Reflectivity of absorbers in 100-200 GHz range

Different types (pyramidal, convoluted, wedge, flat) of absorber panel made by several manufacturers have been measured at 107 and 183 GHz. These measurements revealed a rapid degradation of the quality of absorbers from 107 to 183 GHz: reflectivity increased nearly 10 dB. The best absorbers have reflectivities below -40 dB at 107 GHz and about -35 dB at 183 GHz.<>     

Reflectivity measurements in anechoic chambers in the microwave to millimeter range

A voltage standing-wave ratio (VSWR) measurement method and ensuing reflectivity characteristics are presented for anechoic chambers in the range 1-76.5 GHz. The free space VSWR measurements are performed using a composite antenna technique. By making use of two different anechoic chambers, the dependency of the reflectivity characteristics upon direction in the horizontal plane and absorber's height is analyzed.     

Broad-Band Bias-Current-Tuned IMPATT Oscillator for 100-200 GHz

Broad-band bias-current-tuned IMPATT oscillators rising harmonic oscillations have been realized for the short-millimeter wave-length region (100-300 GHz). The relationship between diode and wave-guide parameters (breakdown voltage, junction diameter, and waveguide cutoff frequency) to obtain broad-band tunable oscillations is investigated theoretically and experimentally. Consequently, a tuning bandwidth of 35 GHz is obtained with IMPATT oscillators in the 160-GHz band, and 30 GHz in the 200-GHz band.     

High-power generation in IMPATT devices in the 100-200-GHz range

A silicon double-drift IMPATT diode with high uniform doping levels was simulated. Simulation results show that it is possible for silicon IMPATT diodes to generate extremely high pulsed output power for frequencies above 100 GHz under high current-density operation. The highest output power matched to a 1-Ω load resistance obtained at 150 GHz is 37.7 W with a DC current density of 200 kA/cm², although the calculated power conversion efficiency is low. It is also shown that the low-power conversion efficiency limits the diode's continuous wave power operation     

Reflectivity level of radio anechoic chambers

A comparison between the antenna-pattern comparison technique and the free-space voltage standing-wave ratio technique for evaluating the reflectivity level of radio anechoic chambers is presented. Based on an analysis of the two techniques, it is pointed out which parameters influence the measured value of the reflectivity level. The comparison is illustrated with experimental results and it is explained why inconsistent and uncorrelated results may be found when the two methods are used. Furthermore, it is demonstrated, by introducing improvements in a chamber, how the reflectivity level can be used to measure the improvements. This work is inspired by the current discussion of finding a figure of merit for anechoic chambers. Based on the results, an evaluation procedure for anechoic chambers is indicated. However, it is pointed out and illustrated by examples that further investigations are necessary before a satisfactory procedure can be outlined.     

Volume integral equation solution of extinction cross section byraindrops in the range 0.6-100 GHz

The validity of the volume integral equation formulation (VIEF) is validated for the purpose of estimating the extinction cross section (ECS) of raindrops. The validations are performed against two well-established models. The validated volume integral equation model is employed to calculate the extinction cross sections of raindrops specified by a modified Pruppacher and Pitter (MPP) model. Data are given for drops with mean radii between 0.25 to 3.5 mm in the frequency range 0.6 to 100 GHz. Our study has showed that results of the extinction cross section for vertical and horizontal polarizations can be used to predict rain attenuation in wireless communications     

An empirical formula for the prediction of rain attenuation infrequency range 0.6 - 100 GHz

