Electrical Length - Antennas

Antennas

Many antenna types are designed to be resonant at their intended frequency of operation; the driven element functions as a resonator with standing waves of current and voltage along it. The advantage of an antenna at resonance is that it presents an impedance to the feedline to the transmitter or receiver which is a pure resistance, while off resonance it presents a reactance, it has capacitance or inductance. The result is that the feedline works most efficiently when the antenna is resonant, transferring the maximum amount of power to the antenna. The condition for resonance in a linear dipole antenna is usually that the electrical length be equal to a multiple of a half-wavelength, λ/2, while for a monopole antenna it is a multiple of a quarter-wavelength, λ/4.

The electrical length of an antenna element is, in general, different from its physical length. For example, increasing the diameter of the conductor, or the presence of nearby metal objects, will decrease the velocity of the waves in the element, increasing the electrical length.

Antennas which are the wrong length to be resonant, or which must operate at a different frequency at which they are not resonant, are often brought into resonance by loading; adding capacitors or inductors in series with them. An antenna which is shorter than its resonant length has capacitive reactance. The capacitance can be compensated by adding an equal value inductance, a loading coil in series. The coil can be thought of as electrically lengthening the antenna. Similarly, an antenna which is longer than its resonant length has inductive reactance, and can be electrically shortened by adding a loading capacitor.