Tunable low-pass filter using dual mode

1. A low-pass filter comprising:

a cylindrical cavity configured to trigger a resonant mode;

a plurality of irises formed on a lateral face of the cylindrical cavity; and

a dummy iris formed on the lateral face of the cylindrical, and

a piston configured to move in the cylindrical cavity,

wherein a location of the piston is adjusted to adjust a center frequency of the low-pass filter,

wherein the resonant mode includes a plurality of modes having a plurality of resonant frequencies, and differences in resonance frequency between the plurality of modes are determined based on angles between the plurality of irises based on an axis of the cylindrical cavity,

wherein the plurality of irises are formed at a point at which one electric field is zero, and

wherein the dummy iris is formed at a position at which an other electric field is zero on the lateral face of the cylindrical cavity.

2. The low-pass filter of claim 1 , wherein the plurality of irises is formed on the lateral face at a predetermined angle.

3. The low-pass filter of claim 1 , wherein the plurality of irises each has a length less than a half-wave length of a used frequency.

4. The low-pass filter of claim 1 , wherein the resonant mode is a transverse electric m1 (TE m1 ) mode, and wherein m is an integer.

5. The low-pass filter of claim 1 , further comprising: a tuning screw configured to compensate for a resonance frequency reduced by the dummy iris.

6. The low-pass filter of claim 5 , wherein the tuning screw and the dummy iris are formed on the cylindrical cavity at angles corresponding to multiples of 180/m degrees (°) based on the axis of the cylindrical cavity and a transverse electric m1 (TE m1 ) mode, and wherein m is an integer.