It helps to eliminate problems that can occur when there is only one emitter/antenna for some scenarios.
My favorite analogy is that it's like sitting behind a support post at the ballpark sports field: To see the whole field, you have to first lean one way (antenna port a), then lean the other way (antenna port b).
With RF, reflections can cause "nulls" where the phase of the reflection cancels out some / most / all of the primary wave.
Or where some surface feature causes a shadow in one spot (like from antenna a), but is covered by the field from antenna b.
The diversity system does not permit both antennas to be working at the same time; first antenna a is active, then antenna b.
It is not acceptable to use one antenna input to cover one area and then use the other antenna to cover a completely different area. Both areas will get really poor coverage and lots of drop-outs.
Just to a note to the great explanation you received from Scott (as always!);
This describes the use of 2 **Identical Antennas in "Diversity" Mode;
The purpose of diversity is to overcome multipath reflections. Diversity antennas that share the same physical housing are placed at an optimum distance apart. The maker of the particular antenna determines that distance based on the characteristics of the antenna. When you use a pair of antennas with matching characteristics to provide diversity for cell coverage in your facility, the guideline is to put those matched antennas at a distance apart from each other that is equal to a multiple of the wavelength of the frequency that is being transmitted. The 2.4 GHz wavelength is approximately 4.92 inches. Therefore, to support diversity on a 2.4 GHz radio with two separate antennas, the antennas should be spaced approximately 5 inches apart. The antenna pair could also be spaced at multiples of 5 inches, but the distance between should not exceed 4 multiples: reflected waves farther apart than that are likely to be so distorted and different in delay spread that the radio could not work with them.Because each antenna is selected by itself, both antennas must have the same radiation characteristics and be positioned to provide similar cell coverage. Two antennas connected to the same access point must not be used to cover two different cells.
"You can relate this to a common occurrence in your car. As you pull up to a stop, you may notice static on the radio. But as you move forward a few inches or feet, the station starts to come in more clearly. By rolling forward, you move the antenna slightly, out of the point where the multiple signals converge.
A diversity antenna system can be compared to a switch that selects one antenna or another, never both at the same time. The radio in receive mode will continually switch between antennas listening for a valid radio packet. After the beginning sync of a valid packet is heard, the radio will evaluate the sync signal of the packet on one antenna, then switch to the other antenna and evaluate. Then the radio will select the best antenna and use only that antenna for the remaining portion of that packet.
On transmit, the radio will select the same antenna it used the last time it communicated to that given radio. If a packet fails, it will switch to the other antenna and retry the packet.
One caution with diversity, it is not designed for using two antennas covering two different coverage cells. The problem in using it this way is that, if antenna no. 1 is communicating to device no. 1 while device no. 2 (which is in the antenna no. 2 cell) tries to communicate, antenna no. 2 is not connected (due to the position of the switch), and the communication fails. Diversity antennas should cover the same area from only a slightly different location."
Diversity Antenna Systems
Diversity antenna systems are used to overcome a phenomenon known as multipath distortion of multipath fading. It uses two **identical antennas, located a small distance apart, to provide coverage to the same physical area.
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