10.7. Encouraging Diversity
There is one more principle that you should know about because it shows up over and over again in wireless. The electrical field of a radio signal, which is called the electrostatic vector, pulses alternately strong and weak. This is an artifact of the flow of charges that lead to its creation and launch into space from its antenna. At a rate equal to its frequency, the electrical vector starts out positive, moves through zero (at the zero crossing), goes negative, and returns to positive. At the same time, there is an electromagnetic field, or vector, which is a result of the antenna's moving charge (remember, as current flows, it produces a magnetic field). This flows in a manner just opposite to the electrostatic, so that when the electrostatic vector is the strongest, the electromagnetic is the weakest, and vice versa. Let's ignore this electromagnetic field for present, but it will come up again when we discuss advanced systems of eavesdropping.
Think for a moment about receiving a signal. Signals bounce as they travel, a condition called multipath. It is possible that any two paths of propagation will overlap, and the distances between the paths will be different enough that the signals null each other out (similar to what happens in a twisted pair cable). On the other hand, about a quarter wavelength away, the two signals are likely to be in phase, making the signal at that point twice as strong. If we place two antennas approximately one quarter wavelength from each other, the signal received at one of them will be at its peak energy while at the other it will be at zero. If there was only one antenna, and it was located at the null point, it might not be able to detect the signal at all.
Because of the extremely small wavelengths of the frequencies we are dealing with (they are after all microwaves) spacing the antennas about four inches apart usually ensures that one or the other or both will always be in signal, and sure enough, many access points use dual antennas at about this spacing. In the roving environment of wireless, the chances that a multipath will occur as the users move with respect to each other is just about certain. Using two or more antennas spaced a quarter wavelength apart to avoid loosing the connection is called diversity reception.