you may build thereʼs going to be a point where one wire attaches, and a point where the other wire attaches. Thereʼs got to be some field orientation in any antenna thatʼs constructed. A dipole radiator is one of the most reasonable approximations to the perfect isotropic radiator; the signal basically radiates out in all directions with, of course, a weaker area directly above and below.

All antennae have some particular field radiation pattern that is not spherical in the way an ideal isotropic antenna would be spherical. This deviation makes the field stronger in some directions, and weaker in others. This is the characteristic of antenna directivity. Physical characteristics of the radiating elements that make up an antenna unit cause the resulting electromagnetic wave to point more in some directions than in others.

Coupling and Re-radiation

By the Reciprocity Theorem we understand that the preceding description of dipole radiation can be thought of in reverse for dipole antenna reception. The electromagnetic field, varying in amplitude in the spatial volume surrounding the dipole receiver induces an electric current into the metal of the antenna. The antenna introduces electrical impedance and a voltage differential is created across the antenna terminals. The energy induced into the antenna can go somewhere. It travels through the wires from the antenna to the receiver circuitry where the signal is demodulated, amplified, and otherwise processed.

Consider now a situation in which the wires connecting the antenna to the receiver circuitry are cut. An accelerating charge in the magnetic field induces a current into the antenna but now thereʼs no place for it to go. As the inducing field decelerates the antenna now radiates the energy back out into space, acting as a transmitter. This effect is called re-radiation and it occurs for any metal object into which a current is induced with no place to “drain off” to earth ground. While the effects of reradiation might be detrimental if they unexpectedly occur because of a metal filing cabinet, they are used productively in the creation of directional antennae.

When two radiators are placed near to each other (within roughly 10-inches for 802.11 communications) they induce current into each other. In this situation the two radiators are said to be coupled. Coupling can be between two active radiators, each having a source of input power, or between an active radiator and a passive radiator (a metal rod acting only as a reradiator). In either case the electromagnetic field produced is the result of two (or more) separate radiating elements. This is the basis for directional antennae like the Yagi in which a single radiator induces current into reradiating elements that are designed to act as either reflecting components or as directing components.

Representing the Direction of Field Propagation

In 1884 a former student of James Maxwell named John Henry Poynting presented a paper entitled “Transfer of Energy in the Electromagnetic Field” at Mason Science College in Birmingham, England. He showed that the direction in which overall energy is flowing in a field could be expressed in a simple formula using the vector product of the electric and magnetic fields. Itʼs indeed amusing that the equations to calculate the direction in which a propagating electromagnetic wave points were produced by a person named “Poynting.” The Poynting vector is measured with dimensions of power per area (Watts/Meter2). The equations for field intensity can be instructive with regard to building a mental picture of whatʼs happening in that invisible (to your eyes) space between the transmitting and receiving antenna.

Copyright 2003 - Joseph Bardwell