‰ NOW 40 WPM ‰ LESS SIGNAL. THIS WOULD SEEM TO BE REINFORCED BY THE RECEIVED POWER GOING UP WITH THE SQUARE OF THE WAVELENGTH IN EQUATION 3, ABOVE. THE FACT OF THE MATTER IS THAT IF THE RECEIVING ANTENNAS AT TWO FREQUENCIES HAVE THE SAME APERTURE AREA THINK THE SAME SIZE, FOR EXAMPLE THE SAME PARABOLIC DISH USED IN EACH CASE THE RECEIVED SIGNAL WILL BE AT THE SAME LEVEL ON BOTH FREQUENCIES. STILL, UNLESS SOMETHING IS DONE, IT IS TRUE THAT LOWER FREQUENCIES WILL HAVE AN ADVANTAGE. INC SPHERE AREA OCCUPIED BY THE PATCH TIMES THE TOTAL POWER TRANSMITTED. LOOKING BACK TO HIGH SCHOOL GEOMETRY, WE RECALL OR COULD LOOK UP THAT THE AREA OF A SPHERE IS. THUS THE ACTUAL RECEIVED ENERGY IS JUST R THE RADIUS OF THE SPHERE, OR EQUIVALENTLY, THE DISTANCE BETWEEN THE TRANSMIT AND RECEIVE ANTENNAS. THE USUAL UNITS ARE WATTS W FOR TRANSMIT AND RECEIVE POWER, METERS M FOR DISTANCE AND SQUARE METERS FOR RECEIVE ANTENNA AREA. ONE IMPORTANT RESULT THAT WE CAN TAKE FROM THIS IS THAT THE RECEIVE POWER IS REDUCED IN PROPORTION TO THE SQUARE OF THE DISTANCE, AN IMPORTANT CONCEPT OFTEN REFERRED TO AS THE INVERSE SQUARE LAW. WHATS THE RECEIVE ANTENNA AREA? THIS IDEA MAKES A GREAT PICTURE, BUT BEFORE WE CAN MAKE MUCH USE OF IT, WE NEED TO KNOW HOW TO DETERMINE THE EFFECTIVE AREA OF A RECEIVE ANTENNA. PERHAPS THE EASIEST TO UNDERSTAND IS A PARABOLIC DISH ANTENNA, OFTEN USED FOR SATELLITE RECEPTION, AS WELL AS POINT TO POINT MICROWAVE LINKS AND RADAR SYSTEMS SEE FIGURE 2. IN THIS CASE THE AREA, PERHAPS NOT SURPRISINGLY, IS CLOSE TO THAT OF THE OPENING OR MOUTH OF THE DISH. REAL WORLD BEING WHAT IT IS, IT CAN NEVER QUITE REACH THE FULL PHYSICAL APERTURE ANTENNA GAIN, PART III HOW MUCH SIGNAL GETS RECEIVED? TRANSMIT GAIN GETS THE SIGNAL LAUNCHED, BUT HOW STRONG WILL IT BE AT THE RECEIVER? JOEL R. HALLAS, W1ZR IN PARTS I AND II OF THIS SERIES WE TALKED ABOUT WHAT ANTENNA GAIN IS, THE DIFFERENT WAYS WE CAN SPECIFY IT, AND WAYS TO GET GAIN FROM ANTENNA STRUCTURESR. IN THIS PART, PART III , WE WILL CONSIDER WHAT HAPPENS TO THE SIGNAL AT THE END OF ITS JOURNEY. WHAT MAKES THE RECEIVED SIGNAL WEAKER? LETS START BY LOOKING AGAIN AT A HYPOTHETICAL ISOTROPICALLY RADIATING ANTENNA IN MYTHICAL FREE SPACE. AS NOTED IN PART I, SUCH AN ANTENNA RADIATES EQUALLY IN ALL DIRECTIONS. IF WE CONSIDER AN IMAGINARY SPHERE SURROUNDING SUCH A SIGNAL SOURCE, THE SIGNAL RECEIVED ANYWHERE ON THE SURFACE OF THE SPHERE WILL BE OF EXACTLY THE SAME STRENGTH AS A SIGNAL RECEIVED ON ANY OTHER PART OF THE SPHERE OF THE SAME AREA. THIS IS SHOWN GRAPHICALLY IN FIGURE 1, IN WHICH THE PATCH SHOWN REPRESENTS A RECEIVING ANTENNA THAT PICKS UP ALL THE SIGNAL COMPONENTS RECEIVED WITHIN THE PATCH AREA. IF THE PATCH IN FIGURE 1 WERE SOMEHOW EXTENDED TO COMPLETELY SURROUND THE SPHERE IN ALL DIRECTIONS, THE RECEIVED SIGNAL WOULD EXACTLY EQUAL THE TRANSMITTED SIGNAL, REGARDLESS OF THE SIZE OF THE SPHERE. FROM THIS IDEA, WE CAN COMPUTE THE ACTUAL POWER THAT CAN BE RECEIVED OVER THE AREA OF THE RECEIVE ANTENNA SURFACE ITS JUST THE FRACTION OF THE TOTAL ‰ END OF 40 WPM TEXT ‰ QST DE W1AW ƒ 