Design 2.4 GHz wide bandpass filter using the closed loop ring resonator with the compound stub

A 2.4 GHz wide bandpass filter using the closed loop ring resonator with the compound stub proposed and implemented using microstrip lines, aiming at high level of miniaturization, low cost, low radiation loss, high quality factor and ease of fabrication. The compound stub is connected to the downer path of the loop filter functioning as the capacitance. As the experimental results, it is found that the bandwidth of this filter is more than 50 percent of the central frequency at the return loss of 20dB. also the size of the filter is smaller than the filter using the conventional open-circuited stub at least a quarter wavelength.