In this letter, a wideband transition from microstrip line-to-empty substrate-integrated waveguide (ESIW) without sharp dielectric taper is proposed. A section of ESIW with an expanded width is employed to replace the commonly used dielectric taper. The dielectric loss of the taper is eliminated, and the low reflection is achieved over a broad bandwidth. A design procedure is presented to provide initial dimensions of the transitions for different frequency bands and different substrates. A tolerance analysis shows that the reflection performance is insensitive to the dimensional parameters related to the printed circuit board cutting process. A Ku -band back-to-back transition prototype has been designed and manufactured using RO4350B substrate of 0.508-mm thickness to validate its performance. The measured return loss is better than 21 dB with an insertion loss of 0.83 ± 0.25 dB (0.415 ± 0.125 dB for a single transition) over the entire Ku -band.

Gap waveguide (GW) is a promising technology for its advantages of self-package and easy assembly without the requirement of contact. In this paper, a miniaturized groove GW (GGW) called slow-wave GGW (SW-GGW) is presented for the first time. The slow-wave feature is implemented by introducing symmetrical capacitive quasi-periodic corrugations inside the GGW. To obtain more design and manufacturing flexibility, all the four walls of the SW-GGW are separated utilizing the gap waveguide technology. A six-order iris SW-GGW band-pass filter is designed, whose length is reduced to 57.1% of the conventional iris rectangular waveguide filter with the same response. The irises of the SW-GGW filter are located on the broad-side walls in half-height form. This configuration not only reduces the manufacturing complexity, but also makes it possible to implement filter with different responses by simply replacing the broad-side walls. The measured fractional bandwidth (FBW) of the SWGGW filter is around 3.88% at 11.87 GHz with the return loss better than 18dB and minimum insertion loss of 0.5dB.

In this letter, a microstrip negative-group-delay (NGD) admittance inverter (J-inverter) is proposed to implement a self-equalized linear-phase absorptive bandpass filter (ABPF). It is revealed that this NGD J-inverter, being composed of a coupled-line loaded with a lumped resistor and an open-circuited stub, has an NGD pit in its frequency response. The superposition of the NGD pits associated with a specific number of the NGD J-inverters is utilized to counterbalance the group delay (GD) peaks at the lower and upper edge of the ABPF passband. A three-order ABPF prototype with a passband of 1.5-2.5 GHz is designed using six NGD J-inverters. Measured GD fluctuation within the 3-dB bandwidth is reduced to 0.18 from 0.59 ns compared with the ABPF using the conventional λ /4 J-inverter. The sharpness of the out-band roll-off has been retained. The measured input-output return loss is better than 12 dB over 0.6-3.5 GHz.

In the field of non-contact sensing, both the continuous-wave (CW) Doppler radar and the frequency-modulated continuous-wave (FMCW) radar have their own superiorities. A combination of these two radar modes would be a suitable choice to accommodate various application scenarios. This paper presents a 60-GHz hybrid FMCW-Doppler radar transceiver fabricated in a 65-nm CMOS process. An on-chip frequency quadrupler driven by a 15-GHz external signal source is utilized for the 60-GHz hybrid waveform generation. Based on the hybrid operating modes, a robust I/Q imbalance calibration method is also proposed. Specifically, in the Doppler radar mode, the imbalance factors are extracted by evaluating the quadrature beat signals during the FMCW period, and then the interferometry signals are calibrated to improve the detection performance. Unlike the trajectory-based ellipse-fitting method, the accuracy of the estimated imbalance factor is independent of the target displacement. Compared with the test-signal-based imbalance factors extraction ahead of the target detection, the real-time extraction during the target detection is more efficient and can also mitigate the estimation errors caused by environmental variations. With the proposed hybrid radar system and the I/Q calibration method, an 80-mm linear displacement is recovered with a 0.016-mm root mean square error (RMSE), and a periodic vibration with a 1-μm amplitude is detected with a 16.4-dB signal-to-noise ratio (SNR). Multiple-target detection is also performed in the FMCW radar mode, and the vibration trajectories are recovered accurately.

This paper presents a wideband dual-polarized L-shaped probe-fed stacked patch antenna element and a 16-element phased array antenna prototype for 60 GHz antenna-in-package (AiP) applications. The primary objective is to overcome the fabrication restrictions and achieve high-performance, cost-effective AiP module using an organic multi-layer industry standard HDI process (100/230 μm minimum via/land size). A new L-shaped probe-fed structure, which functions as a monopole antenna, is implemented to enhance the bandwidth and gain performance of the stacked patch antenna. With help of the comprehensive mode analysis method, the polarization coupling mechanism is highlighted, and an isolated patch is introduced, improving the antenna polarization isolation by up to 7dB. To connect RFIC, considering limited fabrication accuracy and the AiP structure features, a broadband vertical transition structure is designed using the half-wavelength impedance transformation technique, and its working mechanism is analyzed theoretically through a distributed equivalent circuit model. Furthermore, impact of variations in key dielectric properties on important AiP characteristics is analyzed, and the process control guidelines are provided. Thermal simulations using a heatsink solution are performed to address thermal issues. For demonstration, an antenna element and a 16-element antenna array prototype are designed, fabricated, and measured. For both polarizations, measurements indicate that the proposed antenna element has a fractional impedance bandwidth (|S11|<-10 dB) of 23.1%, and its polarization isolation is better than 20 dB. Moreover, their peak gain is above 5.45 dBi. For the 16-element antenna array, each element achieves a |S11| below -10 dB from 51 to 64 GHz with 4±1 dBi gains.

A compact scalable gap waveguide (GW) phased array horn antenna with efficient decoupling network (DN) is proposed. An H-plane GW horn antenna with transverse slots-based slow-wave (SW) structures is designed as the array element. By compensating the phase error along the horn aperture, the antenna length is greatly reduced while the gain is enhanced. To suppress the strong mutual coupling between adjacent elements due to the small spacing, an efficient DN is proposed by introducing a through slot inside the intermediate sidewall between adjacent antenna elements. With this DN, an extra wideband equal-magnitude and anti-phase signal can be generated to cancel the coupling signals. Based on the contactless characteristic of the GW, the presented antenna can be extended to large-scale arrays with high reliability. An 8-element phased array antenna operating at 24-30 GHz is implemented for verification. The antenna length is reduced by 62.2% compared with the optimum horn antenna, and the mutual coupling is reduced by up to 32.7 dB with the DN. The measured array gain is higher than 19.8 dBi and the scan angle reaches ±60° with a scan loss of less than 4.5 dB.