Propagation Losses : Gas absorption : Primarily due to the water vapor and oxygen in the atmosphere in the radio relay region.The absorption peaks are located around 23GHz for water molecules and 50 to 70 GHz for oxygen molecules.The specific attenuation (dB/Km)is strongly dependent on frequency, temperature and the absolute or relative humidity of the atmosphere. Attenuation due to precipitation : Rain attenuation is the main contributor in the frequency range used by commercial radio links Rain attenuation increases exponentially with rain intensity The percentage of time for which a given rain intensity is attained or exceeded is available for 15 different rain zones covering the entire earth’s surface The specific attenuation of rain is dependent on many parameters such as the form and size of distribution of the raindrops, polarization, rain intensity and frequency Horizontal polarization gives more rain attenuation than vertical polarization Rain attenuation increases with frequency and becomes a major contributor in the frequency bands above 10 GHz The contribution due to rain attenuation is not included in the link budget and is used only in the calculation of rain fading Ground Reflection : Reflection on the Earth’s surface may give rise to multipath propagation The direct ray at the receiver may interfered with by the ground-reflected ray and the reflection loss can be significant Since the refraction properties of the atmosphere are constantly changing the reflection loss varies The loss due to reflection on the ground is dependent on the total reflection coefficient of the ground and the phase shift The highest value of signal strength is obtained for a phase angle of 0o and the lowest value is for a phase angle of 180o The reflection coefficient is dependent on the frequency, grazing angle (angle between the ray beam and the horizontal plane), polarization and ground properties The grazing angle of radio-relay paths is very small – usually less than 1o It is recommended to avoid ground reflection by shielding the path against the indirect ray The contribution resulting from reflection loss is not automatically included in the link budget . When reflection cannot be avoided, the fade margin may be adjusted by including this contribution as “additional loss” in the link budget . . . . shimy
Posted on: Thu, 16 Jan 2014 09:03:23 +0000