1 A TECHNICAL BRIEFING FOR AMATEUR RADIO OPERATORS “VOLTAGE STANDING WAVE RATIOS (VSWR): WHY IT DOES NOT MEAN AS MUCH AS YOU THINK” PRESENTED TO: ARASWF NAPLES, FL JANUARY 2015 PRESENTED BY: DR. AL TORRES KP4AQI

2 PRESENTATION OUTLINE WHAT IS VSWR –THE THREE DEMO ANTENNAS –WHY VSWR IS NOT THE WHOLE STORY RELATIONSHIP BETWEEN VSWR AND POWER TRANSFER THREE IMPORTANT ISSUES WITH ANTENNAS –VSWR (POWER TRANSFERRED INTO ANTENNA) –ANTENNA EFFICIENCY (ANTENNA LOSSES) –OPTIMUM RADIATION (IS NOT AT THE LOWEST VSWR) CABLES AND CONNECTORS THE THREE DEMO ANTENNAS: BEST ANTENNA SUMMARY QUESTIONS

3 WHAT IS VSWR THE GENERATION OF STANDING WAVES WITHIN A NETWORK A MISMATCH IMPEDANCE LEVEL THE TRANSITION BETWEEN IMPEDANCE DISCONTINUITIES THE RATIO OF POWER TRANSFERRED TO POWER REFLECTED THE RATIO OF THE FORWARD VOLTAGE TO REFLECTED VOLTAGE REFLECTION COEFFICIENTS [    = VSWR-1/ VSWR+1] RETURN LOSS (S11) RATIO OF S21/S11

4 THE THREE (3) DEMO ANTENNAS ANT 1 ANT 2 ANT 3 VSWR: 2:1 89% VSWR: 1.5:1 96% VSWR: 1.01:1 100%

5 WHY VSWR IS NOT THE WHOLE STORY WE DO NOT KNOW ANYTHING ABOUT THE OPERATING FREQUENCY WE DO NOT KNOW THE CABLE TYPE OR CONNECTORS WE DO NOT KNOW ANYTHING ABOUT THE ANTENNA WE DO NOT KNOW ANYTHING “WHERE” THE ANTENNA IS MOUNTED WE DO NOT KNOW ANYTHING ABOUT THE GROUND REFLECTIVITY WE KNOW VERY LITTLE ABOUT THE “ANTENNA SYSTEM”

6 RELATIONSHIP BETWEEN VSWR AND POWER TRANSFER VSWRPower Transfer (%) 1.0: : : : : : : : :155.6

7 RELATIONSHIP BETWEEN VSWR AND POWER TRANSFER (MORE DETAILS)

8 ANTENNAS AND MATCHING NETWORKS ANTENNA IMPEDANCE IS NOT FLAT AS A FUNCTION OF FREQUENCY EVEN BROADBAND ANTENNAS HAVE A SMALL VSWR VARIABILITY MATCHING NETWORKS ARE DESIGNED TO KEEP THE TRANSMITTER “HAPPY” MATCHING NETWORKS DO NOT KEEP THE ANTENNA IMPEDANCE MATCHED IF YOU USE A MATCHING NETWORK, MAKE SURE SUCH NETWORK IS SYMMETRICAL (e.g. PI NETWORK) A GOOD ANTENNA, PROPERLY MATCHED WILL OUT PERFORM AN ANTENNA (SO-SO) AND A MATCH BOX

9 THREE IMPORTANT ISSUES WITH ANTENNAS VSWR…..POWER TRANSFERED INTO THE ANTENNA ANTENNA EFFICIENCY (ANTENNA LOSSES) OPTIMUM RADIATION IMPEDANCE (NOT THE LOWEST VSWR)

10 VSWR HOW MUCH POWER IS TRANSFERED TO THE ANTENNA MEASURED AS A RATIO OF REFLECTED POWER TO FORWARD POWER SWR = 1 +  (Pr/Pf) / 1 -  (Pr/Pf) COAX LINE ANTENNA POWER FORWARD POWER RFLECTED

11 ANTENNA EFFICIENCY HOW WELL THE ANTENNA RADIATES TRANSFER EFFICIENCY FROM THE ANTENNA IMPEDANCE (TYPICALLY 50 OHMS) TO THE IMPEDANCE OF FREE SPACE (377 OHMS) GAIN = EFFICIENCY x DIRECTIVITY EFFICIENCY IS MADE FROM TWO COMPONENTS: –RADIATION RESISTANCE –OHMIC RESISTANCE GOOD ANTENNAS HAVE A HIGH RADIATION RESISTANCE AND LOW OHMIC RESISTANCE TYPICAL EFFICIENCIES FOR ANTENNAS: –COPPER WIRE MONOPOLE: 93%MOST ANTENNAS: 50-60% –COPPER WIRE DIPOLE: 92% –RUBBER DUCK: 20%

12 OPTIMUM RADIATION NOT AT THE LOWEST VSWR MEASURED WITH A FIELD STRENGTH METER VERY CLOSE TO THE LOWEST VSWR AT ANTENNA RESONANCE, THE ANTENNA REACTIVE VALUES APPROACH ZERO BUT REAL VALUES DO NOT ANTENNA SYSTEM BECOMES BETTER MATCHED WHEN IT IS CLOSE TO THE IMPEDANCE OF THE FEEDLINE (ACCOUNTING FOR CONNECTORS VSWR)

13 CABLES AND CONNECTORS

14 THE THREE (3) DEMO ANTENNAS ANT 1 ANT 2 ANT 3 VSWR: 2:1 89% ANT. EFF.: 93% TOTAL: ~83% VSWR: 1.5:1 96% ANT. EFF.: 20% TOTAL: ~19% VSWR: 1.01:1 100% ANT. EFF.: 1% TOTAL: ~1%

15 SUMMARY START WITH A GOOD EFFICIENT ANTENNA IF ANTENNA HAS GAIN, MAKE SURE THE PERIPHERAL ITEMS (e.g. BALUNS, CONNECTORS, ETC) ARE NOT LOSSY SELECT THE BEST MATCHED FEEDLINE (IMPEDANCE) TO THE IMPEDANCE OF THE ANTENNA USE “MATCH BOXES” TO FINE TUNE YOUR SYSTEM, NOT AS WAY OF MATCHING A LOUSY ANTENNA SELECT A LOW LOSS FEEDLINE, IF OPERATING AT THE HIGHER FREQUENCIES USE A FIELD STRENGTH METER TO OBTAINED THE OPTIMUM RADIATION FROM YOUR ANTENNA LOW VSWR, EFFICIENT ANTENNA, AND FIELD STRENGTH METER (DECLARE VICTORY!)

16 MORE INFORMATION or COPY OF PRESENTATION SEND AN TO THE FOLLOWING URL: CONTACT ME AT: AL TORRES KP4AQI P. O. BOX DAYTON, OH