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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 34119
JANUARY 2015
PRESENTED BY:
DR. AL TORRES
KP4AQI
EMAIL: [email protected]
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PRESENTATION OUTLINE
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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
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QUESTIONS
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WHAT IS VSWR
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THE GENERATION OF STANDING WAVES WITHIN A NETWORK
A MISMATCH IMPEDANCE LEVEL
THE TRANSITION BETWEEN IMPEDANCE DISCONTINUITIES
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THE RATIO OF POWER TRANSFERRED TO POWER REFLECTED
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THE RATIO OF THE FORWARD VOLTAGE TO REFLECTED
VOLTAGE
REFLECTION COEFFICIENTS [    = VSWR-1/ VSWR+1]
RETURN LOSS (S11)
RATIO OF S21/S11
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THE THREE (3) DEMO ANTENNAS
ANT
1
VSWR: 2:1
89%
ANT
2
VSWR: 1.5:1
96%
ANT
3
VSWR: 1.01:1
100%
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WHY VSWR IS NOT THE WHOLE STORY
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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”
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RELATIONSHIP BETWEEN VSWR AND POWER
TRANSFER
VSWR
1.0:1
1.5:1
2.0:1
2.5:1
3.0:1
3.5:1
4.0:1
4.5:1
5.0:1
Power Transfer (%)
100
96
89
81.6
75
69.2
64
59.5
55.6
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RELATIONSHIP BETWEEN VSWR AND POWER
TRANSFER (MORE DETAILS)
VSWR
1.0:1
1.25:1
1.5:1
1.75:1
2.0:1
2.25:1
2.50:1
2.75:1
3.0:1
POWER TRANSFER (%)
100
99
96
93
89
85
82
78
75
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ANTENNAS AND MATCHING NETWORKS
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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
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THREE IMPORTANT ISSUES WITH ANTENNAS
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VSWR…..POWER TRANSFERED INTO THE ANTENNA
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ANTENNA EFFICIENCY (ANTENNA LOSSES)
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OPTIMUM RADIATION IMPEDANCE (NOT THE LOWEST VSWR)
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VSWR
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HOW MUCH POWER IS TRANSFERED TO THE ANTENNA
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MEASURED AS A RATIO OF REFLECTED POWER TO FORWARD
POWER
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SWR = 1 + (Pr/Pf) / 1 - (Pr/Pf)
POWER FORWARD
COAX LINE
POWER RFLECTED
ANTENNA
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ANTENNA EFFICIENCY
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HOW WELL THE ANTENNA RADIATES
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TRANSFER EFFICIENCY FROM THE ANTENNA IMPEDANCE (TYPICALLY
50 OHMS) TO THE IMPEDANCE OF FREE SPACE (377 OHMS)
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GAIN = EFFICIENCY x DIRECTIVITY
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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%
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OPTIMUM RADIATION
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NOT AT THE LOWEST VSWR
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MEASURED WITH A FIELD STRENGTH METER
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VERY CLOSE TO THE LOWEST VSWR
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AT ANTENNA RESONANCE, THE ANTENNA REACTIVE VALUES
APPROACH ZERO BUT REAL VALUES DO NOT
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ANTENNA SYSTEM BECOMES BETTER MATCHED WHEN IT IS
CLOSE TO THE IMPEDANCE OF THE FEEDLINE (ACCOUNTING
FOR CONNECTORS VSWR)
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CABLES AND CONNECTORS
Frequency
Straight
Range
Connectors
DC to 2 GHz
1.15:1
2 to 4 GHz
1.20:1
Two Angle
Connectors
1.20:1
Mixed
Connectors
1.20:1
1.25:1
1.25:1
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THE THREE (3) DEMO ANTENNAS
ANT
1
VSWR: 2:1
89%
ANT. EFF.: 93%
TOTAL: ~83%
ANT
2
VSWR: 1.5:1
96%
ANT. EFF.: 20%
TOTAL: ~19%
ANT
3
VSWR: 1.01:1
100%
ANT. EFF.: 1%
TOTAL: ~1%
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SUMMARY
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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!)
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MORE INFORMATION or COPY OF PRESENTATION
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SEND AN EMAIL TO THE FOLLOWING URL:
[email protected]
CONTACT ME AT:
AL TORRES
KP4AQI
P. O. BOX 24283
DAYTON, OH 45424-0283
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