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This letter presents a wideband sequential-rotated (SR) circularly polarized (CP) 2 × 2 filtenna array with multiple radiation nulls for high out-of-band rejection level. A pair of arc-shaped via walls is introduced to split the degenerate modes TE120/TE210 of the substrate integrated waveguide (SIW) cavity for achieving CP characteristics. SIW cavity modes TE110 and TE220 are generated at the lower and upper band edges, respectively, to form a bandpass filtering response. A CP patch is embedded to enhance the AR bandwidth and create an additional radiation null near the upper band edge. Four filtering slots are further introduced around the radiation aperture to create another radiation null in the upper stopband. This compact filtenna element can achieve high stopband rejection without external filtering circuits. A SR feed network is designed for 2 × 2 array implementation with enhanced gain and AR bandwidth. By integrating a simple filtering structure in the SR feed network, the out-of-band rejection level in the lower stopband can be significantly improved. For demonstration, a 2 × 2 SR filtenna array was fabricated and tested. Good agreements between the measured and simulated results are observed, with a wide AR bandwidth of 18% (centered at 10 GHz), a peak gain of 13.2 dBic, and out-of-band rejection levels of 33 dB and 23 dB in the lower and upper stopbands, respectively. [ABSTRACT FROM AUTHOR]

In this paper, two 24 GHz horizontally polarized $1 \times 8$ patch antenna arrays were developed for automotive radar applications. The proposed antenna arrays offer the advantages of wide fan beams and high gain. The far-field radiation patterns are widened in the E-plane by disturbing the near-field distribution of each driven patch. In Design I, the driven patch is loaded with a parasitic loop, which functions as a director in the E-plane. Due to the directing effect, the E-plane beamwidth can be increased. The experimental verification prepared for Design I showed that a beamwidth of 130° and a gain of 12.2 dBi can be achieved through this method. To further enhance the beamwidth, another array with each patch loaded with parasitic mushroom-like elements was proposed by Design II. The PMLEs are 180° out of phase to their corresponding driven patch. The cancelation effects can be properly managed to slightly sacrifice the gain of the array in return for a wider beamwidth in the E-plane. Design II exhibits a wider beamwidth of 150° and a lower gain of 11.1 dBi. [ABSTRACT FROM AUTHOR]

A millimeter-wave series–parallel patch antenna array is presented, in which the dual-slot feeding structure is handily implemented using the intrinsic field distribution of TE20 mode in substrate-integrated waveguide (SIW). One 28 GHz patch antenna element fed by the TE20-mode SIW is first designed, achieving a 10 dB impedance bandwidth of 10.2% and a simulated peak gain of 6.48 dBi. Based on the antenna element, a $4 \times 4$ array with a compact series–parallel differential feeding network is developed accordingly. Due to the novel compact SIW-based series–parallel feeding network, the antenna array can achieve superior radiation performances, which is the highlight of this communication. The simulation and measurement results of the proposed antenna array are in good agreement, demonstrating a performance of 8.5% impedance bandwidth, 19.1 dBi peak gain, symmetrical radiation patterns, and low cross-polarization levels (−30 dB in E-plane and −25 dB in H-plane) in the operating frequency band of 26.65–29.14 GHz. [ABSTRACT FROM AUTHOR]

In this paper, we developed three designs of dual-band coupled-line couplers with wide separation between bands. Coupler A was constructed using three cascaded coupled-line sections with a band ratio fH/fL of up to 7. Coupler B, with a stub-tapped coupled line, was developed for compactness and has advantages of high band ratio and controllable bandwidth. Couplers A and B are most suited for weak coupling applications. Coupler C was developed for achieving strong coupling while maintaining a high band ratio, and it was constructed using a coupled-line section connected by two uncoupled lines at its two ends. Coupler C has advantages of high band ratio, strong forward-wave coupling, compact size, and easy fabrication. Coupler A, operating at 0.84 and 5.17 GHz with fH/fL = 6.15 / and a coupling of −10 dB, and Couplers B and C, operating at 0.7 and 2.6 GHz with fH/fL = 3.71 and couplings of −11 and −3 dB, respectively, were fabricated for demonstration. The measured results validate their superior dual-band performance. [ABSTRACT FROM PUBLISHER]

