Erwan Fourn

In this article, we systematically reviewed the theory of designing multiband filters based on frequency transformation methods and the design methods based on planar circuits. Firstly, we reviewed the general methods of designing filters based on planar circuits and introduced the concept of designing multiband filters using frequency transformation methods. Secondly, we discussed the mathematical expression of frequency transformation method in depth and provided rigorous mathematical proofs. Furthermore, we presented the topology and design circuit examples of filters based on lumped parameters and coupling matrix synthesis. Finally, we compared the advantages and disadvantages of frequency transformation method and summarized the entire article.

A dual-band slotted waveguide array (SWA) antenna is designed using a single waveguide by placing the radiating slots for 18 GHz and 24 GHz on the same antenna aperture. These bands are excited using a single excitation point through a conventional waveguide feeding technique to radiate unidirectional radiation patterns of same polarization. Theoretical analyses are carried out to study the conductance of the slots to understand their effects on the dual-band performance. Numerical analyses are performed to optimize the antenna by considering mutual coupling. The antenna is fabricated using stereolithography (SLA) and direct metal laser sintering (DMLS) processes to compare its performance in different additive manufacturing techniques. In characterizing the fabricated prototype, an impedance matching of | S ₁₁ | < −10 dB and the boresight antenna gains of 14 dBi and 17.4 dBi are measured in these bands with unidirectional and linearly polarized radiation patterns. A quantitative comparison among dual-band SWA antennas is presented at the end of the manuscript to highlight the uniqueness of the proposed approach over the state-of-the-art solutions.

This article presents a similarity transformation method for multiband bandpass filters (MBPFs) to convert the star-like topology into the in-line topology. A general theoretical technique is introduced, which replaces the traditional step-by-step extraction of $LC$ circuit through filter synthesis with the similarity transformation rotation of coupling matrix, addresses the round-off errors in the parameter extraction process, and improves the accuracy of the theoretical synthesis results. The application of in-line topology significantly enhances the flexibility of filter design, reduces circuit complexity, and simplifies the fabrication of high-order MBPFs. Based on the substrate integrated waveguide (SIW) technology, a series of examples, including tri-band, quad-band, and particularly the first reported quint-band third-order Chebyshev SIW bandpass filters, is designed and implemented. Good agreement is achieved between simulated responses and measured results, validating the designed filter models and the proposed theoretical method.

This article presents a new generalized direct synthesis method to design multiband bandpass filters (MBPFs). Based on the proposed frequency transformation technique, it can be used to synthesize an arbitrary number of bandpass filters starting from a low-pass prototype filter. This synthesis allows determining analytically all the resonant angular frequencies and slope parameters of all the bandpass filters from the low and high cut-off angular frequencies used as initial specifications whatever the number of bands. To validate our method, triple-, quad-, and quint-band third-order Chebyshev bandpass filters are designed and implemented in microstrip technologies. Very good agreements were achieved between simulation responses and measurements.

We present here a design of metal-only unit-cell for transmitarray antenna (TA) applications at Ka-band, and we introduce a solution to reduce the total number of layers by polarization manipulation. The corresponding unit-cell is based on a C-shaped slot radiating element, fabricated by laser ablation on a thin metal sheet and operating in linear polarization. Thanks to the rotation of the slot, polarization of radiated wave can be manipulated to any arbitrary direction, particularly in the orthogonal direction of the incident wave. By combining two unit-cells with opposite rotation directions, we show that metal-only unit-cells made of three layers can be obtained, with a 360° phase shift and insertion loss lower than 1 dB. This unit-cell with reduced profile is used to design several TA prototypes with a main beam direction pointing up to 50°. The experimental results demonstrate an 11% 3-dB fractional gain bandwidth with an aperture efficiency around 50% for a beam at boresight.

Dual-band transmitarray antennas are demonstrated at Ka-band with the capability of forming independent linearly polarized beams with a given polarization in each frequency band, while sharing the same radiating aperture. The proposed three-layer unit-cell is based on identical narrow microstrip patches printed on both receiving and transmitting layers and connected by a metallized via hole. The metal layers are printed on two identical substrates bonded with a thin film, and the designed unit-cell exhibits a 180° phase resolution (i.e., 1-b phase quantization). The dual-band dual-polarized property of the transmitarray is achieved by interleaving unit-cells operating in the down-link and up-link frequency bands. Four different prototypes are characterized to demonstrate the relevance of the proposed concepts. A good agreement is obtained between the radiation patterns, gain curves, and cross-polarization levels measured and computed in both frequency bands and polarizations.

This paper presents a method for synthesizing the equivalent circuit of a single-layer reflect array unit-cell based on filter synthesis techniques. Metallic-ring and slot-ring cells are synthesized respectively by 3rd-order band-stop and band-pass Tchebycheff filters. A phase range of 360° at 12.5 GHz is achieved with an maximum average phase error of 16° for a phase dispersion of 60°/GHz in the band of interest 10.5-14.5 GHz.