The Shape of the Electromagnetic Field

Letʼs try to build a mental picture of what the radiation pattern might look like if we could see radiation in the 802.11 frequencies. To begin we will define an imaginary radiating object, a perfect radiator. From this perfect radiator the electromagnetic field will propagate outwards in all directions equally. This theoretical object is called an isotropic radiator. Seen in cross-section the isotropic radiator would appear as a perfect circle, with the radiating element being a point in the middle and the electromagnetic field expanding equally in all directions. In three-dimensions the electromagnetic field would look like a sphere.

Figure 3.2 The Spherical Radiation Pattern of a Theoretical Isotropic Radiator

A real antenna cannot have an isotropic radiation pattern. As was discussed earlier, the electromagnetic field propagates in a direction thatʼs perpendicular to the radiating wire so, for a vertical antenna, the field isnʼt going to propagate upwards and downwards in the same way it

propagates to the sides. This picture would more resemble a doughnut placed down on the antenna, as shown below.

Figure 3.3 The Doughnut-Shape of the Electromagnetic Radiation Pattern

This picture describes the general shape of the field surrounding an antenna type known as a dipole. Simply put, a dipole antenna has two ends with differing electric polarities. The typical antennae used by an 802.11 access point are normally dipoles. The actual field isnʼt exactly as shown above, but this should give you a starting point for understanding how a simple dipole radiator will behave.

Math and Physics for the 802.11 Wireless LAN Engineer

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Copyright 2003 - Joseph Bardwell