PURPOSE: To obtain a miniaturized resonator by forming a circuit having a characteristic impedance different from a characteristic impedance of a resonance circuit on the way of a cavity resonance circuit so as to reduce the length of the resonance circuit.

CONSTITUTION: The characteristic impedance (Ipd) when viewing from a point (a) toward a short-circuit side (s), Ipd Z1 is expressed in equation I, where W1 is a characteristic impedance of the circuit from the short-circuit side (s) till the point (a) in the TEM mode cavity resonator applied with 3λ/4 (λ is resonance wavelength), l1 is the circuit length, W2 is a characteristic impedance of the circuit from the point (a) to the point (b), l2 is the circuit length, W3 is a characteristic impedance of the circuit from the point (b) to the point (c), and l3 is the circuit length. Moreover, the characteristic impedance Ipd Z2 when viewing from the point (b) toward the short-circuit side (s) is expressed by equation II and the characteristic impedance Ipd Z3 when viewing the point (c) to the short-circuit side (s) is expressed in equation III. The circuit length l1 to satisfy the relation of the total reactance component Zs=+j22.7ohms is expressed in equation IV according to the equations I∼III and the total length (l) of the resonance circuit required is expressed in equation V, where W1=W3=100 ohms. W2=10ohms, l2=l3=500mm. an output static capacitance of power tube Cpg=100pF and the resonance frequency f=70MHz. Thus, the length of the resonance circuit is reduced.