Impedance of feeding cable

Impedance is defined as the ratio of the voltage to current at all points on a feeding cable of unlimited length. Impedance is often represented as Z。. For coaxial cables,

Ｚ。＝〔138/√εr 〕×log(D/d) ohm.

Usually, Ｚ。=50/ 75 ohms, in which D stands for the diameter of the copper net, d, diameter of the conductor, and εr dielectric constant of the isolation medium between the conductor line and the shield net.

From above, we can see, impedance has to do with the diameters of conductors, distance between the conductors and the dielectric constant of the isolation medium, while having nothing to do with the length of the cable, working frequency and the load.

When transmitted in the cable, the signal suffers not only the resistance of the conducting material, but also the media loss of isolation material. These losses increase with the length of the cable and the frequency of the signal. So, it is a must to achieve such designation that the length of cable be kept shortest possible. These losses are represented as attenuation constant, in dB/m or dB/km.

(About the concept of dB: let input power be Ｐ; output power be Ｐ; transmission loss be γ;

γ(dB)＝10×log(Ｐ。/Ｐ　)(dB))

Good Topics.

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