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A near-field phasor beam measurement system is developed for the characterization of heterodyne receiver optics at submillimeter-wave frequencies. The system synthesizes a pair of submillimeter-wave signals as the RF and local oscillator (LO) sources from common reference sources. The synthesized harmonic numbers of the RF and LO sources are arranged with difference by one, which makes this a new configuration with a unitary harmonic diffrence. The coherent RF and LO signal are down-converted by the receiver under test, then mixed with the microwave-frequency common reference signal to generate the second-order IF signal around 100 MHz for amplitude and phase comparison. The amplitude and phase fluctuation of the measurement system at 683 GHz is within [+ or -] 0.2 dB and [+ or -] 4[degrees] in a 1-h period, respectively. The system dynamic range at 683 and 250 GHz can be as high as 43 and 47 dB, respectively. The system is then used to measure the receiver beam patterns at 683 and 250 GHz with different RF transmitting probe antennas. Index Terms--Frequency conversion, Gaussian beams, phase measurement, submillimeter-wave receivers.

A high-directivity combined self-beam/null-steering array for secure point-to-point binary phase-shift keying communications is introduced. The system provides high directivity and reduced probability of interception using just two antenna elements. Using quadrature phase-shift keying modulators allows for compact single-layer fabrication. The 2.4-GHz prototype is tested at interrogation angles of 0[degrees], -10[degrees], and +20[degrees], and demonstrates high signal-to-interference ratio directivity, completely disabling interception [+ or -]20[degrees] from the direction of the interrogator. The system should find various applications where secure communications are required. Index Terms--Digital communication, microwave receivers, phase conjugation, phased arrays, transponders. Digital Object Identifier 10.1109/TMTT.2007.895405

In this work, a new technique to implement the transfer function of polyphase filter with CMOS active components is proposed and analyzed. In the proposed polyphase filter structure, the currents mirrored from capacitors and the transistors in a single-stage are used to realize high-pass and low-pass functions, respectively. The multistage structure expands the frequency bandwidth to more than 20 MHz. Furthermore, a constant-gm bias circuit is employed to decrease the sensitivity of image rejection to temperature and process variations. HSPICE simulations are performed to confirm the performance. With the current-mode operation, the low-voltage version of proposed active polyphase filters was designed. It can be operated at 1-V power supply with similar performance but with only 50% of the power dissipation of the normal-voltage version. The proposed four-stage polyphase filter is fabricated in 0.25-/[micro]m CMOS IP5M technology. The measured image rejection ratio is higher than -48 dB at frequencies of 6.1 MHz ~ 30 MHz. The measured voltage gain is 6.6 dB at 20 MHz and IIP3 is 8 dBm. The power dissipation is 11 mW at a supplied voltage of 2.5 V and the active chip area is 1162 x 813/[micro][m.sup.2]. Index Terms--CMOS technology, double-quadrature architecture, low intermediate frequency architecture, polyphase filter, RF receiver.

We propose a new quasi-optical setup for a nulling interferometer at short millimeter and submillimeter wavelengths employing a single Martin-Puplett interferometer (MPI) for side-band separation. The new design allows the simultaneous use of both sidebands, thus avoiding the loss of sensitivity from conventional sideband filters. Although intended for solar flare observations, the quasi-optical setup can be applied to cancel any extended circular source, while at the same time preserving the full sensitivity toward a point-like source on or near the extended one. Numerical simulations show that the sinusoidal transmission characteristic of the MPI limits the usable relative bandwidth to one-third of the IF. Compared to an instrument employing lossless sideband filters, an increase of the response to a point-like continuum source by a factor of ~ 1.6-2 is expected, depending on the source position and relative bandwidth. Index Terms--Interferometry, radio astronomy, solar radiation, submillimeter-wave receivers, submillimeter-wave technology.

