Balun transformer

1. A balun transformer comprising:

a first conductive pattern having one end provided as an input/output port of an unbalanced signal;

a second conductive pattern electromagnetically coupled to the first conductive pattern and having both ends provided as input/output ports of a balanced signal; and

a first variable capacitor connected between a ground part and a middle part of an electrical length of the second conductive pattern.

2. The balun transformer of claim 1 , further comprising a second variable capacitor connected to the other end of the first conductive pattern.

3. The balun transformer of claim 1 , wherein the first conductive pattern and the second conductive pattern are formed on one surface of a dielectric substrate.

4. The balun transformer of claim 3 , wherein the first conductive pattern is an inductor pattern including a plurality of loop patterns, and the second conductive pattern is an inductor pattern including a plurality of loop patterns, the plurality of loop patterns of the first conductive pattern concentrically alternating with the plurality of loop patterns of the second conductive pattern.

5. The balun transformer of claim 4 , wherein the first conductive pattern is a symmetrical inductor pattern, and the second conductive pattern is a symmetrical inductor pattern, the respective symmetrical inductor patterns of the first and second conductive patterns being symmetrical about one virtual line.

6. The balun transformer of claim 3 , wherein each of the first and second conductive patterns comprises a portion disposed on the other surface of the dielectric substrate and connected with another portion of a corresponding one of the first and second conductive patterns disposed on the one surface of the dielectric substrate through a conductive via hole, so that the corresponding conductive pattern is not short-circuited.

7. The balun transformer of claim 1 , wherein the variable capacitor is a varactor diode.