Improvements of Microstrip Loop Filters

A miniaturized microstrip open-loop filter with dual-mode and defected ground structure is presented in this paper. The proposed filter has the characteristics of wide stopband, reduced size, and high selectivity. The combination of dual-mode and defected ground structure successfully eliminates the nearest two spurious harmonics. The simulated and experimental results show that the transmission coefficients are -30 and -25 dB at 2
^nd
-and 3
^rd
-harmonic frequencies.