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RF and Microwave Engineering: Fundamentals of Wireless Communications by Frank Gustrau

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5.4 S-Parameter Representation of Network Properties

Networks may have special electric properties like reciprocity or loss-lessness. These properties can be directly identified by looking at the scattering parameters as we will show in the following sections.

5.4.1 Matching

A matched port has a zero-valued reflection coefficient. A network that is matched at all ports has only reflection coefficients equal to zero.

5.28 5.28

According to Equation 5.17 a reflection coefficient that is zero means that there is no reflected wave bi at that port. The input impedance Zini at port i equals the port reference impedance Z0i. Matching (i.e. the absence of reflections) is generally desirable in RF circuits. Perfect matching is practically impossible, so we try to keep reflections under a certain limit depending on our application. Typical limits where we consider matching to be sufficient are s11 ≤ − 20 dB (1% reflected power), s11 ≤ − 10 dB (10% reflected power) or s11 ≤ − 6 dB (25% reflected power). The frequency range where we find our requirements satisfied is called (impedance) bandwidth.

Figure 5.4 shows as an example a frequency dependent reflection coefficient of an antenna. In the frequency range from the lower frequency fL, 10 dB to the upper frequency fU, 10 dB the reflection coefficient is −10 dB or less (the return loss is +10 dB or more). So the bandwidth is referred to as 10 dB ...

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