© Kathrein T. Wulff 07/2002

RF System Performance

Kathrein-Werke KGRosenheimPO Box 10 04 44Phone:+ 49 (0)8031 184 - 0Fax: +49 (0)8031 184 991antennas.mobilcom@kathrein.dewww.kathrein.de

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?

© Kathrein T. Wulff 07/2002

• Frequency and Amplitude

© Kathrein T. Wulff 07/2002

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.

© Kathrein T. Wulff 07/2002

VSWR

U [Volt]

t [seconds]0

Negative voltage maximum

Positive voltage maximum

minmax)(

UUsVSWR =

© Kathrein T. Wulff 07/2002

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

© Kathrein T. Wulff 07/2002

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

© Kathrein T. Wulff 07/2002

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

© Kathrein T. Wulff 07/2002

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

© Kathrein T. Wulff 07/2002

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?

© Kathrein T. Wulff 07/2002

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.