7.8 Antenna Considerations

The choice of antenna is critical in the interference analysis of a microwave radio route. For frequency re-use it is the F/B ratio that is the critical parameter, and for overshoot protection it is the sidelobe and cross-polar discrimination aspects that are important. The FIB ratio is defined as the ratio of the gain in the desired forward direction to the gain in the opposite direction out of the back of the antenna, High-performance antennas have excellent F/B ratios, typically 10 dB to 20 dB better than standard antennas. High-performance antennas are significantly more expensive however, and add complexity and cost to the rigging. The towers may also need to be stronger to support the extra weight and windloading. The frequency re-use requirements thus have to be balanced with the other project requirements.