1. A variable resonator, comprising: a line part that comprises one or more lines and has an annular shape; at least two parallel resonant circuits configured to permit a change of a characteristic; and at least three variable reactance blocks configured to permit a change of a reactance value, wherein said at least two parallel resonant circuits are electrically connected to said line part at one end thereof at different positions on the line part, only one parallel resonant circuit of said at least two parallel resonant circuits is selected as a circuit to operate as a switch in an ON state, each remaining parallel resonant circuit of said at least two parallel resonant circuits, which is not selected as the circuit to operate as the switch in the ON state, is selected as a circuit to operate as a switch in an open state, and said at least three variable reactance blocks are electrically connected to said line part at predetermined intervals based on an electrical length at a resonance frequency of the variable resonator.
2. The variable resonator according to claim 1 , wherein said at least two parallel resonant circuits are configured to permit a change of a reactance value, and said at least three variable reactance blocks are the same as said at least two parallel resonant circuits.
3. The variable resonator according to claim 1 or 2 , wherein said one or more lines are formed by one annular line, and said at least three variable reactance blocks are electrically connected to the annular line as a branch circuit at said predetermined intervals based on said electrical length at said resonance frequency at which one wavelength or an integral multiple thereof equals to a perimeter of said annular line.
4. The variable resonator according to claim 3 , wherein said reactance values of said at least three variable reactance blocks are equal and said at least three variable reactance blocks are connected to said annular line at equal electrical distances.
5. The variable resonator according to claim 3 , wherein a total number of said at least three variable reactance blocks is M, where M represents an even number equal to or greater than 4, said reactance values of said at least three variable reactance blocks are equal, a first set of M/2−1 variable reactance blocks of said at least three variable reactance blocks are connected to said annular line at equal electrical distances within a range clockwise from an arbitrarily set position K 1 to a position K 2 spaced apart from the position K 1 by a half of an electrical length of said annular line, where any one of said first set of M/2−1 variable reactance blocks is not connected to the position K 1 and the position K 2 , a second set of M/2−1 variable reactance blocks of said at least three variable reactance blocks are connected to said annular line at equal electrical distances within a range counterclockwise from said position K 1 to said position K 2 , where any one of said second set of M/2−1 variable reactance blocks is not connected to the position K 1 and the position K 2 , and two of said at least three variable reactance blocks are connected to said annular line at said position K 2 .
6. The variable resonator according to claim 3 , wherein a total number of said at least three variable reactance blocks is M−1, where M represents an even number equal to or greater than 4, M−2 variable reactance blocks of said at least three variable reactance blocks have said reactance values equal, the M−2 variable reactance blocks being referred to as a first variable reactance blocks hereinafter, a remaining one of said at least three variable reactance blocks has said reactance value set to half of the reactance value of said first variable reactance blocks, the remaining one variable reactance block being referred to as a second variable reactance block hereinafter, a first set of M/2−1 variable reactance blocks of said first variable reactance blocks are connected to said annular line at equal electrical distances within a range clockwise from an arbitrarily set position K 1 to a position K 2 spaced apart from the position K 1 by a half of an electrical length of said annular line, where any one of said first set of M/2−1 variable reactance blocks is not connected to the position K 1 and the position K 2 , a second set of M/2−1 variable reactance blocks of said variable reactance blocks are connected to said annular line at equal electrical distances within a range counterclockwise from said position K 1 to said position K 2 , where any one of said second set of M/2−1 variable reactance blocks is not connected to the position K 1 and the position K 2 , and said second variable reactance block is connected to said annular line at said position K 2 .
7. The variable resonator according to claim 1 or 2 , wherein said one or more lines are formed by at least three lines, each of said at least two parallel resonant circuits is electrically connected to any one of said at least three lines at one end thereof at a different position, each of said at least three lines has a predetermined electrical length at a resonance frequency at which one wavelength or an integral multiple thereof equals to a sum of the lengths of said at least three lines, and at least one of said at least three variable reactance blocks is electrically serially connected between every adjacent two of said at least three lines.
8. The variable resonator according to claim 7 , wherein a total number of said at least three lines is N, and a total number of said at least three variable reactance blocks is N, where N represents an integer equal to or greater than 3, said reactance values of the at least three variable reactance blocks are equal, said at least three lines have an equal electrical length, and said at least one of said at least three variable reactance blocks includes one variable reactance block connected between said every adjacent two of said at least three lines.
9. A variable filter, comprising: a variable resonator according to claim 1 ; and a transmission line, wherein said variable resonator and said transmission line are electrically connected to each other.
