In this paper, we present the modeling and experimental validation of an efficient, compact, and high-isolation waveguide duplexer operating at 225 GHz. While duplexers based on ferrite circulators are key components of many monostatic radar systems, at W-band frequencies and above the performance of waveguide circulators rapidly deteriorates. The design presented here uses a ferrite-free duplexing concept based on transmitting and receiving in orthogonal circular polarizations. The circular polarization is efficiently generated in waveguide using a septum orthomode transducer polarizer, leading to a compact device with a single horn antenna. The complete duplexer is designed using efficient numerical algorithms, resulting in a fabricated device with a measured 10% fractional bandwidth, a return loss better than 20 dB, isolation of 30 dB, and insertion loss below 1 dB.

In this letter, several bandpass filters operating in the WR-1.5 band (500 to 750 GHz) are presented. The deep reactive ion etching (DRIE) silicon micromachining process is used for the fabrication of the filters. Two canonical filter topologies based on E- and H-plane are implemented. The work presented here has two specific objectives: a) to get important fabrication process parameters, such as tolerances, vertical angles, surface roughness, and repeatability and b) to validate the proper working of the waveguide filters in the terahertz band. These filters do not have any tuning element. Experimental results show better than 10 dB return loss and approximately 1 and 2.5 dB insertion loss (for 6% fractional bandwidth) for the E- and H-plane topology, respectively. The obtained results are in agreement with fabrication tolerances of 2 μm and vertical angles deviations up to 3°.

An improvement to filter synthesis based on a double cross-coupled network is proposed which involves introducing coupling between the input and output ports, i.e. the source and load of the device. Additional coupling allows the designer to obtain N transmission zeros in the case of N-even order symmetric filters. It is therefore possible to achieve all the transmission zeros that an N-order transfer function allows. These zeros may be located on the imaginary axis (±jw_o) for high selectivity specifications, on the real axis (±σ_o), or in the complex quadrant (CQ) (±σ_o ± jw_o) to satisfy equalisation specifications. Filters of different order have been designed to validate the proposed synthesis technique.

Orthomode transducers (OMTs) using folded six-port waveguide junctions are presented for obtaining the maximum compactness in antenna feeds with very wide performance. The proposed junctions are based on the well-known turnstile and Bøifot configurations, having two symmetry planes for keeping the isolation between orthogonal polarizations and the control of higher order modes. The folded arms provide a combined effect: good matching with very significant size reduction, especially in the transversal plane, reducing mass in satellite systems and allowing feeding eventual dense horn antenna arrays. Two Ku-band OMTs are presented in order to illustrate the advantages of the introduced junctions. The first design covers the band 10.4-18.8 GHz (57.5%) with 24 dB return loss. The second OMT, for the 12.60-18.25 GHz band, has measured return loss better than 29 dB in the design band and insertion loss smaller than 0.11 dB for both polarizations.

The design, method of fabrication and experimental validation of a pseudo-elliptical four-poles asymmetric response filter at 100 GHz with 10% of fractional bandwidth is presented. The designed filter is part of an image radar system and must fulfill stringent specifications. The proposed topology and method of fabrication improves the out of band rejection above the pass band in comparison with other classical structures, while keeping a low insertion loss level. A two-fold geometry for implementing a transmission zero is proposed, avoiding the excitation of the first higher order mode and easing the manufacturing without any tuning element. A low-cost milling process is used for the fabrication of the device. The final design fulfills the desired specifications and presents 0.6 dB of insertion loss level at 100 GHz. The excellent agreement between simulations and measurements at this frequency band in both, return loss and insertion loss, is pointed out.