Naked Science Forum
Non Life Sciences => Physics, Astronomy & Cosmology => Topic started by: chris on 17/06/2009 19:56:22
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How does an aerial or antenna work, and how does the length of the antenna relate to the wavelength of the signals being received?
Chris
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Radio signals consist of transverse electromagnetic waves in which the energy flows between the natural capacitance per unit volume of free space charged up to a voltage to the natural inductance of free space flowing as a current to create a magnetic field very much in the way that a swing trades potential energy by being higher at the ends of its travel and stationary with kinetic energy when it is at the bottom of its travel and moving fastest. There are periodic peaks and troughs of voltage alternating across the path they travel with periodic currents peaking in opposite directions a short wire placed in this varying electromagnetic field will have a small voltage and current induced in it as these waves pass. the wavelength is determined by the frequency and the speed of light which is about 300,000 km /sec so waves of a length of 1km have a frequency of about 300,000 cycles per second. this is a bit shorter than the long wave radio 4 frequency.
Now the longer the wire is the larger will be the signal that it will intercept until the wire is half a wavelength long if it is longer than this it is long enough for opposing signals to be induced in it and when it is one wavelength long the two half wavelengths cancel each other out so the ideal length of a radio aerial is about half the wavelength of the signal being received. for a 300Mhz TV signal the wavelength is about a meter and half a wavelength half a metre so the wire cannot be very long
That the most basic story but most TV aerials are much more complicated than that and these complications are dust ways of increasing the signal level locally where the aerial is reflecting signal back onto it so it cannot just go past and ensuring that the aerial element transfers its power most effectively into the cable and the receiver amplifier
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Soul Surfer, would the same concept be applied to UHF signals? I ask this because some people have told me that the longer the antenna is the better the reception is for sailing purposes. What is the role of the amplitude in here?
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i was taught they work by vibrating electrons up and down the aerial, sorry that this isnt quite as in depth as you would like, but i think thats essentially it!
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Soul Surfer, would the same concept be applied to UHF signals? I ask this because some people have told me that the longer the antenna is the better the reception is for sailing purposes. What is the role of the amplitude in here?
This is at risk of being over-simplified and it is a very complicated business. An antenna works because currents flow up and down (electrons vibrating, if you like) the wire, generating electric and magnetic fields. These fields are almost out of phase but there is a small degree of in-phase. This in phase portion represents a loss of power into space - this is the radiated power and the transmitter has to supply the lost power.
A half-wavelength dipole is 'better' than a longer or shorter one because it 'resonates'. It presents a pure resistance to the transmitter. A shorter one 'looks like' there's a capacitor there, too and a longer one 'looks like' an inductor' is there.
We say that it is matched to the feed from the transmitter.
You can match any shape or size of metal object if you add an appropriate matching network but the efficiency (and the usable bandwidth) will be worse.
No antenna is omnidirectional - nothing radiates over a sphere (not for one polarisation, at least). A dippole has a pattern which is like a (ring) doughnut.If you want a more directive antenna (more of the power in a wanted direction), it can be done with a number (an array) of dipoles or a reflector. Size, or aperture, counts for a lot and a good big one will always beat a good little one. The ground acts as a reflector and produces an 'image' of an elevated antenna below the ground. this gives you 'height gain' (an increased aperture), which is why a UHF or VHF antenna on top of a mast gives a better signal. Merely using a longer wire is not usually an advantage because of the problem of matching and efficiency. As I remember, the best you can do with a single radiator is to use a vertical monopole 5/8 of a wavelength long over a good 'earth plane' (a mat of horizontal wires). This used to be a favorite among Radio Amateurs for vhf antennae.
I assume, Erikejah, that, when you use the term 'amplitude' you are referring to length? What I have said about height gain is the answer to your comment, I think.
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Where is the antenna in my car ?..they are all built into the frame somewhere nowadays !
any ideas where it is ?
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It is quite common to use fine wires that are used to heat the rear window as a antenna that is how it is done in my BMW330.
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Not a lot of people know this but a slot in a large metal sheet can act as a very good antenna- in fact any hole in a conductor will radiate. 'Slot' antennas are commonly used in mobile phones these days as it does away with the sticky up conventional antenna. The slot, connected somewhere across its middle, behaves as the dual of a dipole. I know that the window apertures in cars can be used as VHF antennae by connecting and matching them appropriately. The problem is that the pattern of sensitivity is not omnidirectional, like a vertical antenna on the roof.
Loop antennae are also used; the ferrite rod in your MF portable receiver is basically a loop antenna with a high μ core to improve pickup.
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Alma Surfeadora:
Lets say that somebody wants to get a better reception from the radio station transmitting at 90.5Mhz:
which one would be better; a 50cm or a 60cm antenna?
can you show me how to calculated by using:
V=fλ