Physical Measurements Associated With the Polar Coordinate Graph

Itʼs a common misconception that the shape drawn on a polar coordinate graph representing an antennaʼs field strength is somehow actually, and physically in some sense, a cross section of a threedimensional volume in space. It is not. Itʼs convenient to casually think of the three-dimensional shape conjured up from a polar coordinate graph as being some sort of volume growing out of an antenna because that makes for a generally workable mental model. Even earlier in this discussion the convenience of alluding to the model of the polar coordinate volume as some real thing beaming out into a warehouse was helpful in delivering the explanations. Now letʼs fine tune your understanding to see how actual, real-world signal strength (which can be thought of as field density in a volume in space) relates to the polar coordinate model.

The shape drawn on a polar coordinate pattern graph allows you to calculate the estimated field strength at some angle from an antenna. The graph shows dB relative to a theoretical isotropic radiator. For an isotropic radiator the signal propagating at right-angles to the antenna would be the same as that propagating at all other angles. If you were to measure the actual field strength at some distance from the right-angle plane relative to an upright antenna (the horizontal plane), then the polar coordinate graph could be used to estimate how that value would vary at some other angle. In the following figure (4.23) we see the elevation cut (side view) polar coordinate graph overlaid on a real antenna and a user. The polar coordinate graph is conceptual; it doesnʼt have a physical reality in the space around the antenna. It simply shows dB relative to isotropic at various angles.

Figure 4.23 The Polar Elevation Cut as it Relates to a Real-World Situation

Assume that you measure the signal strength at some distance (point A) along the horizontal plane of the antenna. You determine that at that distance (perhaps 100 feet in a typical office environment) the measured signal strength is -65 dBm. You also determine direction from the antenna to the user and this is represented as an angle below the horizontal. If you overlay that angle on the polar

coordinate graph, you can determine the point (point B) where the direction to the user intersects the field strength pattern. The coordinates on the example shown indicate that this point corresponds to the -5 dB attenuation level. You now know that any physical propagation of the field in that direction will be 5 dB less than along the horizontal plane. Since you measured -65 dBm at the lateral point corresponding to the userʼs position, you can calculate that the user would be receiving -70 dBm.

Copyright 2003 - Joseph Bardwell