A dipole antenna, developed by Heinrich Rudolph Hertz around 1886, is an antenna that can be made by a simple wire, with a center-fed driven element for transmitting or receiving radio frequency energy. These antennas are the simplest practical antennas from a theoretical point of view; the current amplitude on such an antenna decreases uniformly from maximum at the center to zero at the ends.
Hertzian (i.e. short or infinitesimal) dipole
The Hertzian dipole is a theoretical dipole antenna that consists of an infinitesimally small current source acting in free-space. Although a true Hertzian dipole cannot physically exist, very short dipole antennas can make for a reasonable approximation.
The length of this antenna is significantly smaller than the wavelength:
The radiation resistance is given by:
where is the impedance of free space.
The radiation resistance is typically a fraction of an ohm, making the infinitesimal dipole an inefficient radiator. The directivity D, which is the theoretical gain of the antenna assuming no ohmic losses (not real-world), is a constant of 1.5, which corresponds to 1.76 dB. Actual gain will be much less due to the ohmic losses and the loss inherent in connecting a transmission line to the antenna, which is very hard to do efficiently considering the incredibly low radiation resistance. The maximum effective aperture is:
A surprising result is that even though the Hertzian dipole is minute, its effective aperture is comparable to antennas many times its size.