Showing papers in "IEEE Transactions on Microwave Theory and Techniques in 2000"

A clinical prototype for active microwave imaging of the breast

Abstract: Despite its recognized value in detecting and characterizing breast disease, X-ray mammography has important limitations that motivate the quest for alternatives to augment the diagnostic tools that are currently available to the radiologist. The rationale for pursuing electromagnetic methods is strong given the data in the literature, which show that the electromagnetic properties of breast malignancy are significantly different than normal in the high megahertz to low gigahertz spectral range, microwave illumination can effectively penetrate the breast at these frequencies, and the breast is a small readily accessible tissue volume, making it an ideal site for deploying advanced near-field imaging concepts that exploit model-based image reconstruction methodology. In this paper a clinical prototype of a microwave imaging system, which actively illuminates the breast with a 16-element transceiving monopole antenna array in the 300-1000 MHz range, is reported. Microwave exams have been delivered to five women through a water-coupled interface to the pendant breast with the participant positioned prone on an examination table. This configuration has been found to be a practical, comfortable approach to microwave breast imaging. Sessions lasted 10-15 min per breast and included full tomographic data acquisition at seven different array heights beginning at the chest wall and moving anteriorly toward the nipple for seven different frequencies at each array position. This clinical experience appears to be the first report of active near-field microwave imaging of the breast and is certainly the first attempt to exploit model-based image reconstructions from in vivo breast data in order to convert the measured microwave signals into spatial maps of electrical permittivity and conductivity. While clearly preliminary, the results are encouraging and have supplied some interesting findings. Specifically, it appears that the average relative permittivity of the breast as a whole correlates with radiologic breast density categorization and may be considerably higher than previously published values, which have been based on ex vivo tissue specimens.

Toward the development of a three-dimensional unconditionally stable finite-difference time-domain method

Abstract: In this paper, an unconditionally stable three-dimensional (3-D) finite-difference time-method (FDTD) is presented where the time step used is no longer restricted by stability but by accuracy. The principle of the alternating direction implicit (ADI) technique that has been used in formulating an unconditionally stable two-dimensional FDTD is applied. Unlike the conventional ADI algorithms, however, the alternation is performed in respect to mixed coordinates rather than to each respective coordinate direction, Consequently, only two alternations in solution marching are required in the 3-D formulations. Theoretical proof of the unconditional stability is shown and numerical results are presented to demonstrate the effectiveness and efficiency of the method. It is found that the number of iterations with the proposed FDTD can be at least four times less than that with the conventional FDTD at the same level of accuracy.

3-D ADI-FDTD method-unconditionally stable time-domain algorithm for solving full vector Maxwell's equations

Abstract: We previously introduced the alternating direction implicit finite-difference time domain (ADI-FDTD) method for a two-dimensional TE wave. We analytically and numerically verified that the algorithm of the method is unconditionally stable and free from the Courant-Friedrich-Levy condition restraint. In this paper, we extend this approach to a full three-dimensional (3-D) wave. Numerical formulations of the 3-D ADI-FDTD method are presented and simulation results are compared to those using the conventional 3-D finite-difference time-domain (FDTD) method. We numerically verify that the 3-D ADI-FDTD method is also unconditionally stable and it is more efficient than the conventional 3-D FDTD method in terms of the central processing unit time if the size of the local minimum cell in the computational domain is much smaller than the other cells and the wavelength.

High-isolation CPW MEMS shunt switches. 1. Modeling

Abstract: This paper, the first of two parts, presents an electromagnetic model for membrane microelectromechanical systems (MEMS) shunt switches for microwave/millimeter-wave applications. The up-state capacitance can be accurately modeled using three-dimensional static solvers, and full-wave solvers are used to predict the current distribution and inductance of the switch. The loss in the up-state position is equivalent to the coplanar waveguide line loss and is 0.01-0.02 dB at 10-30 GHz for a 2-/spl mu/m-thick Au MEMS shunt switch. It is seen that the capacitance, inductance, and series resistance can be accurately extracted from DC-40 GHz S-parameter measurements. It is also shown that dramatic increase in the down-state isolation (20/sup +/ dB) can be achieved with the choice of the correct LC series resonant frequency of the switch. In part 2 of this paper, the equivalent capacitor-inductor-resistor model is used in the design of tuned high isolation switches at 10 and 30 GHz.

