the antenna heights will remove the cancellation. This can be done if the reflection point is constant and does not change. If the antenna is increased in size the midpoint illumination is decreased. With a narrower beam antenna the power drops off sharply. 3. Another method of reducing the amount of reflection is to tilt each antenna up. This can be done if the loss in power can be tolerated. In conclusion, it can be seen that 23GHz paths may not always be simply 'aim and shoot'. The design of these paths should be done as carefully as the lower frequency paths. Fresnel zones and reflection point should be carefully considered. Figure 50.16 Pictorial of Fresnel zones: (a) cross-section of (b) through the beam axis 50.2.2 Outage considerations The effects at the receiving antenna of having multiple unob- The main reason for being concerned with path design to begin with structed Fresnel zones are shown in Figure 50.17. It will be noted is to be sure the path availability will meet the requirements placed that the nulls correspond to the even numbered zones while the on the overall system. Assuming the path has been designed for the peaks correspond to odd numbered zones. It can be seen that with a proper Fresnel clearance etc. the main contributors to outage large number of zones unobstructed and a low reflective surface in become multipath fading and rain induced fading. For analog radios the path that the path attenuation is equivalent to free space loss. above 10GHz the predominant effect is due to rain induced fading. As an indication of what happens when various zones are blocked However, due to the high data rates used, digital radios at these refer to Figures 50.16(a) and 50.17. If the towers of Figure 50.16(a) frequencies may be limited by dispersion caused by multipath fades. Signal strength with 50.2.2.1 Dispersive fading Fresnel zone maximum reflection Signal strength with obstruction no reflection One mechanism that causes multipath fading is refraction in the atmosphere. Certain atmospheric condition