This paper considers specular coherent and noncoherent optimal detection for unresolved multipath Ricean fading channels with known delays. The focus is on receiver structures and performance. Specular coherent detection employs the carrier phase of the Ricean specular component, while noncoherent detection does not. Therefore, a specular coherent detector must be augmented with a carrier phase estimator for the specular component. The structures considered in this paper are generalization of the well-known Rake receiver to the unresolved multipath case. It is shown that both optimal structures perform a decorrelation operation before combining, which is essential to eliminating error floors under multipath unresolvability conditions. Furthermore, the noncoherent optimal receiver includes an inherent estimator for the specular component phasor. It is shown that the specular coherent and noncoherent structures converge at high SNR. This result is confirmed through analytical and numerical performance evaluation. Little performance gains can be obtained by the use of specular coherent detection for orthogonal frequency-shift keying and to a lesser extent for differential phase-shift keying over mixed mode Ricean/Rayleigh fading channels, making noncoherent demodulation attractive in these cases. Index Terms--Decorrelators, detection, estimation, multipath fading channels, noncoherent, quadratic receivers, specular coherent, wireless communications.

This paper provides an overview of the microwave applications of high-temperature superconductor (HTS) technology. The main characteristics of HTS materials are outlined, highlighting the differences between superconductors and normal conductors. This paper presents recent progress in the development of HTS filters, multiplexers, cryogenic receivers, delay lines, and antennas. A brief summary of cryocooler technology and cryopackaging requirements is presented. The benefits of using HTS technology in wireless and satellite systems are also discussed. The intent of this paper is to provide perspective to newcomers in the field and to empower the potential end-users with current status and performance capabilities of HTS microwave devices. Index Terms--Cryogenic receivers, high-temperature superconductors, HTS technology, microwave filters and multiplexers, microwave superconductivity, satellite payloads, wireless base-stations.

This paper reports the impact of digital signal processing on microwave receiver technology. The majority of modern receiver designs are based on digital technology. Wide-and narrow-band receivers will be presented. The wide-band receivers cover approximately 1-GHz instantaneous bandwidth and are used to intercept radar pulses. Current narrow-band receivers cover up to 50-MHz instantaneous bandwidth and are primarily used for receiving communication signals. Two approaches for wide-band receiver design will be discussed. One is the conventional digital receiver. The other one is called the monobit receiver, which has slightly inferior performance in some respects, but can be built on a single chip. Narrow-band receivers are best implemented in software because they can more adapt to changes. Two types of receivers will be discussed. One is the software global positioning system receiver. The other one is called a transform-domain communication system. The object of this system is to avoid interference in a hostile communication environment. Index Terms--Digital receiver, digital signal processing, digital technology, GPS receiver, microwave receiver, monobit receiver, transform-domain communication.

A single sideband superconductor-insulator-superconductor (SIS) receiver developed for 0.85 mm wavelength is tested at the 30 m diameter IRAM radio telescope. The observation speedup withy SSB tuning is between three and two for a reasonably good DSB SIS receiver noise, below 80 K. The best radio telescope system operation improvement with the SSB tuning takes place for the same DSB receiver noise temperature of about 30-40 K.

Research was conducted to examine Superconducting Integrated Receiver (SIR) made on silicon substrates and designed for a nine-pixel Superconducting Imaging Array Receiver at 500 GHz. The characteristics of the integrated lens antenna were investigated using an externally pumped reference SIS mixer. Results show that the computer system IRTECON is a suitable instrument for both the operation of a SIR and chip qualification.

A low-cost buried-mesa avalance photodiode (APD) primarily targeted for 2.5-Gb/s lightwave applications was developed. A simple batch process that generates a robust and reliable device with potentially high yield and low cost was used to produce APD's. One epitaxial step was used to grow the entire base structure of the InGa-As-InP APD while four simple steps that included a mesa etch, isolation, one epitaxial overgrowth and metallization were used for the remaining process. The APD's were observed to have high reliability under rigorous stress testing.

