Series resonance

Chapter Series resonance

Teach Yourself Electricity and Electronics Third Edition Book
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Teach Yourself Electricity and Electronics Third Edition Book

  • This loss occurs at heat dissipation. It is true power. Any true power that goes intoheating up a transmission line is wasted, because it cannot be dissipated in the load.The additional loss caused by standing waves, over and above the perfectly-matchedline loss, is called standing-wave loss.IThe greater the mismatch, the more severe the standing-wave loss becomes. Themore loss a line has to begin with (that is, when it is perfectly matched), the more lossis caused by a given amount of mismatch. Standing-wave loss increases with frequency.It tends to be worst in long lengths of line at VHF, UHF, and microwaves.Line overheatingA severe mismatch between the load and the transmission line can cause another prob-lem: physical destruction of the line!A feed line might be able to handle a kilowatt (1 kW) of power when it is perfectlymatched. But if a severe mismatch exists and you try to feed 1 kW into the line, the ex-tra current at the current loops can heat the conductors to the point where the dielec-tric material melts and the line shorts out.It is also possible for the voltage at the voltage loops to cause arcing between theline conductors. This perforates and/or burns the dielectric, ruining the line.When a line must be used with a mismatch, derating functions are required to de-termine how much power the line can safely handle. Manufacturers of prefabricatedlines can supply you with this information.Series resonanceOne of the most important phenomena in ac circuits, especially in radio-frequency en-gineering, is the property of resonance. You’ve already learned that resonance is a con-dition that occurs when capacitive and inductive reactance cancel each other out.Resonant circuits and devices have a great many different applications in electricity andelectronics.Recall that capacitive reactance, XC, and inductive reactance, XL, can sometimesbe equal in magnitude. They are always opposite in effect. In any circuit containing aninductance and capacitance, there will be a frequency at which XL−XC. This is reso-nance. Sometimes XL−XC at just one frequency; in some special devices it can occurat many frequencies. Generally, if a circuit contains one coil and one capacitor, therewill be one resonant frequency.Refer to the schematic diagram of Fig. 17-11. You might recognize this as a seriesRLC circuit. At some particular frequency, XL−XC. This is inevitable, if L and C arefinite and nonzero. This is the resonant frequency of the circuit. It is abbreviated fo.318 Power and resonance in ac circuits17-11A series RLC circuit.At fo, the effects of capacitive reactance and inductive reactance cancel out. Theresult is that the circuit appears as a pure resistance, with a value very close to R. If