DESCRIPTION OF THE MICROWAVE BEAM:
1. The microwave beam behaves like a light beam and it tends to follow a straight line and azimuth. 2. The changes on temperature, pressure and relative humidity tends to refract the microwave beam. So it will follow on a slightly curved path. The radio horizon exists. 3. It is diffracted when grazing over an obstacle. A small shadow area created when some of the energy is redirected. 4. The beam can be reflected from relatively smooth terrain and water surfaces. The criterion of smoothness depends on the wavelength of the signal. 5. At 6 to 8GHz frequency band, rain attenuation is not a problem. But for frequencies 11GHz and above, rain attenuation is very serious. The amount of attenuation depends upon the rate of rainfall, the size of the drops and length of exposure. 6. Atmospheric absorption due to oxygen and water vapor also exists and its magnitude is a function of frequency and path length.
Things to consider in designing terrestrial microwave comm. System: * Site Considerations
* Sources of Path Data
* Path Profile
* Interferences and Restrictions
* Routes and Sites to be Avoided
* Effects of Fading
* Diversity Methods to apply
* Reliability Objectives
1. A full description of each site by geographical coordinates, political subdivision, access roads and physical objects. 2. Any unusual weather conditions to be expected in the area including wind velocity, temperature and relative humidity. 3. A description of the physical characteristics of the site, indicating the amount of leveling required, removal of rocks, trees or other structure. 4. Relationship of the site to any airports.
5. The mean sea level elevation of the site and the path. 6. If possible the nearest location where commercial power is available. 7. The accessibility of the site for maintenance purposes.
SOURCES OF PATH DATA:
* Aeronautical charts
* Topographic maps
* Political maps
2. Aerial Photography
* Often useful in rough terrain to show more the details of the path.
After the tentative antenna sites have been selected and the relative elevation of the terrain and between the sites has been determined, a path profile can now be prepared.
1. CURVATURE & EQUIVALENT EARTH RADIUS
* The relative curvature of the earth and the microwave beam is an important factor when plotting a profile chart. The beam is normally bent downward a slight amount by atmospheric refraction. * Incorporating both earth’s curvature and signal bending will result to an equivalent earth radius, K.
The curvature for various value of K;
h - the change in the vertical dist. from reference line; (feet) d1 – the dist. from a point to one end of the path; (miles ) d2 – the dist. from the same point to the other end of the path; (miles) K – equivalent earth radius factor
Various values of K:
K = ∞ : flat earth
K = 1 : reference
K = 4/3 : super refraction; K > 1(Cases where a hot body of land occurs next to a relatively cool body of water.) K = 2/3 : sub-refraction; K <1 (cases where a cold land next to a relatively warm sea)
h(∞) = 0 ; K = ∞
h(2/3) = d1d2; K = 2/3
h(1) = 2/3 (d1d2) ; K = 1
h(4/3) = ½ (d1d2); K = 4/3
2. Drawing SCALES:
Horizontal scale: 1 mile : 1 inch...