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Half Wave Dipole Antenna

Wavelength and Frequency

Antenna wave diagram

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.

Antenna Calculations

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.

Electromagnetic Waves

Antenna Poalrisation

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.

FM Radio Polarisation

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.

Impedance Matching

The antenna impedance must match the feeder cable impedance.

When the impedance is matched ...

Antenna Dipole UHF Radio Controlled Orange Orchard Irrigation

Dipole Antenna Impedance

Measure the voltage and current at the feed point (where the co-axial cable is connected).

Antenna dipole impedance

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

Satellite Tracking

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. I believe this school was the first UK site to decode the weather pictures, even beating the Met' Office.

Satellite tracking at Douai School in the 1970s

Yagi Antenna

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.

Yagi Antenna

And here 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.

Vertically polarised Yagi antenna

Quarter Wave "Vertical" Antenna

Quarter Wave Antenna

This antenna is a quarter of a wavelength long and is often vertical. The antenna relies on a mirror effect to work. Here the mirror is the users own body. This is safe for low powers. Metal car bodies work well and for large antennas, the earth itself. 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 and a bit shorter for DAB radio.

Rubber Duck

The antenna is coiled and encased in rubber. It's springy and bendy. It behaves as a quarter wave but the coiling means it takes up less space. It's a bit less efficient than a straight wire quarter wave.

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