An integrated quasioptical receiver is proposed for 420-530 GHz frequency range applications. The device consists of a superconducting flux-flow oscillator, superconductor-insulator-superconductor mixer and a planar double-dipole antenna. A singleNb-AlOx-Nb trilayer was used to integrate the receiver components on a 4mm x 4mm x 0.2 mm crystalline quartz substrate. The low mass, size and power consumption of the receiver makes it ideal for space applications.

A study was conducted on different quasi-optical superconductor-insulator-superconductor mixer designs operating at frequency ranges between 300-1200 GHz. The mixers employ a two-junction tuning circuit, twin-slot antenna and an antireflection-coated silicon hyperhemispherical lens. Fourier transform spectrometry was utilized in measuring the broad-band frequency response.

A 6 GHz hybrid superconductive/semiconductive microwave receiver was fabricated. The device consists of a cryogenic LNA, high-temperature superconductor microstrip preselect filter, superconducting local oscillator and a commercially-available mixer. The receiver has an IF frequency of 4.483 GHz and a 500 MHz operating bandwidth centered at a 5.9 GHz radio frequency. A 10.383 GHz local oscillator facilitates downconversion in the device.

A detailed analysis of the microwave and direct current (DC) properties of Nb-AlOx-Nb flux flow oscillators for integration with superconductor-insulator-superconductor mixers was conducted. A Josephson effect-based model was developed for the analysis of experimentally measured DC I-V curves. A new reliable approach based on harmonic mixing was used to measure the spectral linewidth of the radiation emitted by the oscillators.

This paper refers to quantitative reconstruction of the dielectric and conductive property distributions of biological objects by means of active microwave imaging. An iterative reconstruction algorithm based on the Levenberg-Marquardt method is tested with synthetic data. The influence of the receiver geometry is investigated and two methods for choosing the regularization parameter, an empirical method and generalized cross validation (GCV) method, are examined. Index Terms - Microwave imaging.

The current status of InP-based high electron mobility transistor (HEMT) technology for low noise amplification at frequencies up to more than 100 GHz is presented. Following a review of recent advances industry-wide in both device and circuit performance, two issues which will pace the rate at which this new technology can be inserted into microwave systems - material/process maturity and long-term reliability - are discussed.

A direct digitization approach greatly reduces the hardware requirements in traditional front end design. Further, the hardware that has been eliminated is typically the source of a number of potential difficulties including age-based, temperature-based, and/or nonlinear performance. This paper presents a case study on the design and implementation of direct digitization Global Positioning System (GPS) receiver front end. First, sensitivity and dynamic range issues for a generic front end are discussed with particular attention given to the unique requirements in the direct digitization approach. Second, two GPS front end implementations are compared. The first is the direct digitization of the input signal at radio frequency (RF) as is the case in the true digital receiver or software radio. The second uses a more standard approach of downconverting the input signal to an intermediate frequency (IF) for further processing or digitization. Experimental data is presented which characterizes the relative signal-to-noise ratio for both implementations as well as the results of initial acquisition processing of true GPS data.

The three-dimensional monolithic microwave/millimeter wave integrated circuits (MMIC) structure that places thin polyimide-film layers on wafers significantly increases the integration level of MMIC's. We newly develop 9.2-12 GHz receiver and 9.5-14 GHz transmitter chips with 20 dB gain using the three-dimensional MMIC technology. The receiver chip includes a four-stage front-end amplifier, a local oscillator (LO) amplifier, and an image-rejection mixer in a 2 x 2 mm chip. The transmitter chip also includes an IF amplifier with balanced outputs, an LO amplifier, an RF buffer amplifier, and a balanced upconverter in a 1.9 x 1.9 mm chip. The integration levels of these chips are nearly three times higher than those of conventional planar devices. The design method of each function block, such as the amplifier and mixer, is also described.

