Reflectance Measurement of Lossy Metal Meshes in the Millimeter-Wave Range
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Reflectance Measurement of Lossy Metal Meshes in the Millimeter-Wave Range Boris Kapilevich
Received: 26 March 2008 / Accepted: 10 June 2008 / Published online: 28 June 2008 # Springer Science + Business Media, LLC 2008
Abstract The method of power reflectance measurement of small lossy metal meshes is suggested by evaluating the measured reflection coefficient in the vicinity of a resonance mode of quasi-optical resonator. The resonator is described by the equivalent circuit that includes coupling ohmic losses of the mesh. The multi-points reconstructing algorithm is suggested to restore power reflectance. This particular technique is suitable for reflectance measurements of lossy metal meshes and wire grids having overall small dimensions which are not relevant when employing standard free space techniques. Its validity is illustrated by reflectance measurements in W-band. Keywords Lossy circuits . Millimeter wave measurements . Reflection . Quasi-optical resonators
1 Introduction Metal meshes, perforated plates and wire grids are widely used as elements of quasi optical devices such as polarizing filters, deflectors, quasi optical gratings, semi-transparent mirrors, different coupling elements etc. [1–5]. Recently, the interest in such structures has been stimulated by their potential applications associated with metamaterials [6]. The most important parameter of such structures is reflectance and transmittance characteristics. Basically, direct free space measurement techniques using reflection or transmission modes are employed [7] to determine these parameters of a metal mesh. However, free space techniques require mesh samples with overall dimensions much larger in comparison with a wavelength. Thus, it can not be applied for small meshes employed in some quasi optical devices. Indeed, the situation is becoming more dramatic when the metal mesh is used as the mirror of a quasi-optical resonator being the RF part of mm-wave Free Electron Laser (FEL) [8]. In order to provide interaction of the electron beam with resonating mode, the resonator is installed inside the wiggler with intermittently placed dc-magnets as shown in B. Kapilevich (*) The Ariel University Center of Samaria, Ariel, Israel e-mail: [email protected]
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Int J Infrared Milli Waves (2008) 29:871–878
Fig. 1a. Since the wiggler’s length is less than the length of the resonator, active interaction with beam takes place on the part of the resonator – about 70% of the total length in our case. The input mirror and output mesh coupler forms mm-wave Fabry-Perot resonator. Accidentally, the output mesh can be easily damaged by the electron beam if it is slightly declined from the axis of the resonator. The example of such damaged mesh extracted from the FEL after completing its operation is shown in Fig. 1b. Burned and damaged surface of the mesh introduce additional ohmic losses and leads to decreasing reflectance resulting in considerable reduction of the generated RF power. Since the dimensions of the mesh are quite small, the conventional
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