Contents I. Introduction
During the last two decades, a considerable research progress on the development of RFmicrowave filters with multi-band operational characteristics has been conducted. This has been largely motivated by their potential need for emerging multi-purpose RF-wireless-communications and remote-sensing applications, in which the exploitation of several spectral bands at the same time is advantageous [1]. In this manner, a vast plurality of multi-band bandpass filters for the simultaneous acquisition of multiple desired RF signals corresponding to different services has been reported [2], [3], [4], [5], [6], [7], [8]. The same applies to RF multi-band bandstop or multi-notched-band filters, as a hardware mechanism that may allow the implementation of the required protection for RF systems to unwanted jamminginterference RF signals in highly-congested electromagnetic (EM) environments [9], [10], [11]. Note that although the existing variety of prior-art RF multi-band filters in terms of underlying RF operational principles and circuit structures is very rich, those approaches using single-to-multi-band frequency transformations deserve special mention [12], [13], [14], [15], [16]. This is owing to their intrinsic simplicity, since they mostly involve the replacement of the basic resonators of a pre-designed bandpass filter counterpart—or of the normalized unitary capacitors of an equivalent normalized lowpass filter prototype for normalized-lowpass-to-multi-band frequency transformations—by sets of resonators to shape the distinct bands. Lately, more-evolved frequency transformations have been devised, such as those in [11] and [17] allowing to embed multiple stopbands inside a wide passband range for the design of RF pre-selection broad-band filters with co-integrated multi-interference suppression.