A fixed-tuned superconductor-insulator-superconductor (SIS) receiver has been designed to operate in the 250-350-GHz frequency band. This receiver has a doubleside-band noise temperature of between 35 and 45 K, or about 3hv/[k.sub.B], over its entire operating band. Extensive characterization of the receiver has been carried out using techniques developed for submillimeter waves. The input noise, side-band ratio, 1 dB compression point, optimum LO drive level, and vector near-field beam profile have all been measured experimentally. The measurement techniques and results are presented and discussed.

We present a novel approach to the problem of quasi-optical LO injection in a 100 GHz SlS imaging receiver. The use of a specially engineered molecular beam epitaxy material as an all-optical millimeter wave modulator is presented. This optically controlled modulator, used together with a kinoform to generate and control an array of Gaussian beams, is an efficient, completely solid-state, rugged and physically small solution. The working principle of the optically controlled modulator lies in the fact that the conductivity of a semiconductor can be changed by the generation of free carriers under photonic excitation, thus changing its refractive index.

This paper proposes monolithically integrated photoreceivers with tunable filtering functions using HBT-compatible HPT's for band-limited microwave subcarrier multiplexed transmission and optical/microwave signal processing systems. The filter photoreceiver, which consists of the HPT with a positive feedback LC resonance circuit and a Q-damping resistor, realizes the high quality factor (Q-factor) of 37.7 with the photo-response peak frequency of 13.6 GHz. Furthermore, it yields not only 24.1 dB and 16.2 dB higher gain but also over 7.5 dB and 3.5 dB improvement in sensitivity at 13.6 GHz without any spurious oscillation, in comparison with a standard photodiode and a discrete HPT, respectively. The tuner photoreceiver is constructed based on the filter photoreceiver. By adapting the HBT-compatible varactor diode in the resonance circuit, we realize tunable filtering functions, i.e., a very-wide photo-response peak frequency tuning range of 11.0-14.6 GHz with a Q-factor of over 20.2. The chip size is as small as 1.0 mm x 0.75 mm. The developed photoreceivers promise to realize compact, cost-effective hardware for subcarrier multiplexed optical/microwave or /millimeter-wave systems.

The signal to noise performance of grounded source amplifier and distributed amplifier optical receiver configurations has been theoretically investigated. This predicts that an improvement of signal to noise ratio up to 12.7 dB is achievable for the distributed amplifier configuration compared with the grounded source in a bandwidth of 40 GHz. Our preliminary experimental results support the theoretical analysis. An optical receiver with 3 dB bandwidth of 20 GHz was fabricated by embedding the p-i-n photodiode into a T-network at the gate line in the distributed amplifier and an average equivalent input noise of 20 pA/[Hz.sup.1/2] was achieved over a bandwidth of 2-18 GHz.

InP-based pin-HEMT receiver-OEIC's with different circuit layouts for bit rates up to 10 Gb/s are simulated, realized and characterized. The circuits under investigation are a high impedance amplifier, a common-gate circuit, and a transimpedance-cascode circuit. The high frequency behavior of all circuits is compared by means of on wafer-characterization. All circuits show a bandwidth of more than 5 GHz, the transimpedance circuit has the highest responsivity (12.9 dB A/W) and a very low average noise current of 11.5 pA/[square root of Hz] when assembled in a module. The receiver sensitivity of the transimpedance circuit in the module is measured to be as high as -19.2 dBm.

Optoelectronic mixers can exhibit a very wide bandwidth of operation with significantly reduced third-order intermodulation products. A 20-GHz bandwidth optoelectronic mixer has been constructed and characterized. The third order intermodulation terms were demonstrated to be more than 70 dB below the I.F. output. The mixer was then incorporated into a simple superheterodyne receiver architecture, which was shown to have a tangential sensitivity of -66 dBm and a compressive dynamic range of 44 dB. System limitations and possible improvements are discussed.