A new quasi-Yagi antenna for planar active antenna arrays

Abstract: In this paper, a novel broadband planar antenna based on the classic Yagi-Uda dipole antenna is presented, and its usefulness as an array antenna is explored. This "quasi-Yagi" antenna is realized on a high dielectric-constant substrate, and is completely compatible with microstrip circuitry and solid-state devices. This antenna achieves a measured 48% frequency bandwidth for voltage standing-wave ratio <2, better than a 12-dB front-to-back ratio, smaller than -15 dB cross polarization, and 3-5-dBi absolute gain. Mutual coupling of the antenna in an array environment is investigated. Finally, three simple arrays are presented, demonstrating the usefulness of the antenna as an array element. This novel antenna should find wide application in wireless communication systems, power combining, phased arrays, and active arrays, as well as millimeter-wave imaging arrays.

Synthesis of cross-coupled resonator filters using an analytical gradient-based optimization technique

Abstract: We propose a general approach to the synthesis of cross-coupled resonator filters using an analytical gradient-based optimization technique. The gradient of the cost function with respect to changes in the coupling elements between the resonators is determined analytically. The topology of the structure is strictly enforced at each step in the optimization thereby eliminating the need for similarity transformations of the coupling matrix. For the calculation of group delays, a simple formula is presented in terms of the coupling matrix. A simple recursion relation for the computation of the generalized Chebychev filtering functions is derived. Numerical results demonstrating the excellent performance of the approach are presented.

A varactor-tuned RF filter

Abstract: An electronically tunable filter at 1 GHz is presented in this paper. The filter uses a suspended substrate design and commercially available varactors for filter tuning. The filter has a 60% tuning range from 700 MHz to 1.33 GHz with a low insertion loss (better than 3 dB from 1 to 1.33 GHz). This paper discusses the effects of the varactor series resistance and the electrical length of the distributed resonator on the overall resonator quality factor and filter insertion loss. The input third-order intermodulation product intercept point was measured to be better than 17 dBm across the entire tuning range.

Microwave detection of breast cancer

Abstract: Breast cancer affects many women, and early detection aids in fast and effective treatment. Mammography, which is currently the most popular method of breast screening, has some limitations, and microwave imaging offers an attractive alternative. A microwave system for breast tumor detection that uses previously introduced confocal microwave imaging techniques is presented in this paper. The breast is illuminated with an ultrawide-band pulse and a synthetic scan of the focal point is used to detect tumors; however, the geometric configuration and algorithms are different from those previously used. The feasibility of using small antennas for tumor detection is investigated. Signal processing algorithms developed to mitigate the dominant reflection from the skin are described, and the effectiveness of these skin subtraction algorithms is demonstrated. Images of homogeneous and heterogeneous breast models are reconstructed with various numbers of antennas. Both the influence of antenna spacing and the suitability of simplified models for system evaluation are examined.

High-isolation CPW MEMS shunt switches. 2. Design

Abstract: For pt.1 see ibid., vol.48, no.6, p.1045-1052 (2000). In this paper, the second of two parts, the equivalent RLC model of the shunt switch is used in the design of tuned two- and four-bridge "cross" switches from 10 to 40 GHz. The cross switch attained an insertion loss of less than 0.3-0.6 dB, a return loss below -20 dB from 22 to 38 GHz in the up state, and a down-state isolation of 45-50 dB with only 1.5 pF of down-state capacitance (C/sub d/). Also, an X-band microelectromechanical system (MEMS) switch with an insertion loss of less than 0.2 dB and an isolation of 35 dB is presented. This is done by inductively tuning the LC series resonance of the shunt switch. The MEMS bridge height is 1.5-2.5 /spl mu/m, resulting in a pull-down voltage of 15-25 V. Application areas are in low-loss high-isolation communication and radar.

