Historically, an introduction of bandpass electrical filters invented independently by Karl W. Wagner and George A. Campbell played a vital role in their successful development in high-frequency wireless telegraphy, wireless telephony, or multiplex wire telephony transmitting systems when a maximum number of channels could simultaneously operate on a given circuit with a minimum frequency interval between adjacent channels [1,2]. In addition to separating the various channels from each other in multiplex line, it was found convenient from an operating standpoint to separate, within the toll offices, the carrier frequencies as a group (using a high-pass filter) from the frequencies intended for ordinary telephony and telegraphy (using a low-pass filter), and transmit them by separate lines .
This chapter discusses the basic types of radio frequency (RF) and microwave filters based on the low-pass or high-pass sections, and bandpass or bandstop transformation. Classical filter design approaches, using image parameter and insertion loss methods, are given for low-pass and high-pass LC filter implementations. The quarterwave-line and coupled-line section, which are the basic elements of microwave transmission-line filters, are described and analyzed. Different examples of coupled-line filters including interdigital, combline, and hairpin bandpass filters are given. Special attention is paid to microstrip filters with unequal phase velocities that can provide unexpected ...