An analysis of the relationship between a linear amplifier chain and an analog-to-digital converter (ADC) in a digital microwave receiver, with respect to sensitivity and dynamic range issues, is presented. The effects of gain, third-order intermodulation products and ADC characteristics on the performance of the receiver are illustrated and design criteria for the linear amplifier chain (given a specified ADC) are developed. A computer program is included which calculates theoretical receiver performance based on gain and third-order intermodulation product selections. Experimental results are also presented and compared with theoretical values.

A 250 Ghz monolithic Schottky-diode receiver based on a double-slot antenna is presented. The double-slot antenna is placed on an extended hemispherical high-resistivity silicon substrate lens. The measured DSB conversion loss and noise temperature at 258 GHz are 7.8 [+ or -] 0.3 dB and 1600 [+ or -] 100K for the antenna-mixer, respectively. A nonoptimal polyethylene [Lambda][subscript d]/4 matching-cap layer for the silicon lens improves the conversion loss and noise temperature by 1 dB, and another 0.7 dB improvement could be obtained with the use of a more optimal matching cap layer. The uniplanar double-slot antenna receiver is less than 0.3 X 1 mm in size including the IF filter and represents the first fully monolithic 250 GHz receiver to date. The measured performance is within 2-3 dB of the best 200+ GHz waveguide receivers using planar Schottky divides.

We propose a compact quasi-optical scheme to achieve image separation using two mixers. With this scheme, an image rejection ratio of better than 12 dB has been obtained over 25 GHz in W-band without any mechanical tuning. This diplexing scheme should find applications in remote sensing and radio astronomy.

High-frequency transistors are known for their ability to efficiently perform signal processing functions that reach beyond mere amplification. Among such functions are oscillation, signal mixing, frequency multiplication, and optical signal detection. The current investigation focuses specifically on these four functions as they pertain to retrieving signal information impressed on optical carriers of 1.3-[[micro]meter] wavelength. The main demonstration object is a 40-GHz hybrid-integrated heterodyne receiver circuit that employs an InP-based HEMT in a four-function role to simultaneously provide carrier demodulation, self-generation of a local oscillation signal, internal frequency doubling thereof, and signal downconversion to a lower intermediate frequency band.

In this paper, interference caused by multiple uncoordinated low-power wireless access system transmitters to fixed point-to-point microwave receivers is analyzed. The analysis shows that the density of wireless access system users that can share spectrum with point-to-point microwave systems depends critically on the propagation relation, the threshold of detecting energy from point-to-point transmitters, and the allowable interference into point-to-point receivers. We also show that, for reasonable assumptions of point-to-point link, wireless access system, and propagation parameters, a wireless access system employing measurement-based interference avoidance must detect energy of point-to-point transmissions far below the thermal noise floor of the wireless access system receivers. Meeting this requirement could increase the complexity of wireless access systems. This work is targeted towards understanding the implications for local exchange networks of wireless system alternatives that could provide access to those networks.

We present a simple new technique to determine the noise temperature of the rf input section of a superconducting quasiparticle heterodyne receiver. This quantity is difficult to measure by existing methods. The new technique uses standard hot/cold-load measurements, and the precision should be as good as the hot/cold-load determination of receiver noise temperature. For most receivers, correction terms will be much smaller than the quantum temperature h bar-Omega/k.

A new broadband frequency selective limiter, which provides over 14 dB of limiting across more than an octave bandwidth, is described. The limiter is fabricated with epitaxially grown YIG films in a stripline configuration and has a threshold power level of below 0 dBm. Multiple FSL's have been cascaded with amplifiers to allow compression of microwave signals with a power range of 60 dB into a range of less than 5 dB.

