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This gives a feel for what happens in an antenna. In the animation, the electron movement is greatly exaggerated. In real life, each electron moves fractions of a millimetre but there are huge numbers of electrons moving.d
Calculating Wavelength and the Length of a Dipole
C = F λ
C is the speed of light = 300 000 000 metres per second (3x108).
F is the frequency - for example 100 MHz (the FM radio band in the UK)
λ is the wavelength. This is the Greek letter Lambda.
λ = C / F
λ = 300 000 000 / 100 000 000 = 3 Metres wavelength.
The half wave dipole will be 1.5 metres long for the 100MHz FM radio band.e
This diagram shows the electric and magnetic components at right angles to each other and in phase. The electromagnetic radiation is produced by the acceleration of charge. The dipole antenna used to radiate this would also be vertical.
A vertical car radio antenna will receive vertically polarised waves best. In fact the FM band transmissions use circular polarisation so the antenna orientation does not matter.
Some TV transmissions are horizontal and some are vertical. This allows regional transmitters to be placed closer together because a vertically polarised transmitter will not interfere with horizontally polarised antennas.g
Here is an nice antenna for sending or receiving circularly polarised waves. The metal plane near the mast is a reflector. The antenna itself is a helix of wire on a plastic pipe. Thank you Remco at http://helix.remco.tk/ This antenna is for computer WiFi signals and is probably not legal in the UK.
This is transmitted from the satellite with one block of channels horizontally polarised and another vertically. In the receiver dish low noise block (LNB) there are two antennas. One picks up the horizontal transmissions and the other the vertical. This simple trick doubles the capacity of the radio link. It is an example of space division multiplexing.i
Light can also be polarised. Light reflected off water or road surfaces is partially polarised. Polaroid sun glasses selectively filter out the reflected polarised light waves. This reduces the glare effect of the reflected light.j
The antenna impedance must match the feeder cable impedance.
When the impedance is matched ...
UHF Radio Controlled Orange Orchard Irrigationk
Measure the voltage and current at the feed point (where the co-axial cable is connected).
The result will be about 50 or 75 Ohms depending on the antenna and the type of co-axial cable.
Similar measurements can be made along the co-axial cable. If a matched cable is being used, the result will be about 50 or 75 Ohms.
If the wrong type of cable is used ...
Other Impedance Matching Examples
A circularly polarised small-boy-steerable antenna for satellite tracking in the 136 - 138 MHz VHF satellite band. This was used at Douai School to receive weather pictures from one of the low earth orbit satellites in around 1970.
These are based on the half wave dipole but the other elements make the antenna directional. In the forward direction, they work much better than a dipole. In other directions they are much less efficient than a dipole. The connection of the co-axial cable is usually more complex than shown to maximise the transfer of energy from the antenna to the feeder cable.
On the left, is a vertically polarised Yagi antenna used for remote power line switching. This antenna is quite directional. It works better in the wanted direction as well as rejecting signals from the wrong direction.
This antenna is a quarter of a wavelength long and is usually vertical. The earth acts as a mirror and there is a mirror image quarter wave underground. The antenna and the mirror image behave very like a half wave dipole. Telescopic radio aerials work on this principle. For example a car's aerial will be about a quarter wave long and the car body acts as the earth to provide the mirror image. For the 100 MHz FM radio band, the antenna will be about 75 cm long.
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