RF-signal power reflections produced by impedance mismatches remain a critical limitation in high-frequency systems, degrading their operational performance, robustness, and RF-signal integrity. Typical RF-protection mechanisms, such as inter-RF-stage attenuators, isolators$\boldsymbol {/}$circulators, or RF-signal-cancellation loops, often impose penalties in terms of increased in-band insertion-loss levels, size, cost, DC-power consumption, and design complexity. To overcome these challenges, reflectionless or absorptive RF circuits have emerged as an alternative paradigm. They intrinsically absorb undesired stopband$\boldsymbol {/}$non-transmitted RF signals, thereby providing broad-band impedance matching. This overview paper reviews the latest advances on RF reflectionless circuits, covering their fundamental RF design principles and more-recent developments. Beyond the well-established case of RF reflectionless filters illustrated through several state-of-the-art implementations in 3-D and planar technologies, other emerging designs are also addressed. They include reflectionless beyond-transmission-band-flat-group-delay single-ended bandpass filters and balanced lowpass filters for high-data-rate digital communications in planar and integrated RF platforms, and on-chip two-port-absorptive bandstop filters. Furthermore, as key examples of reflectionless RF active devices, millimeter-wave input-absorptive RF amplifiers for beyond-5G wireless scenarios are presented. Cutting-edge applications in antennas, electromagnetic compatibility (EMC), and quantum engineering are also discussed, highlighting the versatility of the reflectionless design philosophy.

This paper presents a high-order multilayer RF filter with wideband quasi-flat group-delay response and sharp multi-transmission-zero (TZ) stopbands. It is composed of three in-series-cascaded stages of low-order/single-unit input-absorptive wideband bandpass filters (BPFs). Its constituent BPF unit, which is based on a modified two-layer microstrip-to-microstrip vertical transition and a shunt resistivelyterminated microstrip line and exhibits quasi-flat group-delay response and broadband input-absorptive capabilities, is firstly discussed. Due to the employed short-circuit-ended two-section microstrip stub in the modified vertical transition, two close-to-passband TZs are realized to obtain a sharper-rejection BPF response. To achieve highly-attenuated multi-TZ stopbands, a high-order wideband quasi-flat-group-delay BPF with three in-series-cascaded single-unit BPFs using different impedanceparameter values for the short-circuit-ended microstrip stubs is then studied. For experimental-validation purposes, a 2-GHz proof-of-concept microstrip prototype in a two-layer substrate is simulated, built, and tested. The measured input-absorptive BPF is centered at 1.956 GHz with 3-dB absolute bandwidth of 0.828 GHz and maximum group-delay variation of ± 0.05 ns from 1.387 to 2.618 GHz (i.e., 1.231-GHz absolute bandwidth).

The theoretical design and practical development of RF analog filtering devices with co-integrated lowpass, single$\bm{/}$multi-band bandpass, and highpass transfer functions is reported. For this purpose, two different classes of generalized frequency transformation that convert the equivalent normalized lowpass filter prototype into the desired RF filter with several co-designed filtering actions are proposed. They realize a frequency mapping of the reactance of a normalized unitary capacitor into that of a one-port single$/$multi-resonance cell with added lowpass and highpass filtering capabilities. For these devised frequency transformations, the theoretical foundations and various illustrative filter examples designed at the ideal-circuit-model level are presented. In addition, higher-selectivity filter architectures based on the generation of additional out-of-band transmission zeros (TZs) by means of cross-coupling techniques are shown. Afterwards, the extension of this design methodology to RF multi-functional filtering components, such as input-reflectionless$\bm{/}$absorptive filters based on complementary-diplexer circuit networks and two-way filtering power-distribution circuits, is also demonstrated. Furthermore, two design examples of distributed-element and inverterless lumped-element RF filters are provided. Besides, for the distributed-element circuit, a proof-of-concept microstrip prototype is manufactured and measured as experimental validation.

A class of reflectionless RF bandpass filters (BPFs) with wideband quasi-constant in-band group-delay response in transmission for high-data-rate digital-communication systems is presented. These BPFs are developed by exploiting multilayer wideband microstrip-to-microstrip vertical transitions and lossy multi-section microstrip lines in complementary-diplexer-based architectures. A two-port-reflectionless wideband BPF with low-in-band-variation equi-ripple-type group-delay characteristics is firstly realized by using two resistively-terminated one-section microstrip lines. To further flatten its passband group delay, its counterpart based on lossy two-section microstrip lines is then conceived to attain extremely-flat wideband group-delay pattern. Subsequently, by means of a modified wideband microstrip transition with a short-circuit-ended two-section microstrip line and a resistively-terminated two-section microstrip line, an input-reflectionless BPF with two close-to-passband transmission zeros (TZs) and wideband in-band flat group-delay profile is reported. In order to obtain a higher-order flat-group-delay wideband BPF response with more TZs, its structure shaped by two in-series-cascaded units of the previous input-absorptive BPF section with two TZs is engineered, in which different impedance values for their constituent short-circuit-ended microstrip lines are utilized. The operational foundations of all the proposed wideband flat-group-delay RF BPFs are described in detail. Furthermore, for practical-validation purposes, three 2-GHz microstrip prototypes of these reflectionless wideband RF BPFs with quasi-constant group-delay responses beyond their 3-dB absolute bandwidths (BWs) are designed, simulated, manufactured, and characterized.