Digital radio transmission systems use complex modulation schemes that require powerful signal-processing techniques to correct channel distortions and to minimize bit error rates. Acousto-optic processors are used, in conjunction with adaptive digital equalizers, to reduce the bit error rate of the digital radio receiver. The acousto-optic processor implements an inverse channel filter that is rapidly adaptable to time-varying distortions. A specific architecture is identified and a laboratory system is tested to verify the ability of the processor to track and correct time-varying channels. Computer simulations are used to show that the hybrid acousto-optic and digital equalizer permits a fourfold increase in the modulation capacity of radio, relative to all digital equalization, while improving the bit error rate performance.

A novel planar, quasi-optical SIS receiver operating at 230 GHz is described. The receiver consists of a 2 X 5 array of half wave dipole antennas with niobium-aluminum oxide-niobium SIS junctions on a quartz dielectric-filled parabola. The 1.4 GHz intermediate frequency is coupled from the mixer via coplanar strip transmission lines and 4:1 balun transformers. The receiver is operated at 4.2 K in a liquid helium immersion cryostat. We report here accurate measurements of the performance of single untuned array receiver elements. A mixer noise temperature of 89 K DSB, receiver noise temperature of 156 K DSB and conversion loss of 8 dB into a matched load have been obtained. This mixer noise temperature is approximately a factor of two larger than that of current state of the art waveguide mixers using untuned single junctions at the same frequency.

We have used the quantum theory of mixing for extensive numerical calculations to determine the mixer source conductance G(sub s) required to optimize a superconductor-insulator-superconductor (SIS) quasiparticle heterodyne receiver. The optimum G(sub s) matches an empirical formula which can be understood by a simple derivation. Previous work indicated that G(sub s) should vary inversely with frequency, and this implies that the critical current density of SIS junctions used for mixing should increase as frequency squared, a stringent constraint on the design of submillimeter SIS mixers. On the contrary, we find that G(sub s) is more weakly dependent upon frequency, and we discuss the implications for the design of submillimeter SIS mixers.

This paper presents a new type of electron bolometric ('hot electron') mixer. We have demonstrated a three order of magnitude improvement in the bandwidth compared with previously known types of electron bolometric mixers, by using the two-dimensional electron gas (2DEG) medium at the hetero-interface between AlGaAs and GaAs. We have tested both in-house MOCVD-grown material, and MBE material, with similar results. The conversion loss (L(sub c)) at 94 GHz is presently 18 dB for a mixer operating at 20 K, and calculations indicate that L(sub c) can be decreased to about 10 dB in future devices. Calculated and measured curves of L(sub c) versus P(sub LO), and I(sub DC), respectively, agree well. We argue that there are several different configurations of electron bolometric mixers, which will all show wide bandwidth, and that these devices are likely to become important as low-noise THz receivers in the future.

An integrated planar receiver has been developed and tested over the 82-112 GHz bandwidth. The quasi-integrated antenna used in the receiver has a high gain, a high Gaussian coupling efficiency and a wide bandwidth. The novel mixer design consists of a planar GaAs Schottky diode placed at the feed of a dipole-probe suspended inside an integrated horn antenna. The diode uses an etched surface channel and a planar air bridge for reduced parasitic capacitance. At 92 GHz, the room temperature antenna-mixer exhibits a double sideband conversion loss and noise temperature of 5.5 + or - 0.5 dB and 770 K + or - 50 K, respectively. The measured DSB conversion loss and noise temperature over a 20 GHz bandwidth (86 GHz-106 GHz) remain less than 6.2 dB + or - 0.5 dB and 1000 K + or - 50 K, respectively. The low cost of fabrication and simplicity of the design makes it ideal for millimeter- and submillimeter-wave receivers.

The implementation of a wideband real-time spectrum-analysis receiver is presented. A chirp-transform algorithm and superconductive YBa2Cu3O7-x chirp filters are the bases for the high-Tc spectrum analysis receiver used in the study. The system, which has a bandwidth of 3 gigahertz, has undergone partial testing, with full testing expected to be completed in future experiments.

