Stub‐loaded square loop bandpass filter with equiripple response in both delay and magnitude

It is shown that a stub loaded square loop filter, composed of six sections of commensurate transmission line, can achieve equiripple response in both magnitude and group delay simultaneously, when the admittances of the lines are properly chosen. Design formulas are given to calculate the admittances by given specifications such as fractional bandwidth and in‐band magnitude ripple. An interesting property of this filter is that the group delay can be adjusted while the in‐band magnitude response keeps almost unchanged. For verification, a filter sample is designed, fabricated and measured. In experiment, it exhibits a delay variation of 0.22 ns and minimal insertion loss of 0.21 dB within its passband (|S₁₁| < ?13.8 dB) from 0.48 to 1.52 GHz, which agree well with the theoretical predictions.