Hua Chi

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. >

Millimeter wave complex refractive index, complex dielectric permittivity and loss tangent of extra high purity and compensated silicon

Single crystal high resistivity (11,000 ohm-cm) boron doped silicon was found to exhibit lowest absorption loss at room temperature (25 C) in the entire millimeter wave region. At 140 GHz its loss tangent value is as low as 40 microradians. The study of dielectric properties of silicon as a function of resistivity reveals that the low frequency free carrier absorption present in all silicon (and other semiconductors) vanishes with increasing resistivity. It is then possible to use such a silicon in substrate applications in microwave integrated circuitry. The unique broadband dispersive Fourier transform spectroscopic technique was utilized for these measurement.

Millimeter wave complex refractive index, complex dielectric permittivity and loss tangent of high purity and compensated silicon

Single-crystal high-resistivity (11000- Omega -cm) boron-doped silicon was found to exhibit the lowest absorption loss at room temperature (25 degrees C) in the entire millimeter-wave region. The millimeter-wave absorption coefficient values for the compensated silicon are at least one order of magnitude less than values obtained with undoped pure silicon. At 140 GHz, the loss tangent value of the compensated silicon is as low as 40 mu rad. The study of the dielectric properties of silicon as a function of resistivity reveals that the low-frequency free-carrier absorption present in all silicon (and other semiconductors) vanishes with increasing resistivity. The dispersive Fourier transform spectroscopic technique was utilized for the measurements.<<ETX>>

Window materials for high power gyrotron

The room temperature application of sapphire as window material at higher frequencies is not feasible since its absorption coefficient increases almost linearly with increasing frequency in the millimeter wavelength region. At cryogenic temperature the absorption coefficient value decreases only by a few factors (factor of 2 to 3) in the 90 – 200 GHz region. The earlier reported temperature squared dependence (decrease) in the absorption coefficient or the loss tangent value is totally absent in our broad band continuous wave data we are reporting here (at 6.5 K, 35K, 77K and 300K) and one we reported at conferences earlier. Our results are verified by another technique. We utilize our precision millimeter wave dispersive Fourier transform spectroscopic techniques at room temperature and at cryogenic temperatures The extra high resistivity single crystal compensated silicon is no doubt the lowest loss material available at room temperature in the entire millimeter wavelength region At higher millimeter wave frequencies an extra high resistivity silicon window or an window made with extra high resistivity silicon coated with diamond film would certainly make a better candidate in the future. A single free standing synthetic diamond window seems to have higher absorption coefficient values at millimeter wavelength region at this time although it is claimed that it possesses good mechanical strength and higher thermal conductivity characteristics. It certainly does not rule out the use of diamond film on a single crystal high resistivity silicon to improve its mechanical strength and thermal conductivity

An automated 60 GHz open resonator system for precision dielectric measurement

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.<<ETX>>

A 60 GHz Open Resonator System for Dielectric Measurement

A precision automated open resonator system is designed and constructed for the measurement of loss tangent and dielectric permittivity of low absorbing materials at 60 GHz. The high Q hemispherical Fabry-Perot cavity together with highly stabilized synthesized phase locked Gunn oscillator sources and the super heterodyne receiver enable us to measure loss tangent value as low as 10 micro-radians.

Millimeter- and submillimeter-wave transmission and dielectric properties of radome materials

Transmission, absorption coefficient, refractive index, real and imaginary parts of dielectric permittivity and loss tangent spectra of polytetrafluorethylene (teflon), Gore-Tex and another teflon based radome memebrane materials are presented as a continuous function of frequency over the range 60-2,400 GHz (5 mm-0.125 mm in wavelength). The new data are better resolved and cover a much wider frequency range. The transmission reduces significantly with increasing frequency for most radomes. The absorption coefficient or loss tangent values increase almost exponentially with increasing frequency for all these radome materials. The refractive index or the real part of permittivity values are alomst similar to teflon since these radomes are teflon based.

High-resolution absorption coefficient and refractive index spectra of carbon monoxide gas at millimeter and submillimeter wavelength region

The use of dispersive Fourier transform techniques in interferometry allows the measurement of absorption coefficient and refractive index spectra with great precision. This paper presents the absorption coefficient and refractive index spectra of carbon monoxide gas at millimeter and submillimeter wavelengths. In order to assess rotational lines of carbon monoxide, the gas was measured at four different pressures, giving insight into the behavior of the spectral lines with varying parameters. Our measurements demonstrate that varying the pressure of the gas affects only the amplitude of the absorption lines and not their exact position. This is critical in air pollution studies when trying to single out a specific gas from a field sample with unknown constituents

Observation of a doublet ferromagnetic resonance in cubic and hexagonal ferrites at millimeter wavelengths

Ferromagnetic resonance from two individual magnetic sublattices of ferromagnetic materials has been observed in the laboratory. Separate g-factor (gyromagnetic ratio) values were measured from the resonance experiment. A high signal-to-noise-ratio, millimeter-wave, broadband Fourier transform spectrometer and a high-intensity-field (140000-G) DC magnet were utilized for this measurement.<<ETX>>

Window materials for 110-GHz and 280-GHz gyrotron

At higher frequencies it is not easy to find a suitable material for output window applications in a high power gyrotron. The room temperature application of sapphire as a window material at higher frequencies is not feasible since its absorption coefficient increases almost linearly with increasing frequency in the millimeter wavelength region. At cryogenic temperature the absorption coefficient value decreases only by a few factors (factor of 2 to 3) in the 90-200 GHz region. The earlier reported temperature squared dependence (decrease) in the absorption coefficient of the loss tangent value is totally absent in our broad band continuous wave data we are reporting here (at 6.5 K, 35 K, 77 K, and 300 K). We utilize our precision millimeter wave dispersive Fourier transform spectroscopic techniques at room temperature and at cryogenic temperatures. The extra high resistivity single crystal compensated silicon is no doubt the lowest loss material available at room temperature in the entire millimeter wavelength region. At higher millimeter wave frequencies an extra high resistivity silicon window or a window made with extra high resistivity silicon coated with diamond film would certainly make a better candidate in the future. A single free standing synthetic diamond window seems to have higher absorption coefficient values at millimeter wavelength region at this time although it is claimed that it possesses good mechanical strength and higher thermal conductivity characteristics. It certainly does not rule out the use of diamond film on a single crystal high resistivity silicon to improve its mechanical strength and thermal conductivity.