MICROWAVE PROPAGATION

HOW THE SYSTEM WORKS ……..

SITUATION: CONSIDER YOU ARE A COMMUNICATION

ENGINEER WORKING IN A COMPANY YOU ARE ASSIGN WITH TASK TO

ESTABLISH A COMMUNICATION LINK BETWEEN TWO STATIONS

TOOLS PROVIDED TO YOU ARE MICROWAVE TRNSMITTER AND RECEIVER

FOLLOWING INFORMATION ABOUT MICROWAVE IS KNOWN

MICROWAVE CAN PENETRATE THROUGH ATMOSPHERE

IN FREE SPACE MICROWAVE PROPAGATES IN STRAIGHT LINE AND WITHOUT ATTENUATION

USING MICROWAVE HIGHLY DIRECTIONAL ANTENNAS CAN BE DESIGNED

PROBLEM NOW YOU STARTED TRANSMISSION FROM

POINT A TO POINT B WHICH ARE ALONG THE LINE OF SIGHT ……..

FOR YOUR SURPRISE YOU RECEIVE COMPLAIN FROM POINT C THAT THEY ARE RECEIVING YOUR SIGNAL INSTEAD OF OTHER SIGNAL DESIGNATED AT THAT FREQUENCY…….

NOW YOU ARE IN STATE OF CONFUSION BECAUSE SIGNAL IS GOING BEYOND THE LINE OF SIGHT REGION

SO LETS THINK WHAT IS REALLY HAPPENING

REASON :THE RELATIVE PERMITTIVITY OF THE

ATMOSPHERE IS CLOSE TO UNITY ,BUT IS ACTUALLY A FUNCTION OF AIR PRESSURE , TEMPERATURE AND HUMIDITY .

AT MICROWAVE FREQUENCY

THIS RESULT SHOWS THAT PERMITTIVITY GENERALLY DECREASES AS ALTITUDE INCREASES .

• THIS CHANGE IN PERMITTIVITY WITH ALTITUDE CAUSES RADIO WAVES TO BEND TOWARDS EARTH(REFRACTION).

THERE IS ONE MORE REASON : WEATHER CONDITION CAN PRODUCE A TEMPERATURE INVERSION ,WHERE THE TEMPERATURE INCREASES WITH ALTITUDE.

THIS CONDITION CAN LEAD TO DUCTING WHERE WAVE CAN PROPAGATE A LONG DISTANCE PARALLEL TO EARTH SURFACE VIA DUCT

IS IT A ADVANTAGE OR DISADVANTAGE ???

ADVANTAGE : COMMUNICATION RANGE IS INCREASED

DISADVANTAGE: IN RADAR SYSTEM REFRACTION EFFECT LEADS TO ERROR AND DETERMINING THE ELEVATION OF TARGET CLOSE TO HORIZON .

PROBLEM 2 AT PARTICULAR FREQUENCY B IS NOT

ABLE TO RECEIVE THE SIGNAL SPECIALLY IN BAD WEATHER.

AT FREQUENCY SUCH AS 22.2 GHz,183GHz, 60 AND 120GHz

REASON ?????

THIS IS CAUSED PRIMARILY BY ABSORPTION OF MICROWAVE ENERGY BY WATER VAPOUR AND MOLECULAR OXYGEN LEADING TO ATTENUATION.

NOW AGAIN …IS IT ADVANTAGE OR DISADVANTAGE ?????

ADVANTAGE : REMOTE SENSING OF ATMOSPHERE IS OFTEN DONE WITH RADIOMETERS OPERATING NEAR 20 OR 55 GHz TO MAXIMIZE THE SENSING OF ATMOSPHERIC CONDITION .

DISADVANTAGE: IN COMMUNICATION SYSTEM

HOW TO OVERCOME ……...

BY USING THE FREQUENCY NEAR 35 ,94 AND 135 GHz WHERE COMMUNICATION SYSTEM AND RADAR OPERATE WITH MINIMUM LOSS.

WHATEVER WE DICUSSED TILL NOW IS RELATED TO ATMOSPHERIC EFFECTS …

REFRACTION AND ATTENUATION

GROUND EFFECTS THE MOST OBVIOUS EFFECT OF THE

PRESENCE OF THE GROUND ON MICROWAVE PROPAGATION IS REFLECTION FROM THE EARTH’S SURFACE .

The presence of reflectors in the environment surrounding a transmitter and receiver create multiple paths that a transmitted signal can traverse. As a result, the receiver sees the superposition of multiple copies of the transmitted signal, each traversing a different path. Each signal copy will experience differences in attenuation, delay and phase shift while travelling from the source to the receiver. This can result in either constructive or destructive interference, amplifying or attenuating the signal power seen at the receiver. Strong destructive interference is frequently referred to as a deep fade and may result in temporary failure of communication due to a severe drop in the channel signal-to-noise ratio.

ANOTHER GROUND EFFECT IS DIFFRACTION, WHEREBY A RADIO WAVE SCATTERS ENERGY IN THE VICINITY OF LINE OF SIGHT BOUNDARY AT THE HORIZON ,THUS GIVING THE RANGE SLIGHTLY BEYOND THE HORIZON .

IN RADAR SYSTEM UNWANTED REFLECTIONS OFTEN OCCURS FROM TERRAIN, VEGITATION, BUILDINGS AND SURFACE OF THE SEA .THIS EFFECT DEGRADE OR MASK THE RETURN OF TWO TARGET

IN REMOTE SENSING SUCH REFLECTION ARE THE DESIRED SIGNAL .

NOW CONSIDER ONE MORE INTERESTING PROBLEM FACED IN SPACECRAFT .

WHEN SPACE CRAFT REENTERS THE ATMOSPHERE IT LOOSES COMMUNICATION CONTACT WITH GROUND FOR SOME PERIOD OF TIME .

REASON ???

PLASMA EFFECT A PLASMA IS A GAS CONSISTING OF

IONIZED PARTICLE.THE IONOSPHERE CONSISTS OF

SPHERICAL LAYERS OF ATMOSPHERE WITH PARICLES WHICH HAVE BEEN IONISED BY SOLAR RADIATION .

PLASMA IS CHARACTERIZED BY IONS PER UNIT VOLUME . DEPENDING ON THIS DENSITY AND FREQUENCY WAVE MIGHT BE REFLECTED , TRANSMITTED OR ABSORBED .

A VERY DENSE PLASMA IS FORMED ON SPACE CRAFT AS IT REENTERS THE ATMOSPHERE, DUE TO HIGH TEMPERATURE PRODUCED BY FRICTION .

THE SPACECRAFT WILL ABLE TO COMMUNICATE AFTER ITS VELOCITY DECREASED BELOW PARTICULAR LIMIT.

THANK YOU