I.INTRODUCTION SPACE SIGNALS TRAVEL OR PROPOGATES FROM TX TO RX THE ENERGY LEVELS DECREASES RAPIDLY WITH DISTANCE OF TX THE ELECTROMAGNETIC WAVE ALSO AFFECTED BY OBJECTS ALONG THE WAY (EG. TREES, BUILDINGS AND STRUCTURES PATH OF EM SIGNALS ACTUALLY DEPENDS ON MANY FACTORS SUCH AS FREQUENCY OF SIGNAL, ATMOSPHERIC CONDITION AND TIME OF THE DAY WITH THESE FACTORS, THE QUALITY OF SIGNAL CAN BE IDENTIFIEDII. BASIC MODED OF WAVE PROPAGATION1.GROUND (SURFACE) WAVE PROPAGATION METHOD A GROUND WAVE IS AN EM WAVE THAT TRAVELS ALONG THE EARTH SURFACE IT IS ACTUALLY FOLLOW THE CURVATURE OF THE EARTH IT MUST HAVE A VERTICAL POLARIZATION TO PROPAGATE FROM ANTENNA BECAUSE THE ELECTRIC FIELD IN A HORIZONTALLY POLARIZED WAVE WOULD BE PARALLEL TO THE EARTHS SURFACE. THE HORIZONTALLY POLARIZED WAVES ARE ABSORBED BY THE EARTH( SHORT CIRCUITED TO THE GROUND) USED IN WORLD-WIDE COMMUNICATIONS IN THE VLF AND LF BANDS AND FOR BROADCASTING IN THE MF BANDS GENERALLY EFFECTIVE ONLY UP TO 2 MHZ.

FIGURE 1 GROUND WAVE PROPAGATIONTHE FIELD STRENGTH AT A DISTANCE FROM A TRANSMITTING ANTENNA IS GIVEN BY:

WHERE:120-CHARACTERISTIC IMPEDANCE OF FREE SPACEHT AND HR-EFFECTIVE HEIGHT OF THE TRANSMITTING AND RECEIVING ANTENNA, RESPECTIVELYL-ANTENNA CURRENTR-DISTANCE FROM THE TRANSMITTING ANTENNAL-WAVELENGTHTHE SIGNAL RECEIVED AT THAT DISTANCE IF A RECEIVING ANTENNA IS IN PLACE IS:V= ,,hrA.APPLICATIONS SHIP TO SHIP COMMUNICATION SYSTEM SHIP TO SHORE COMMUNICATION RADIO NAVIGATION MARITIME MOBILE COMMUNICATIONB. ADVANTAGES GIVEN ENOUGH AMOUNT OF POWER FOR TRANSMISSION, GROUND WAVE CAN BE USED TO COMMUNICATE BETWEEN ANY TWO LOCATION IN THE WORLD GROUND WAVES ARE RELATIVELY UNAFFECTED BY CHANGING OF THE ATMOSPHERIC CONDITIONC. DISADVANTAGES REQUIRE HIGLY TRANSMISSION POWER LIMITED TO VLF,LF AND MF, THEREFORE REQUIRED LARGE ANTENNA GROUND WAVE LOSSES VARY CONSIDERABLY WITH SURFACE MATERIALS.2. SKY-WAVE (IONOSPHERIC) PROPAGATION METHOD SIGNAL REFLECTED FROM IONIZED LAYER OF ATMOSTPHERE BACK DOWN TO EARTH SIGNAL CAN TRAVEL A NUMBER OF HOPS, BACK AND FORTH BETWEEN IONOSPHERE AND EARTHS SURFACE REFLECTION EFFECT CAUSED BY REFRACTION GENERALLY EFFECTIVE ABOVE 2MHZ UP TO 30MHZ. EXAMPLES ARE AMATEUR RADIO, CB RADIO AND LONG DISTANCE BROADCAST A RADIO WAVE THAT ENTERS THE E LAYER WITH AN ANGLE OF INCIDENT 0 WILL CONTINIOUSLY BE REFRACTED AWAY FROM THE NORMAL. IF THE VALUES OF THE ELECTRON DENSITY N AND FREQUENCY F ARE SUCH THAT

WHERE: N-NUMBER OF FREE ELECTRONS PER M^3n=FREQUENCY OF RADIO WAVE (HZ)

