ge of 100 to 500. Short haul and millimetre path design 50/7 Top-top While it is theoretically possible to achieve some improvement in Top-bottom propagation availability by mounting the receive antennas 'edge to edge' where space is limited, little experience is available to dem- onstrate that the formulas are valid for small spacings. The general feeling is that such a configuration would provide an improvement Bottom receive antenna over non diversity operation, however. As stated, space diversity is an effective method of improving Top receive antenna propagation reliability where multipath or reflective type of fading Total is expected. Diversity will not provide improvement where earth cancellation bulge, ducting, or similar effects that block or steer the signal away from the receiving antenna are present. Partial cancellation 50.2 Short haul and millimetre path design Figure 50.13 Space diversity Microwave path design at 18GHz and higher follows a very similar procedure to path design at 2GHz and 6GHz. However, due to the high frequencies involved, some of the parameters evaluated along the way may be quite different than at the lower frequencies. To Top antenna also used demonstrate this we have selected a few path design topics and for transmitting evaluate their significance at 18GHz and 23GHz. 50.2.1 Fresnel zones and obstructions One of the considerations in microwave path design is the effect of Receiver A Receiver B reflections and obstructions of the microwave path. The effects of reflections and obstructions usually get sufficient attention at the lower frequencies since the paths are much longer Comparator and clearances are costly to achieve. It will be noted that the Fresnel zones are larger at the lower frequencies. At the higher frequencies such as 18GHz and 23GHz the paths are much shorter and it is much simpler to establish line of sight since Both baseband switches both ends of the paths usually can be seen. normally closed Figure 50.15 shows a polar plot of an antenna pattern. The main l