An empirical formula for calculating the extinction cross section (ECS) by raindrops over a broad frequency range is first derived based on extensive calculations made on a widely varying in mean radius of modified Pruppacher and Pitter (MPP) raindrop models ranging from 0.25 to 3.5 mm. The expansion coefficients in the empirical formula are determined by least-squares curve fitting of numerical data obtained by the volume integral equation formulation (VIEF). The formula satisfies the frequency and raindrop size dependence. Numerical results obtained from the empirical formula for calculating the ECS are generally in good agreement with those calculated by the VIEF for raindrops with mean radius varying from 0.25 to 3.5 mm in the frequency range from 0.6 to 100 GHz. The average error in the ECS is less than 10%. The formula thus provides a simple and inexpensive method for calculating the ECS of raindrops, which otherwise requires complicated and expensive methods of calculation. By implementing this empirical formula of ECS into the rain attenuation equation, a new numerically empirical formula for calculating the specific rain attenuation is also proposed. The validity of the empirical formula for calculating the specific rain attenuation is also checked by comparing the obtained results of specific rain attenuation with those obtained from Li et al.'s (1995) solution, Yeo et al.'s (1993) measurement, and Olsen et al.'s (1978) power-law equation     

Efficient detection of resonances in anechoic chambers using thematrix pencil method

In a previous paper we have presented a method for evaluating the performance of anechoic chambers by analyzing the S-parameters of a system comprising two antennas facing each other in an anechoic chamber using the matrix pencil method. In this work, we present an improvement of this resonance detection technique using only the transmission parameter S₂₁. The propagating components of the transmission parameter S₂₁ are derived over small frequency intervals using the matrix pencil method and then removed from S₂₁ in a two-level decomposition procedure. The resonances are clearly identified from the residual signal. Two examples of resonance detection in two different anechoic chambers illustrate the proposed method     

Electromagnetic wave absorbers and anechoic chambers through the years

A historical summary of the development of microwave absorbing materials and anechoic chambers is presented.     

A planar wideband 80-200 GHz subharmonic receiver

A wideband planar subharmonic mixer has been designed for millimeter-wave operation. The receiver consists of a back-to-back Schottky-diode pair integrated at the base of a wideband log-periodic antenna and placed on a silicon lens. The wideband planar receiver results in state-of-the art-performance at 90 GHz (and 182 GHz) with a double-sideband conversion loss and noise temperature of 6.7 dB (and 8.5 dB) and 1080 K (and 1820 K), respectively. These results are about 3 dB higher than the results for best tuned waveguide subharmonic mixers using planar diodes. The design is well suited for higher frequencies (up to 1 THz) and for the inclusion of biased back-to-back planar diodes to ease the LO power requirements. The planar subharmonic approach results in an inexpensive wideband receiver, and the design can be easily extended to receiver arrays     

A 90-100 GHz double-folded slot antenna

A double-folded slot antenna (DFS) has been designed, fabricated, and tested at 90-100 GHz. The antenna shows a very wideband impedance around 20 Ω from 85 to 110 GHz. The low impedance is compatible with superconductor-insulator-superconductor (SIS) junctions, Schottky diodes or high electron mobility transistor (HEMT) amplifiers, which require a low impedance at millimeter wave frequencies. The antenna is placed on a dielectric lens to synthesize a semi-infinite substrate and realize high-directivity patterns. The measured radiation patterns agree very well with theoretical calculations and demonstrate symmetric main beams and sidelobe levels below -15 db over a 10% bandwidth. The double folded slot antenna is an attractive candidate for low-cost wideband millimeter-wave monolithic microwave integrated circuits (MMIC) front ends     

A Very Low-Noise Single-Sideband Receiver for 200-260 GHz

A cryogenic Schottky diode mixer receiver has been built for the 230-GHz region with true single-sideband operation and a receiver noise temperature as low as 330 K. Local oscillator power is provided by a frequency tripler, with LO injection and sideband filtering accomplished through quasi-optical interferometers. The image sideband is terminated in a cryogenic load with an effective temperature of 33 K. The IF bandwidth is 600 MHz with nearly flat noise, and the RF band is nearly flat over 50 GHz using backshort tuning of the mixer.     

A solid-state frequency source for radio astronomy in the 100 to 1000 GHz range

A compact Gunn oscillator-multiplier combination can provide the for SIS receivers necessary LO power up to 1000 GHz. Frequency multiplication factors of 2 to 8 are accomplished in a single step using a Schottky mixer diode in the varistor mode.     

