Antenna design: The half-wave dipole
 
A frequently met form of antenna is a half-wave dipole, consisting of two metal rods or wires which are held in line with each other but separated by means of insulated supports. Each rod has a length which is one-quarter of the wavelength to be monitored by the radio receiver. The feed from the antenna to the receiver is taken by cable from the ends of the rods at the centre of the antenna. A typical dipole is illustrated in figure 1 which also shows its far-field polar diagram of directional sensitivity.
The half-wave dipole polar diagram in three dimensions can be obtained by rotating the twodimensional diagram about the antenna’s axis. It can be seen that the dipole is insensitive to radiation which arrives from a direction along its axis. However, it can also be seen that it has poor directional sensitivity and, consequently, its absolute power gain is low, being 1·64. By itself, this antenna is of little use to act as a radio telescope.

Figure 1. The polar diagram of a half-wave dipole antenna.