Chebyshev filter. It contains a Butterworth-style (or maximally flat) pass band, a moderate group delay, and an equiripple stop band. The bandstop prototype is characterised by the equivalent network shown in Figure 4(a). To apply the proposed EBG structure to the filter design, we use admittance inverters ( J inverters) to transform the series connected parallel resonators into the shuntconnected series LC resonator, as illustrated in Figure 4(b). The filter depicted in Figure 5 comprises the proposed EBG cells acting as resonators. The J inverters were implemented by transmission lines with a quarter wavelength at the cut-off frequency f c .The filter was designed at f c ϭ 2.1 GHz on a two-layer Roger4003 substrate with relative permittivity r ϭ 3.38 and heights h 1 ϭ h 2 ϭ 32 mil. The LC element values selected in the prototype are L 01 ϭ 6.78 nH, C 01 ϭ 0.78 pF, L 02 ϭ 7.39 nH, C 02 ϭ 0.83 pF, L 03 ϭ 8.15 nH, and C 03 ϭ 0.85 pF. The filter is fabricated on a 52 mm ϫ 20 mm ϫ 64 mil board. The actual patch dimensions and positions of vias in the three cells are listed in Table I. The distances between the centers of adjacent EBG cells are 20 mm, which corresponds to a quarter wavelength. Figure 6 shows the simulated and measured frequency response of the notch filter. It is evident that this filter can provide a wide stopband. The filter performances are center frequency: 2.1 GHz, 20-dB stopband: 1.93-2.28 GHz (17%), and minimum insertion loss in stopband: 50 dB. CONCLUSIONWe have proposed an EBG structure for microstrip applications in the S band. This proposed structure utilises a resonator coupled to the microstrip to provide a distinguished bandgap property. A lumped LC-equivalent circuit model has been established to approximate its operation. Based on the simulation and equivalent circuit model, a wideband notch filter has been theoretically and experimentally developed. The numerical and experimental results agree very well. Furthermore, this two-layer design features a smaller size and the capability of preventing the other integrated parts from unwanted radiation arising from the slots etched onto the ground plane of the planar back-defected bandgap structures. This study has demonstrated that the proposed structure with its equivalent circuit model is capable of providing a new way to design more efficient circuits for microwave and RF applications, such as filter, phase shifter, and other passive circuits. CORRECT ANALYSIS OF GIRES-TOURNOIS ABSORBING
Due to small wavelengths, the bandwidths of short millimeter and submillimeter waves substantially differ from both UHF and optical bandwidths. In structures of this bandwidth, application of waveguide technologies that are traditional for UHF is complicated due to small dimensions of both waveguides and structural elements. These difficulties led to the application of optical methods in combination with elements of UHF devices and resulted in creation of so-called quasi-optical devices.This article offers a model of quasi-optical detector section constructed and tested in 2.1-2.4 mm bandwidth. The detector section represents a 2-mirror elliptical resonator of open structure [l]. The lower elliptical mirror is asymmetric and enables feed of in-focus wave beam to the point of resonator's lower focus. Due to ellipse's property, after several interreflections the beam is constricted towards the ellipsoid axis. A sonde with a diode is installed on the axis of the elliptical resonator. The sonde represents a rigid coaxial structure with a diode of "cellular" structure, i.e. multitude of diodes on a single substrate, soldered to its central conductor. Upon an appropriate selection of resonator size, stationary wave is formed on the axis. By shifting of the sonde along the resonator axis, the sonde is fixed in wave loop, thus providing conditions for maximum energy take-off.
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