FIGURE 2

FIGURE 2 SKY WAVE PROPAGATION3. SPACE WAVE (TROPOSPHERIC) PROPAGATION METHOD DIRECT WAVE AND GROUND REFLECTED WAVE ALSO KNOWN AS LINE OF SIGHT PROPAGATION THE DIRECT WAVE IS THE MOST COMMONLY USED MODE IN ANTENNA COMMUNICATION THE PROPAGATED WAVE IS DIRECT FROM TRANSMITTING ANTENNA TO RECEIVING ANTENNA AND DOES NOT TRAVEL ALONG THE GROUND, MEANS THE EARTHS SURFACE DOES NOT ATTENUATE IT. BECOMES COMPULSORY WHEN FREQUENCY GENERALLY EXCEEDS 30 MHZ AND BEYOND UP TO 300GHZ. TWO GENERAL COMPONENTS ARE:(1) ONE OF WHICH TRAVELS IN A VERY NEARLY STRAIGHT LINE BETWEEN THE TRANSMITTING AND RECEIVING ANTENNAS(DIRECT SPACE WAVE), AND(2) THE OTHE WHICH TRAVELS BY MEANS OF A SINGLE REFLECTION FROM THE EARTH( GROUND REFLECTED SPACE WAVE). USED FOR SOUND AND TELEVISION BROADCASTING FOR RADIO RELAY SYSTEMS, AND FOR VARIOUS MOBILE SYSTEMS OPERATING IN THE VHF, UHF AND SHF BANDS WAVE IS LIMITED BY(1) EARTHS CURVATURE (LINE OF SIGHT DEPENDENT) AND(2) HEIGHTS OF TRANSMITTING AND RECEIVING ANTENNAS MAXIMUM RADIO RANGE (LINE OF SIGHT DISTANCE BETWEEN TRANSMITTER AND RECEIVER TOWERS) IS GIVEN BY:d=dt+dr=WHERE: D-DISTANCE BETWEEN ANTENNAS(KM)HT-TRANSMITTING ANTENNAS HEIGHT IN METERHR-RECEIVING ANTENNAS HEIGHT IN METER

FIGURE 3 SPACE WAVE PROPAGATIONIII. OTHER MODES OF WAVE PROPAGATION1. FREE SPACE PROPAGATION RADIO SIGNALS TRAVELS IN FREE SPACE, OR AWAY FROM OTHER OBJECTS WHICH INFLUENCE THE WAY IN WHICH THEY TRAVEL. IT IS ONLY THE DISTANCE FROM THE SOURCE WHICH AFFECTS THE WAY IN WHICH THE FIELD STRENGTH REDUCES. ENCOUNTERED WITH SIGNALS TRAVELLING TO AND FROM SATELLITES.2. PROPAGATION BY SATELLITES COMMUNICATION SATELLITES ARE ORBITING AROUND THE EARTH AT APPROXIMATELY 22, 300 MILES ABOVE. RECEIVED A SIGNAL FROM AN EARTH STATION, AMPLIFY IT AND THEN TRANSMIT IT AT A DIFFERENT FREQUENCY TOWARDS THE EARTH. ACTING AS RADIO REPEATER IN OUTER SPACE COMMONLY USED TO CARRY INTERNATIONAL TELEPHONY SYSTEMS AND TELEVISION SIGNALS

FIGURE 4 PROPAGATION BY SATELLITES3. TROPOSPHERIC SCATTER METHOD CONSIDERED A SPECIAL CASE OF SKY WAVE A TROPOSPHERIC-SCATTER RADIO LINK OPERATES WITH ITS DISTANT TERMINAL WELL BEYOND THE RADIO HORIZON A VERY SMALL PROPORTION OF THE RADIATED ENERGY IS FORWARD SCATTERED BY THE TROPOSPHERE AND DIRECTED DOWNWARDS TOWARDS THE RECEIVING ANTENNA MOST OF THE TRANSMITTED ENERGY CONTINUES UPWARDS, PASSED THROUGH THE IONOSPHERE AND RADIATED INTO SPACE. POSSIBLE AT MOSTLY FREQUENCY ABOVE 500MHZ MAIN APPLICATION IS A COMMUNICATION OVER HOSTILE TERRAIN WHERE COMMUNICATION SYSTEM WOULD NOT BE ECONOMICAL PROVIDES A BANDWIDTH OF SEVERAL MHZ AND IS EMPLOYED TO CARRY WIDEBAND TELEPHONY SYSTEM USEFUL ENERGY RETURNED TO EARTH TYPICALLY 1KM OR 2KM ABOVE THE EARTH BOTH THE TRANSMITTER LAUNCH ANGLE AND THE SCATTER ANGLE SHOULD BE SMALL, USUALLY LESS THAN 4

FIGURE 5 TROPOSPHERIC SCATTER PROPAGATION METHOD

REFERENCES: HTTP://WWW.SLIDESHARE.NET/JAYLEONG111/CHAPTER-8-WAVE-PROPAGATIONHTTP://WWW.SLIDESHARE.NET/ISHANEGI35/ANTENNAS-WAVE-AND-PROPAGATION