Quasi-reflectionless microstrip bandpass filters with improved passband flatness and out-of-band rejection

A communication system that includes an absorptive coupled-line bandpass filter, comprising;

• a transmitter;
• a receiver;
• the absorptive coupled-line bandpass filter, which is located in the transmitter and/or receiver, wherein the absorptive coupled-line bandpass filter comprises,
• a first port,
• a second port,
• a coupled-line bandpass section coupled between the first and second ports, wherein the coupled-line bandpass section comprises a set of one or more parallel strip line resonators, which are coupled together in series and are coupled to the first and second ports through overlapping coupled-line sections,
• wherein the first port is coupled to a first end of the coupled-line bandpass section, and
• wherein the second port is coupled to a second end of the coupled-line bandpass section,
• a first absorptive stub coupled to the first port, and
• a second absorptive stub coupled to the second port.

A method for operating an absorptive coupled-line bandpass filter, comprising:

• receiving a signal-to-be-filtered at a first port;
• feeding the signal-to-be-filtered through an absorptive coupled-line bandpass filter to produce a filtered signal, wherein the absorptive coupled-line bandpass filter includes a coupled-line bandpass section coupled between the first port and a second port, wherein the coupled-line bandpass section comprises a set of one or more parallel strip line resonators, which are coupled together in series and are coupled to the first and second ports through overlapping coupled-line sections, wherein at a center frequency of a passband for the absorptive coupled-line bandpass filter, the first and second absorptive stubs appear as open circuits, and wherein outside of the passband, the first and second absorptive stubs appear as matched loads to ground and contribute to absorption of out-of-band signals;
• and
• outputting the filtered signal through the second port.