because it needs to be large enough for the long radio waves to pass into it. Is this true? Why are antennas for radio stations so tall and slender? – LW, Blacksburg, VA

A vertical pole radio antenna receives a radio wave by allowing that wave to push electric charges up and down the antenna. The radio senses this moving charge and is thus aware of the passing radio wave. The ideal length of a vertical receiving antenna is a quarter of the wavelength of the radio wave it’s trying to receive–in which case, charge that the radio wave’s electric field pushes up and down the antenna has just enough time to reach the end of the antenna before it has to reverse directions.

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But the waves used in FM radio transmission have much shorter wavelengths–typically 3 meters–so that antennas of about 75 centimeters are all that’s needed. The vertical pole radio antenna on your car is designed to receive these FM waves. The antennas of FM radio stations are also rather short, but they are usually mounted high up on a pole so that the whole structure looks like an AM radio antenna. However, if you look near the top of an FM radio tower, you’ll see the actual FM antenna as a much smaller structure.

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4

It’s one whole lot of things. The crucial elements (as I remember) are how much signal is collected and how much noise gets included. Making a collector bigger will add to the signal. Of course, a casual user has little way of figuring out how much noise is introduced. Buying an expensive antenna will get you darn good results. Putzing around with some wire isn’t going to get you very far.

5

and here’s another interesting point…

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How can an antenna be short and still work as well as a long one?

The length of an antenna is very important. If the antenna is too short, the charges will reach its end too soon and the charge will not flow very smoothly back and forth in it. If the antenna is too long, the charges will not reach its end before it is time for them to reverse directions and some of the antenna will not be used (it will actually cause more trouble than help). Thus there is an ideal length for the antenna and this length depends on the frequency of the radio wave it is trying to create. But it is also possible to shorten an antenna by delaying the flow of charge to its ends. Adding a coil to the antenna (an inductor) will slow the flow of current through the antenna and make a short antenna behave like a longer antenna. Most portable AM radios use a coiled antenna that behaves as though it were much longer than its physical length. FM radios work best with antennas that are about 1 meter long.

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