M.N. Afsar

A novel open-resonator system for precise measurement of permittivity and loss-tangent

An open-resonator system is constructed for the precision dielectric permittivity and loss-tangent measurement of low-absorbing materials at 60 GHz. In this new inexpensive simple system, only one frequency source is used together with a liquid helium-cooled InSb detector for the full cavity-length variation technique. This new simple open resonator system provides data with high measurement accuracy although it only uses one fixed frequency source.

A novel W-band spectrometer for dielectric measurements

A new spectrometer for the precision measurement of dielectric permittivity and loss tangent is presented. The new instrument is capable of providing the high resolution data for the first time over extended W-band (68-118 GHz) frequencies for specimens with a large range of absorption values, including highly absorbing specimens which otherwise would not be possible. A novel technique based on the unbalanced bridge is developed for the measurement of the phase of the wave passed through the specimen in free space (quasi-optical) with reference provided by a waveguide arm. Specially constructed precision waveguide and quasi-optical components allowed reliable broadband operation. A number of common dielectrics are measured and results are compared with previously reported data.

A new 60 GHz open-resonator technique for precision permittivity and loss-tangent measurement

A new open-resonator technique is described for the determination of loss tangent and dielectric permittivity of low-absorbing materials at 60 GHz. Micrometer and the stepping-motor-driven flat mirror in the Fabry-Perot cavity call be scanned at a very small step (to change the cavity length or mirror spacing) to provide a complete interferogram over the entire resonance peak profile. This method is termed here as a new, full cavity-length variation technique. The minimum step size is 20 nm and the variation range of the cavity length is about 25 mm, which can cover eight fundamental resonance modes. With this technique, all resonance curves in the variation range of the cavity can be studied more completely than with any other previous technique. The technique requires only fixed frequency sources, and eliminates the need for a broader band, stable, continually tunable frequency source.

New method for measurement of complex magnetic permeability in the millimeter-wave range, part II: hexaferrites

Magneto-optical methods were applied for the first time in millimeter wavelength range for characterization of anisotropic ferrites. The principles of the free-space magneto-optical method are presented and the new experimental procedures leading to the determination of millimeter-wave permeability and permittivity in anisotropic ferrite materials are described. The measurements were performed with a computer-controlled W-band (70-120 GHz) quasi-optical-waveguide bridge. A backward-wave oscillator was used as a source of tunable millimeter wave radiation. The oriented Sr-hexaferrite ceramic was selected for the verification of millimeter-wave magneto-optical method. The magneto optical measurements in transverse configuration revealed strong anomalous dispersion in the millimeter-wave refractive index spectrum for Sr hexaferrite, mainly due to the frequency variation of magnetic permeability. Computer simulations revealed a good agreement of measured parameters with known data for Sr hexaferrite. It was shown that the free-carrier absorption (/spl sigma//spl sim/0.05 /spl Omega//sup -1/ cm/sup -1/) and magnetic permeability contribute to the relatively high millimeter-wave losses in hexaferrite ceramics. The frequency dependence of Faraday rotation in the millimeter-wave range was measured, and the experimental results are discussed. The results presented in this paper demonstrate that this new magneto-optical method is capable of providing accurate dielectric and magnetic data in the millimeter-wavelength range.

A new open-resonator technique at 60 GHz for permittivity and loss-tangent measurement of low-loss materials

This paper describes a new open resonator system at 60 GHz, in which a new, full cavity-length variation technique is utilized. This new technique provides a 20-nm resolution for the variation of the cavity length. The introduction of this high resolution in the new system allows to measure the permittivity and loss tangent with slightly better than that using the variation technique. Additionally the system becomes much simpler when a complete study of resonance peak profiles are made in the full cavity-length variation method.

Quasi-optical waveguide W-band spectrometer for precision dielectric measurement of absorbing materials

A quasi-optical waveguide bridge spectrometer is designed and constructed for the precision measurement of dielectric permittivity and loss tangent of medium and highly absorbing materials over an extended W-band frequency range. An electronically sweeping backward wave oscillator (BWO) is used as a source of tunable coherent radiation in the frequency range 70-118 GHz. The high output power of BWO (typically 50-100 mW at each frequency) and high sensitivity receiver system employing a liquid helium cooled InSb detector enable the accurate transmission measurement of highly absorbing materials. Data for very absorbing materials such as the low resistivity silicon and germanium are presented and compared.

A waveguide bridge/quasi-optical W-band spectrometer for dielectric measurement of absorbing materials

A spectrometer system is designed and constructed for the precision measurement of dielectric permittivity and loss tangent of absorbing materials. The new instrument is capable of providing the high resolution data for the first time over an extended W-band (68-118 GHz) frequencies for specimens with large range of absorption values, including highly absorbing specimens which otherwise would not be possible. The technique is based on the unbalanced bridge which is developed for the measurement of the phase of the wave passed through the specimen in free space (quasi-optical) with reference provided by a waveguide arm. Precision waveguide and quasi-optical components allowed reliable broad band operation. A number of common dielectrics are measured and data for the real and imaginary parts of dielectric permittivity are presented.

A novel quasi-optical resonator for the surface near-millimeter waves

The theory and experimental investigations are presented for a new type of quasi-optical near-millimeter-wave prismatic resonator, which provides frequency selective transformation of the plane waves into the surface waves. Theoretical expressions are obtained for the Q-factor and finesse, which consider dielectric loss and the effect of prism finite aperture. The theoretical evaluations are verified by experimental measurements in the frequency range 300-500 GHz using a prismatic resonator made from the optical quality fused silica glass. The increase of intensity of surface waves by a factor of 10-25 is demonstrated for the prismatic resonator made from an extremely low-loss dielectric.

A new open resonator method for the measurement of dielectric permittivity and loss tangent of low absorbing materials at 60 GHz

An open resonator system is constructed for the measurement of real and imaginary part of complex dielectric permittivity and the loss tangent of low absorbing materials at 60 GHz. A full cavity length variation technique was employed. The cavity length of the Fabry-Perot hemispherical open resonator was varied in small steps (20 nano meters) to record the resonance mode patterns for a number of modes. It is no longer necessary to have any expensive tunable frequency source as the fine frequency tunability is substituted with the cavity length variation in ultra fine steps. The cavity length variation method has an additional advantage since it provides less noise in the resonance interferogram as a function of the cavity length compared to the equivalent interferogram recorded as a function of fine frequency sweeping. The measurements are made on a number of fused silica and polymeric specimens and dielectric permittivity and loss tangent results are compared with other published results.

A W-band spectrometer for precision dielectric measurements

A new spectrometer for the precision measurement of dielectric permittivity and loss tangent is presented. The new instrument is capable of providing the high resolution data for the first time over an extended W-band (68-118 GHz) frequencies for specimens with large range of absorption values, including highly absorbing specimens which otherwise would not be possible. A novel technique based on the unbalanced bridge is developed for the measurement of the phase of the wave passed through the specimen in free space (quasi-optical) with reference provided by a waveguide arm. Specially constructed precision waveguide and quasi-optical components allowed reliable broad band operation. A number of common dielectrics are measured and results are compared with previously reported data.