Quality and performance in wireless microphones depend not only on the type of transmitter, but also on the type of receiver.
Historically when High Band VHF transmitters were the dominant type of wireless system, a relatively simple single antenna was sufficient. But with the greater capability of UHF transmitters a better antenna was needed to take advantage of the greater potential of the system. Thus the birth of the diversity receiver.
Diversity is a reception technique by which two antennae are utilized to eliminate dropouts that occur when multiple signals arrive at the receiver at different times. A dropout can be the result of a weak signal, causing a hiss, or a lapse in the silencing circuitry, which results in a "pop". A diversity system constantly monitors the antennae to see which is providing the stronger signal at any given moment so that the receiver can utilize the stronger signal.
A true diversity system goes one step further by using two separate receivers housed in a single unit each of which is paired with one of the antennae. Whichever receiver detects the stronger signal is the one that is used.