The performance of Pb/Bi/In and Nb/Al junctions to be used as components of a 345 gigahertz open structure astronomical receiver was evaluated. The junctions, which were fabricated to compensate for the tunnel barrier capacitance in the receiver, were tested for their voltage versus infrared power and noise temperature versus local oscillator frequency characteristics. Results show that the junctions are very useful for measuring molecular line emission at 345.8 gigahertz in external galaxies.

The feasibility of using parallel-in-series biased junction arrays to enhance the performance of superconductot-insulator-superconductor (SIS) mixers was evaluated. Experimental receivers were tested in a liquid helium cryostat at a mixer ambient temperature range of 4.2 to 5.0 degrees Kelvin. The receivers' noise temperature, current-voltage characteristics and input signals were then measured under the presence of an applied magnetic field. Results show that the use of the SIS junctions enable the mixers to operate in the sub-millimeter region with higher efficiency.

This paper presents a novel multireceiver configuration for the purpose of carrier arraying and/or signal arraying. Such a problem arises for example, in NASA Deep Space Network where the same data-modulated signal from a spacecraft is received by a number of geographically separated antennas and the data detection must be efficiently performed on the basis of the various received signals. The proposed configuration is arrived at by formulating the carrier and/or signal arraying problem as an optimal estimation problem and consists of two stages. The first stage optimally estimates various phase processes received at different receivers with coupled phase-locked loops (PLL) wherein the individual PLL's acquire and track their respective receivers' phase processes but are aided by each other in an optimal manner via low frequency error signals. The coupled PLL estimator is followed by a linear or nonlinear combining of the quadrature-phase components of the mixers from various receivers for the purpose of data detection. The proposed configuration results in the minimization of the effective radio loss at the combiner output and thus maximization of energy per bit to noise power spectral density ratio is achieved. The paper also presents a novel adaptive algorithm for the estimator of the signal model parameters when these are not known a priori. Essentially these parameters are the various noise covariance matrices and the received signals' amplitudes.

A proposed millimeter and submillimeter wavelength receiver uses Nb-AlOx-Nb tunnel junctions and a Nb-SiO fix-tuned broadband matching circuit. The receiver possesses high sensitivity properties and uses quasi-optical mixers which are easily fabricated and applied. Coverage of the 200-475 Ghz bandwidth is possible with a two-section impedance transformer connected to an end loaded stub and without external tuners as compared to existing waveguide mounts.

A layer of vegetation over the soil surface absorbs some of the radiation emitted from the soil and emits at its own temperature. This results in a reduction of the information in the microwave radiation about the soil surface. To study this problem further we use the Ulaby and El-Rayes model for the dielectric constant of vegetation to estimate the absorption loss and optical depth, tau, of plant canopies for frequencies between 1 and 40 GHz. We treated tau as the product of a vegetation parameter b and vegetation water content, VW. We compared both the linear and square root (refractive) mixing models with the observed data in terms of the b parameter. These data were obtained from published reports on the values of tau and VW for crops ranging from prairie grass to corn and soybeans. The data fit the curve for the refractive model quite well. For the refractive model the value of b was independent of VW, while for the linear model there was some dependence on VW. For both models b is roughly proportional to the frequency.

A new junction fabrication procedure which incorporates planar tuning circuits during integration is proposed for superconductor-insulator-superconductors. The latter are widely used in heterodyne receivers operating in the millimeter wavelength. In the study, the Nb/Al trilayer structure rendered the subsequent receiver noise temperature to be significantly reduced while the tuning elements provided a larger junction region with lower current densities.

The development of a 230 GHz superconducting insulator superconducting heterodyne receiver is presented. The receiver, made of Nb/AlOx/Nb tunnel junctions whose current concentration is around 8500 A persquare cm, includes an integrated broadband matching network for enhanced intermediate frequency coupling efficiency up to 96%. Upon testing, it manifested a minimum noise temperature at 239 GHz with 48 K DSB at 4.2 K and 40K DSB at 2.1 K.