Specific absorption rate and temperature increases in the head of a cellular-phone user

Abstract: In this paper, a complete electromagnetic and thermal analysis has been performed considering the head of a subject exposed to various kinds of cellular phones available on the market, and focusing the attention on important organs like the eye lens and brain. Attention has first been posed on a particular phone model, and a comparison between the absorbed power distribution and steady-state temperature increases has been carried out. The influence of different antennas (dipole, monopole, whip, and planar inverted F antenna) on the power absorption and on the consequent tissue heating has then been analyzed. The obtained results show for a radiated power of 600 mW, maximum SAR values, averaged over 1 g, from 2.2 to 3.7 W/kg depending on the considered phone. The maximum temperature increases are obtained in the ear and vary from 0.22/spl deg/C to 0.43/spl deg/C, while the maximum temperature increases in the brain lie from 0.08/spl deg/C to 0.19/spl deg/C. These steady-state temperature increases are obtained after about 50 min of exposure, with a time constant of approximately 6 min. Finally, the results evidence a maximum temperature increase in the external part of the brain from 0.10/spl deg/C to 0.16/spl deg/C for every 1 W/kg of SAR, averaged over 1 g of brain tissue.

New cross-coupled filter design using improved hairpin resonators

Abstract: Stepped-impedance resonators have been thoroughly studied in this paper. Two equations for odd- and even-mode resonance are derived from a new network model. The size and resonant frequencies of the resonator could then be designed based on these two equations. A new resonator-embedded cross-coupled filter, constructed by stepped-impedance hairpin resonators and miniaturized hairpin resonators is presented. This new filter is very compact and has lower spurious response. A 0/spl deg/ feed structure, which adds two transmission zeros to the filter response, is also studied. The two zeros are so close to the passband that the selectivity and out-of-band rejection of the filter are significantly increased. The design has been verified by experiment results.

Design of highly selective microstrip bandpass filters with a single pair of attenuation poles at finite frequencies

Abstract: This paper presents the design of a class of highly selective microstrip bandpass filters that consist of microstrip open-loop resonators that exhibit a single pair of attenuation poles at finite frequencies. A practical design technique for this class of filters is introduced, including tables and formulas for accurate and fast filter synthesis. Two design examples of a six-pole filter with a fractional bandwidth of 7.331% at 955 MHz and an eight-pole filter with a fractional bandwidth of 10.359% at 985 MHz are described. Theoretical and experimental results are presented. The compact size and the excellent performance of this class of filters have been demonstrated.

Improvement of the quality factor of RF integrated inductors by layout optimization

Abstract: A systematic method to improve the quality (Q) factor of RF integrated inductors is presented in this paper. The proposed method is based on the layout optimization to minimize the series resistance of the inductor coil, taking into account both ohmic losses, due to conduction currents, and magnetically induced losses, due to eddy currents. The technique is particularly useful when applied to inductors in which the fabrication process includes integration substrate removal. However, it is also applicable to inductors on low-loss substrates. The method optimizes the width of the metal strip for each turn of the inductor coil, leading to a variable strip-width layout. The optimization procedure has been successfully applied to the design of square spiral inductors in a silicon-based multichip-module technology, complemented with silicon micromachining postprocessing. The obtained experimental results corroborate the validity of the proposed method. A Q factor of about 17 have been obtained for a 35-nH inductor at 1.5 GHz, with Q values higher than 40 predicted for a 20-nH inductor working at 3.5 GHz. The latter is up to a 60% better than the best results for a single strip-width inductor working at the same frequency.

The dependence of electromagnetic energy absorption upon human head tissue composition in the frequency range of 300-3000 MHz

Abstract: The requirements for testing compliance of cellular phones with electromagnetic safety limits demand evaluation of the maximum exposure that may occur in the user group under normal operational conditions. Under these conditions, the tissues of the ear region are most exposed, the tissue composition of which is complex and varies considerably from user to user. The objective of this paper is to derive head tissue equivalent dielectric parameters that enable the utilization of one generic homogeneous head for testing compliance for the entire user group, i.e., granting no underestimation, but also not greatly overestimating the actual maximum user exposure. As a primary study, a simple analytical model of an infinite half-space layered tissue model exposed to a plane wave was utilized to investigate the impact of impedance matching standing waves, etc. On the spatial-peak specific absorption rate. The tissue layers were varied in composition and thickness, representing the anatomical variation of the exposed head region covering the user group including adults and children ( 90% percentile). Based on the worst-case tissue layer compositions with respect to absorption at each frequency, head tissue equivalent dielectric parameters for homogeneous modeling were derived, which result in the same spatial-peak absorption. The validity of this approach for near-field exposures was demonstrated by replacing the plane wave by different near-field sources (dipoles and generic phones) and the layered structure with magnetic-resonance-image-based nonhomogeneous human head models.

