Impedance and VSWR
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© Kathrein T. Wulff 07/2002 RF System Performance Kathrein-Werke KG Rosenheim PO Box 10 04 44 Phone:+ 49 (0)8031 184 - 0 Fax: +49 (0)8031 184 991 [email protected] www.kathrein.de Torsten Wulff Kathrein-Werke KG Rosenheim Germany www.kathrein.de Phone: +049 8031 184 - 0
Transcript of Impedance and VSWR
© Kathrein T. Wulff 07/2002
RF System Performance
Kathrein-Werke KGRosenheimPO Box 10 04 44Phone:+ 49 (0)8031 184 - 0Fax: +49 (0)8031 184 [email protected]
Torsten Wulff
Kathrein-Werke KGRosenheimGermany
www.kathrein.dePhone: +049 8031 184 - 0
© Kathrein T. Wulff 07/2002
Impedance
VSWR
Return Loss
Mismatch Loss
Communication Market
© Kathrein T. Wulff 07/2002
Communication Market
• Impedance throughout the communication market is specified with 50 Ohm [Ω].
• Therefore all components and devices of the entire system need to match the impedance of 50 Ohm.
© Kathrein T. Wulff 07/2002
Impedance, what does it stand for?
Definition
• Impedance: The total passive opposition offered to the flow of electric current.
• While Impedance is determined by the particular combination of resistance, inductive reactance, and capacitive reactance in a given circuit
• While Impedance is a function of frequency, except in the case of purely resistive networks.
© Kathrein T. Wulff 07/2002
Impedance out of a VSWR value
Two different impedance values calculate out of a given VSWR. Fundamental impedance (Z0) refers to
50 Ohm.
Upper Impedance: ZU = Z0 x VSWRLower Impedance: ZL = Z0 / VSWR
E.g.: VSWR 1.15 equals VSWR 1.4 equalsZU = 57.5 Ohm ZU = 70 OhmZL = 43.5 Ohm ZL = 35.7 Ohm
© Kathrein T. Wulff 07/2002
• Voltage Standing Wave Ratio
... and what does it mean?
VSWR, what does it stand for?
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• Frequency and Amplitude
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Amplitude is the height of the wave relative to the zero line
t [sec]
1 second
Zero
Max.
Frequency describes the number of cycles within 1 second.
1 Hertz is shown within this example.
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VSWR
U [Volt]
t [seconds]0
Negative voltage maximum
Positive voltage maximum
minmax)(
UUsVSWR =
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Input signal
Reflected signal
Termination
A generator will generate a frequency and send it to a termination.
Generator
The termination may not accept the entire input power (green line), and therefore will reflect some of the input power (red line)
back to the generator.
Loss
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Termination100 % Forwarded Power
... some reflected power
Generator
We can measure the power that is rejected from the termination, and may compare it with the power forwarded from the generator.
Loss
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Return Loss
Return Loss• Return power is the power that can’t enter the
termination! It will “return” to the generator!
• The power is no longer available in the termination and is “lost”.
• It is a “return loss” travelling back to the source.
Termination100 % Forwarded Power
return loss
Generator
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We may state the performance of a RF device with three different words:
Impedance
VSWR
Return lossBut they all express’ the same!
Conclusion
Loss of power caused by the performance of a RF device
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We name the loss of power caused by reflections as:
Mismatch Loss
The term “mismatch loss” describes the loss of energy (power) due to reflections at the input port to
the termination.
Thus the termination is NOT sufficiently matched to the source.
© Kathrein T. Wulff 07/2002
How does the mismatch loss effect our site installation?
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VSWR and Mismatch
Antennas
Cables
VSWR of antennas as internationally specified with min 1.5
A mismatch loss of 0.18 dB occurs out of this specification
VSWR of cables depends on the manufacturer and is usually better than 1.1
A mismatch loss of 0.044 dB occurs out of this specification
© Kathrein T. Wulff 07/2002
Mismatch LossComparison of different VSWR values
• VSWR 1.5 => -14.0 dB return loss => 0.18 dB mismatch loss 4% power loss
• VSWR 1.4 => -15.7 dB return loss => 0.13 dB mismatch loss 3% power loss
• VSWR 1.3 => -17.7 dB return loss => 0.08 dB mismatch loss 2% power loss
• VSWR 1.1 => -26.5 dB return loss => 0.04 dB mismatch loss 0.6% power loss
© Kathrein T. Wulff 07/2002
Summary
Once a single component within the communication system is off the 50 Ohm
impedance, a so named “mismatch loss” will occur, which will result in additional losses due
to reflections.
Additional losses eventually will add to cable losses, and insertion losses of particular
components to degrade to overall performance.
