versity paths can be based on space, frequency, and/or time diversity. The simplest and preferred form of diversity for LOS microwave is space diversity. Such a configuration is illustrated in Figure 9.15. The two diversity paths in space diversity are derived at the receiver end from two separate receivers with a combined output. Each receiver is connected to its own antenna, separated vertically on the same tower. The separation distance should be at least 70 wavelengths and preferably 100 wavelengths. In theory, fading will not occur on both paths simultaneously. Frequency diversity is more complex and more costly than space diversity. It has advantages as well as disadvantages. Frequency diversity requires two transmitters at the near end of the link. The transmitters are modulated simultaneously by the same signal but transmit on different frequencies. Frequency separation must be at least 2%, but 5% is preferable. Figure 9.16 is an example of a frequency-diversity configuration. The two diversity paths are derived in the frequency domain. When a fade occurs on one frequency, it will probably not occur on the other frequency. The more one frequency is separated from the other, the less chance there is that fades occur simultaneously on each path. Frequency diversity is more expensive, but there is greater assurance of path reliability. It provides full and simple equipment redundancy and has the great operational advantage Combiner Output Input Figure 9.15 A space diversity configuration with LOS microwave application. The vertical distance between the upper and lower antennas is of key importance. Input Output signal signal Combiner Figure 9.16 A frequency diversity configuration. CONCEPTS IN TRANSMISSION TRANSPORT of two complete end-to-end electrical paths. In this case, failure of one transmitter or one receiver will not interrupt service, and a transmitter and/or a receiver can be taken out of service for maintenance. The primary disadvantage of frequency diversity is that it doubles the amount of frequ