Various families of flat-in-band-group-delay RF planar bandpass filters (BPFs) with multiple out-of-band transmission zeros (TZs) in their filtering transfer function are presented. As the first approach, a type of BPFs with theoretically-constant group-delay profile and perfect two-port-reflectionless behavior is proposed. The theoretical sufficient conditions to obtain a frequency-constant group delay in a generalized form of transfer function for these BPFs are derived. They are then particularized in transmission-line-based transversal-circuit realizations with flat-in-band-group-delay and quasi-reflectionless characteristics. Afterwards, a class of reflective-type BPFs based on directional power couplers that are arranged in transversal-mode topologies are addressed toward its practical demonstration for the first time. These BPFs, which exhibit flat in-band group delay, are compared with their classic Bessel-type BPF counterparts without TZs. Moreover, discrete-time models of such BPFs are derived, analyzed, and discussed. For experimental-validation purposes, proof-of-concept microstrip prototypes centered at 2.5-GHz of the engineered quasi-absorptive/reflective-type flat-in-band-group-delay BPFs are designed, manufactured, and characterized.

Two classes of frequency-reconfigurable multi-band negative-group-delay (NGD) circuit networks that feature low-input-power-reflection capabilities are reported. They consist of lossy-complementary-diplexer architectures, in which the NGD properties are obtained within the stopband regions of their lossy multi-band bandstop-filter (BSF) channel. Their complementary lossy multi-band bandpass-filter (BPF) branch absorbs in its terminating resistor the RF-input-signal energy that is not transmitted by the lossy multi-band BSF channel within its stopbands. In this manner, the input-reflectionless/absorptive behavior is realized. The theoretical foundations of the devised lossy-multi-band-BSF-based NGD structures using a coupling-routing-diagram formalism and single-to-multi-band admittance transformations are described. For the first-order case as illustration, guidelines for the synthesis in the bandpass frequency domain are provided. Furthermore, the extension of these multi-band NGD approaches to higher-order and in-series-cascade multi-stage realizations for more-general and wider-band NGD patterning, as well as to two-port/symmetrical designs, is shown. In addition, the conception of multi-functional passive components with NGD characteristics, such as wide-band BPFs and power directional couplers with embedded NGD regions, is also addressed. For experimental-demonstration purposes, an electronically-reconfigurable microstrip prototype of a two-stage-in-series-cascade dual-band NGD circuit is manufactured and measured.

Inverse synthetic aperture radar (ISAR) is a coherent high-resolution radar technique capable of providing range-Doppler images of non-cooperative targets. Conventional ISAR systems just consider a single reflection of transmitted waveforms from targets. Nevertheless, today's new applications force ISAR systems to work in much more complex scenarios such as urban environments. Consequently, multiple-bounce returns are additionally superposed to direct-scatter echoes. We refer to these as ghost images, since they obscure true target image and lead to poor visual quality, making target detection particularly difficult. By applying time reversal concept to ISAR imaging, it is possible to reduce considerably (or even mitigate) ghosting artifacts, recovering the lost quality due to multipath effects. Nevertheless, before applying this innovative technique, it is essential to estimate the distance between radar and target for each transmitted ramp and for each target scatterer. To that end, a pre-processing algorithm based on detecting the prominent points of a conventional ISAR image corrupted by multipath, followed by a windowing process, is used. Both simulated and real data are used to verify the proposed method.

It is widely known that signals with sparse spectrums may be properly acquired when using specific sub-Nyquist sampling frequencies. This alleviates the sampling requirements in advanced acquisition systems, such as the direct-sampling solution to the software-defined radio paradigm. Unfortunately, beyond the theorem of bandpass sampling, analytical expressions for these valid sub-sampling rates are unavailable. Previously, the authors derived the analytical rules which guarantee that the minimum sub-Nyquist sampling frequency avoids aliasing. However, these equations are only valid for the restricted case of evenly-spaced equal-bandwidth multichannel systems, being useless in a broader framework with arbitrary-bandwidth channels. In this work, the general dual-band case is exhaustively analyzed. Given the channel bandwidths, the spectrum parameters which assure that the minimum sub-Nyquist frequency does not give rise to aliasing may be found. For more-than-two channels, some simple guidelines are also provided. Finally, simulation examples are shown to confirm the correctness of the deduced equations.

Inverse synthetic aperture radar (ISAR) is an all-weather radar technique which may generate high-resolution images of noncooperative targets. The standard range-Doppler algorithm (RDA) is usually employed for image generation. However, the images obtained with RDA are usually blurred because of the relative motion between radar and target. As a consequence, motion compensation techniques should be used to improve the imagery quality. The range-instantaneous-Doppler (RID) technique based on time-frequency transforms has been proposed for obtaining a sequence of focused ISAR images without the need of using motion compensation techniques. However, in this letter, it is clearly shown that the migration of the target scatterers in slant range indirectly induces Doppler spreading of the scatterers' point spread function in each of the ISAR images obtained by the RID technique. This Doppler spreading means blurring. It is important to highlight that the migration in slant range may be caused not only by the radial component of the translational motion but also by the rotational motion. The application of motion compensation techniques prior to the use of the RID technique allows us to mitigate the Doppler spreading, as shown here both for simulated and live data acquired by a high-resolution coherent radar.

Traffic surveillance is an important civilian application of radars. The current high-resolution radars give new opportunities so that the traffic application may be redefined. In this paper, a traffic scenario with a high-resolution radar is presented. A range-bin alignment method, the Global Range Alignment, which comes from the focusing techniques in inverse synthetic aperture radar, is applied to obtain further capabilities than the usual velocity measurement: distinction between vehicle types via length estimation and adequate management in situations with simultaneous targets. Preliminary results from a real scenario using a high-resolution linear frequency-modulated continuous-wave millimeter-wave radar are shown.