Figure of merit for low frequency anechoic chambers based onabsorber reflection coefficients

Return loss as a function of frequency and angle of incidence is studied to determine the effectiveness of the absorbing material used in an anechoic chamber. This alone is not enough to determine a figure of merit for an anechoic chamber or to compare the figure of merit for one anechoic chamber to that of another. While the information gained from return loss calculations and measurements as a function of angle of incidence is valuable, an overall measure of anechoic chamber effectiveness is necessary in order to compare different designs. In this paper, a new chamber figure of merit which is based on the decay time of the chamber is introduced. This decay time is, in turn, based on the average power absorbed by the chamber walls. The resulting model is simple and does not require intensive numerical computation. Calculations of the figure of merit for anechoic chambers which contain different types of absorbing materials are shown, and calculated and measured values of decay time for a primary standards calibrations facility are compared     

Measurement of electrically large array antennas in small anechoic chambers [Measurements Corner]

A post-processing procedure that yields accurate array-antenna radiation-pattern estimations is described. By resorting to this strategy, measurements of large array antennas inside small anechoic chambers become feasible. The method employs an adequate phase-correction derived from an optical ray approach, under the assumption that the elementary radiators themselves are electrically small and can be measured under far-field conditions inside the relevant anechoic chamber. The effectiveness of the proposed technique is demonstrated by examining the case of a nonuniform linear array antenna.     

Low-loss sapphire waveguides for 75-110 GHz frequency range

Low-loss dielectric waveguides are promising for use instead of metal ones, but problems in transitions have to be overcome. A simple and effective structure made of a monocrystalline sapphire waveguide has been designed. Experimental results at 75-110 GHz indicate good matching with metal waveguides (VSWR ⩽1.13) and low insertion loss (0.05-0.35 dB for 47 mm dielectric section)     

Spherical near-field facility for microwave coupling assessments inthe 100 MHz-6 GHz frequency range

This paper describes a spherical near-field facility set up at the Centre d'Etudes de Gramat (CEG) to perform low-power microwave coupling assessments. Specifically, this facility has been designed to determine the coupling cross section of a complex object. However, it can also be used for the characterization of any radiating system in electromagnetic compatibility (EMC) or high-power microwave (HPM) environments. The near-field approach is shown to be complementary and to offer increased flexibility when compared to other more conventional measurement techniques. More particularly, the use of a probe array in the upper part of the frequency band significantly speeds up the near-field measurement process. Consequently, broadband and multiparameter acquisitions can be performed within acceptable duration. The examples given in this paper provide a broad illustration of the capabilities of near-field techniques for EMC and HPM applications     

Atmospheric emission and attenuation in the range 100 to 600 GHz measures from a mountain site

Further measurements of atmospheric emission from a site in the Canary Islands (Izana, âltitude 2.4 km) have been made during the month of August 1980. The measurements were made with a polarising Interferometer and a composite Ge bolometer. An independent measurement of the precipitable water vapour was made using Infrared Hygrometers. The experimental details are described and the spectra obtained are compared with model spectra using the measured precipitable water vapour.     

Model for estimating rain attenuation at frequencies in range 6-30GHz

The raindrop size distribution (RSD) for various types of rainfall over Guwahti, India is presented. The observed RSD parameters are compared with those given by other models. By employing the observed RSD, the values of a and b in the relation A=aR^b for estimating the rain attenuation at frequencies from 6 to 30 GHz are given. The significance of the type of rainfall in controlling attenuation is discussed     

Microwave (1-100 GHz) dielectric model of leaves

A semiempirical formula for the complex dielectric permittivity of leaves from different plants is found from a comparison of published measurements covering the frequency range from 1 to 100 GHz. The explicit parameters are the dry-matter fraction m_d of the leaf and the permittivity ϵ_SW of saline water with a salinity of about 1 percent. The physical part of the formula is its basis on ϵ_SW while the empirical part is its linearity with m_d. The formula is applicable to fresh leaves; their m _d values are in the range 0.1d<0.5. A test indicates that besides the m_d variation and the spectral dependence the formula also describes the temperature variation correctly