Mohammed N. Afsar

Single and dual band 77/95/110 GHz metamaterial absorbers on flexible polyimide substrate

Ultra thin millimeter-wave absorbers on flexible polyimide substrate utilizing metamaterials are implemented for single and dual frequency bands in an emerging frequency spectrum of 77, 95, and 110 GHz. The dual band absorber is designed using a novel approach of imbedding high frequency resonator inside low frequency resonator capable of absorbing electromagnetic energy at both 77 and 110 GHz bands simultaneously. The total thickness of the absorber is just 126 μm (almost 1/20th of the wavelength). Measured peak absorptions for single frequency absorbers are 92, 94, and 99% at 77.2, 94.8, and 109.5 GHz, respectively, and for dual band absorber 92% at 77 GHz and 94% at 109.8 GHz.

Millimeter-wave magnetooptics: New method for characterization of ferrites in the millimeter-wave range

This paper presents the application of the magneto-optical method for the first time in the millimeter wave range for the characterization of ferrites. The magneto-optical measurements were realized with a specially built millimeter wave spectrometer. Backward wave oscillators were used as a source of tunable coherent radiation. It is demonstrated that this new method is capable of generating accurate millimeter wave dielectric and magnetic data for any type of ferrite material.

Measurement of Complex Permittivity of Liquids Using Waveguide Techniques

Complex permittivity of a number of liquids and binary mixtures has been studied by measurement using the waveguide techniques at the X and Ku band. Particular pieces of WR90 and WR62 waveguides were designed for the measurement of liquid materials. The custom designed TRL calibration kits are applied for calibration of the waveguide system. The measured results of complex permittivity of liquid dielectrics, such as methanol, propyl alcohol, ethyl alcohol, chlorobenzene, dioxane, cyclohexane and binary mixtures, are presented. Particular pieces of open-ended waveguides for the X and Ku bands were also designed for holding liquids and the measured data using the open-ended waveguide technique were compared with those measured using the waveguide technique. Some of the measured results are also compared with calculated data using the Debye equation and published data measured by the Fourier transform spectroscopy.

An automated 60 GHz open resonator system for precision dielectric measurements

An automated open resonator system designed and constructed for precision measurement of loss tangent and dielectric permittivity of low absorbing materials at 60 GHz is reported. The use of a high-Q hemispherical Fabry-Perot cavity together with highly stabilized synthesized phase-locked Gunn oscillator sources and the superheterodyne receiver enabled a loss tangent value as low as 10 mu rad to be measured. The system is automated by means of a precision lock-in amplifier, a V-band Hewlett-Packard spectrum analyzer and a Hewlett-Packard Vectra computer system with analog-to-digital conversion accessories. The synthesizer allows the collection of data at very small steps over the complete Gaussian beam, and, together with a statistical fitting, the Q determination can be made very accurately. >

MEASUREMENT OF COMPLEX PERMITTIVITY OF LIQUIDS USING WAVEGUIDE TECHNIQUES

Complex permittivity of a number of liquids and binary mixtures has been studied by measurement using the waveguide techniques at the X and Ku band. Particular pieces of WR90 and WR62 waveguides were designed for the measurement of liquid materials. The custom designed TRL calibration kits are applied for calibration of the waveguide system. The measured results of complex permittivity of liquid dielectrics, such as methanol, propyl alcohol, ethyl alcohol, chlorobenzene, dioxane, cyclohexane and binary mixtures, are presented. Particular pieces of open-ended waveguides for the X and Ku bands were also designed for holding liquids and the measured data using the open-ended waveguide technique were compared with those measured using the waveguide technique. Some of the measured results are also compared with calculated data using the Debye equation and published data measured by the Fourier transform spectroscopy.

Complex permittivity and permeability of barium and strontium ferrite powders in X, KU, and K-band frequency ranges

This paper presents accurate results for the complex permittivity, e=e′−je″ and permeability, μ=μ′−jμ″ of barium ferrite powder (BaFe12O19) and strontium ferrite powder (SrFe12O19), in the frequency range from 8.0to26.5GHz. The complex permittivity and permeability are determined via the waveguide transmission/reflection (TR) technique and the waveguide cavity resonator (CR) technique at 25°C and relative humidity <75%. Measurements reveal that the real permittivities of BaFe12O19 and SrFe12O19 are, respectively, 2.497<eBa′<2.678, and 2.597<eSr′<2.712. BaFe12O19 has an average real permeability μBa′=1.078 and SrFe12O19 has an average real permeability μSr′=1.063. The imaginary permittivities are respectively eBa″<0.081 and eSr″<0.077. The imaginary permeabilites are respectively μBa″<0.095 and μSr″<0.106.

The millimeter wave magneto-optics: new method for characterization of ferrites in the millimeter wave range

This paper presents the application of the magneto-optical method for the first time in the millimeter wave range for the characterization of ferrites. The magneto-optical measurements were realized with a specially built millimeter wave spectrometer. Backward wave oscillators were used as a source of tunable coherent radiation. It is demonstrated that this new method is capable of generating accurate millimeter wave dielectric and magnetic data for any type of ferrite material.

Complex permittivity and permeability of strontium ferrites at millimeter waves

Magneto-optical approach for the measurements of complex dielectric permittivity and magnetic permeability of solid and powdered strontium ferrite materials has been performed in the Q-, V-, and W-band frequency ranges. Free-space quasioptical millimeter-wave spectrometer equipped with a backward-wave oscillator as a tunable source of coherent radiation provides the transmittance spectra in transverse magnetic field up to 7.5kOe. Frequency dependences of dielectric and magnetic parameters of strontium ferrites have been calculated by matching theoretical curves to the experimental transmittance spectra. Shifting of the ferromagnetic resonance to higher frequencies and broadening of the zone of strong absorption in magnetic field for solid ferrite materials have been observed.

Analysis and measurement of a broadband spiral antenna

A broadband spiral antenna (2 to 18 GHz) is modeled using the FDTD method. The inner turn of the antenna is an Archimedean spiral, while the outer turn is an Archimedean spiral with a zigzag shape. The current-density distribution along the spiral arms is first presented in the time domain, interestingly manifesting the impulse current propagation from the feed point to the outer end of the spiral arms. The performance of the broadband spiral antenna is also measured in an anechoic chamber. The simulated and measured results showed the sensitivity of the axial ratio and radiation pattern to the current distribution. The simulated linear gain, radiation patterns, and axial ratios agree well with the measured data over all of the frequency range from 2 to 18 GHz.

Cavity-Perturbation Measurement of Complex Permittivity and Permeability of Common Ferrimagnetics in Microwave-Frequency Range

This paper investigates complex permittivity and permeability of several commercially available yttrium iron garnet (YIG) and nickel-ferrite ceramics in the demagnetized state. The measurements were realized by using the rectangular cavity resonators in the frequency range 4.5-26.5 GHz