6不同媒质中的波数和波阻抗(15-1).doc

7. Intrinsic Constants Intrinsic constants in general media As mentioned previously, the wave number (the intrinsic wave number) and the intrinsic impedance (the intrinsic wave impedance) are defined by and where and To solve and for and , one obtains the complex impedance and the complex admittance as and [Proof] and Therefore a knowledge of the wave number and the intrinsic impedance is equivalent to the knowledge of the complex impedance and the complex admittance , or to the knowledge of , in other words the knowledge of and specifies the characteristics of the medium. Both the wave number and the intrinsic impedance are complex, and may be written as in terms of which, the +z-propagated traveling plane wave becomes and where is termed the phase constant, which gives rise to a variation of the wave phase, . is termed the attenuation constant, which causes an exponential attenuation of the wave amplitude, . In addition to the wave number , another parameter defined by and called the propagation constant is often utilized in some textbooks. It is easy to show that is the attenuation constant, and is the phase constant. In fact Namely, the wave number k and the propagation constant are related by and It is physically understood that for a z-propagated and attenuated plane wave, both the attenuation constant and the phase constant should be positive, and this is why the wave number is denoted by rather than by , and the propagation constant is denoted by rather than by . Intrinsic constants in lossy media For lossy media, the material parameters are assumed to be independent of frequency, The complex impedance and the complex admittance are The wave number is where The phase constant and the attenuation constant are determined as follows. then then then then then then then The intrinsic impedance is The intrinsic resistance R and the intrinsic reactance X are determined as follow