Antennas
An antenna is a conductor designed to convert electrical signals into radiated electromagnetic waves and back again. On transmission it takes the alternating current supplied by a transmitter and launches it into space as a radio wave; on reception it does the reverse, intercepting passing waves and producing a small voltage at its terminals. The same antenna usually works equally well in both directions, a property known as reciprocity.
The simplest and most instructive example is the half-wave dipole, a straight conductor about half a wavelength long and fed at its centre. Current is largest in the middle and falls to zero at the ends, and the oscillating charges radiate most strongly broadside to the wire and not at all along its axis. This gives the dipole its familiar doughnut-shaped radiation pattern. More elaborate antennas, such as arrays and reflector types, shape this pattern to concentrate energy in a chosen direction.
Key properties describe how an antenna distributes energy. The radiation pattern maps the relative field strength in each direction, directivity measures how focused that pattern is, and gain combines directivity with efficiency. Polarisation describes the orientation of the radiated electric field, which must match between transmitter and receiver for good reception. Because antenna behaviour is a three-dimensional field phenomenon, animations are valuable for showing current distribution along the conductor and the field lines detaching and propagating away as a wave.
Frequently asked questions
- Why is a dipole made about half a wavelength long?
- At that length the antenna resonates at the operating frequency, producing a favourable current distribution and a real, easily matched feedpoint impedance for efficient radiation.
- What is antenna gain?
- Gain measures how strongly an antenna concentrates radiated power in its preferred direction compared with a reference, taking both its directivity and its efficiency into account.
- Why does polarisation matter?
- Polarisation is the orientation of the radiated electric field. If the transmitting and receiving antennas are not aligned in polarisation, the received signal is weakened.