This paper presents a 60-GHz $4 \times 4$ differential -fed patch antenna array using low-temperature cofired ceramic (LTCC) process. Wideband patch with L-shaped feeding scheme is adopted as antenna element, while differential substrate integrated waveguide feeding network with low insertion loss is applied for the integration of antenna array. The differential-fed structure improves the symmetry of radiation patterns and reduces the cross-polarization level significantly. To further suppress the surface wave and improve the gain of the antenna array, one kind of soft surface structure is proposed. The equivalent circuit model of the soft surface is developed for calculating its dispersion diagram and analyzing stopband characteristics. The simulated results indicate that the antenna gain enhancement can be up to 2 dB because of the proposed soft surface. For demonstration, one prototype using LTCC process is fabricated and measured. The measured 10-dB impedance bandwidth of the antenna array is 11.7%. The measured antenna peak gain of 18.62 dBi at 61.5 GHz and symmetrical radiation patterns with a low cross polarization of −25 dB across the whole operating frequency are achieved. [ABSTRACT FROM AUTHOR]

In this paper, compact multilayered helical resonators and various coupled resonator pairs with a mixed electric and magnetic coupling structure were developed. A multilayer structure is suitable for miniaturization and enhances mutual coupling. A pair of transmission zeros (TZs) can be created using mixed coupling between two coupled helical resonators, achieving superior frequency selectivity. An equivalent circuit model was proposed to analyze the creation of TZs. The planar helical structure provides design flexibility in control of TZs. Eight coupled resonator pairs were proposed for synthesizing various filter responses. An experimental second-order helical filter was fabricated using low-temperature cofired ceramic technology. The measurement results of this second-order filter showed the center frequency to be 0.976 GHz, with two TZs located at 0.8 and 1.4 GHz. The insertion loss was lower than 1.96 dB, and the return loss was 25 dB at f0 . The size of this filter was only 5.7~\textrm mm \times 3.4 mm ( 0.051~\lambda g\times 0.031~\lambda g ), which is highly compact compared with filters from other published studies. In addition, the proposed coupled pairs have potential in the construction of high-order filters with superior frequency selectivity by cascading multiple coupled helical pairs. [ABSTRACT FROM AUTHOR]

This paper presents a cross-coupled bandpass filter with stacked substrate-integrated waveguide cavities on low-temperature cofired ceramic substrates. The proposed filter has a local multipoint distribution service band with a novel same-side-feed input/output structure. Conventionally, a cross-coupled structure generates only a single pair of transmission zeros. The proposed filter can generate two pairs of transmission zeros beside the passband, thereby providing an excellent cutoff rate in the stopband and improved frequency selectivity. The additional pair of transmission zeros is created by the same-side-feed structure, which constructs an additional source–load coupling path without increasing the circuit size. A multipath coupling diagram is used to illustrate the conformation of the second pair of transmission zeros and predict its behavior. The experimental filter exhibits responses centered at 27.95 GHz with an insertion loss of -2.8~dB, and a bandwidth of 9%. Two pairs of transmission zeros (at 26.3 and 29.6 GHz, and at 23.2 and 37 GHz) around the passband were obtained, achieving excellent selectivity and a wide stopband. [ABSTRACT FROM AUTHOR]

This paper presents a wideband 60-GHz antenna array with a dual-resonant slot-patch structure. A multilayered low-temperature co-fired ceramic substrate was used for antenna fabrication. A half-wavelength resonant slot was designed with a backed substrate-integrated waveguide (SIW) cavity to enhance the front radiation. The inverted microstrip center-fed structure was designed for easy signal excitation and superior impedance matching. A parasitic patch was also applied to enhance for bandwidth and gain enhancement. The effects of the SIW cavity-backed slot antenna with and without parasitic patches were empirically examined. The simulated results show that adding parasitic patches increased the resonance of the poles and improved antenna gain by 1.85 dB and bandwidth by 9%. A 2\,\times\,2 dual-resonant slot-patch antenna array was designed to further enhance the gain and bandwidth. The equipment setup for on-chip measurements of gain and radiation patterns was established. The measured S11 showed a wide bandwidth of 23%. The measured gain for the four-element antenna array was 9 dBi with slight fluctuations over the 57–64-GHz frequency range. [ABSTRACT FROM AUTHOR]

This work presents a miniaturized balun consisting of two re-entrant coupled line sections with \sim\lambda/8 length connected by a redundant line with \sim 0.04\lambda length fabricated by planar PCB technology. The structure of the balun was analyzed to obtain the appropriate lengths of couplers and redundant line for the balun realization. An experimental balun operated at S-band was designed and fabricated to verify the proposed structure of balun. The prototype demonstrates a miniaturized dimension of \sim 0.29\lambda\times 0.05\lambda and less than 0.36 dB amplitude imbalance associated with 1.6^\circ phase difference in the frequency range of 2.09 \sim 3.06 GHz (37.7%). The return loss in the operation band is less than -10 dB. The length of the prototype is almost shrunk to nearly half of the traditional balun. [ABSTRACT FROM PUBLISHER]