CAD-oriented equivalent-circuit modeling of on-chip interconnects on lossy silicon substrate

Abstract: A new, comprehensive CAD-oriented modeling methodology for single and coupled interconnects on an Si-SiO/sub 2/ substrate is presented. The modeling technique uses a modified quasi-static spectral domain electromagnetic analysis which takes into account the skin effect in the semiconducting substrate. Equivalent-circuit models with only ideal lumped elements, representing the broadband characteristics of the interconnects, are extracted. The response of the proposed SPICE compatible equivalent-circuit models is shown to be in good agreement with the frequency-dependent transmission line characteristics of single and general coupled on-chip interconnects.

Optimization of distributed MEMS transmission-line phase shifters-U-band and W-band designs

Abstract: The design and optimization of distributed micromechanical system (MEMS) transmission-line phase shifters at both U- and W-band is presented in this paper. The phase shifters are fabricated on 500 /spl mu/m quartz with a center conductor thickness of 8000 /spl Aring/ of gold. The U-band design results in 70/spl deg//dB at 40 GHz and 90/spl deg//dB at 60 GHz with a 17% change in the MEMS bridge capacitance. The W-band design results in 70/spl deg//dB from 75 to 110 GHz with a 15% change in the MEMS bridge capacitance. The W-band phase-shifter performance is limited by the series resistance of the MEMS bridge, which is estimated to be 0.15 /spl Omega/. Calculations demonstrate that the performance of the distributed MEMS phase shifter can be greatly increased if the change in the MEMS bridge capacitance can be increased to 30% or 50%. To our knowledge, these results present the best published performance at 60 and 75-110 GHz of any nonwaveguide-based phase shifter.

Mobile ranging using low-accuracy clocks

Abstract: Our position-location technique provides location information within milliseconds and is integrated in a handheld direct-sequence spread-spectrum (DSSS) communications system. We use a standard DSSS waveform, with a state of-the-art chipping rate to provide a position location capability to an accuracy of less than 1 m in a severe multipath environment. We use a two-way time-of-arrival (TOA) measurement technique with 1-ppm clocks that eliminates the need to synchronize master and reference radio clocks. Several techniques improve TOA and, therefore, range accuracy. A loop back calibrates internal system delay. Frequency diversity orthogonalizes multipath with respect to direct path, and leading edge curve fitting of the direct path reduces the effect of multipath. Applications include location of urban war fighters, firefighters, police, and medical personnel and resources.

On-chip spiral inductors suspended over deep copper-lined cavities

Abstract: A silicon micromachining method has been developed to fabricate on-chip high-performance suspended spiral inductors. The spiral structure of an inductor was formed with polysilicon and was suspended over a 30-/spl mu/m-deep cavity in the silicon substrate beneath. Copper (Cu) was electrolessly plated onto the polysilicon spiral to achieve low resistance. The Cu plating process also metallized the inner surfaces of the cavity, forming both a good radio-frequency (RF) ground and an electromagnetic shield. High quality factors (Qs) over 30 and self-resonant frequencies higher than 10 GHz have been achieved. A study of the mechanical properties of the suspended inductors indicates that they can withstand large shock and vibration. Simulation predicts a reduction of an order of magnitude in the mutual inductance of two adjacent inductors with the 30-/spl mu/m-deep Cu-lined cavity from that with silicon as the substrate. This indicates very small crosstalk between the inductors due to the shielding effect of the cavities.

High-power time-domain measurement system with active harmonic load-pull for high-efficiency base-station amplifier design

Abstract: A measurement system combining vector corrected waveform measurements with active harmonic load-pull extends, for the first time, real-time experimental waveform engineering up to the 30-W power level. The vector correction procedure is presented in this paper. A novel harmonic load-pull approach based on the real-time measurement capability of the system is demonstrated on a 4-W LDMOS device. A 20% increase in maximum output power to 4.7 W without degrading gain and efficiency is realized. Waveform analysis at various drive and load conditions directly identifies nonlinear capacitance effects being a key design issue for the design of highly efficient power amplifier.

Microwave MEMS-based voltage-controlled oscillators

Abstract: A microwave voltage-controlled oscillator (VCO) based on coupled bonding wire inductors and microelectromechanical system (MEMS)-based variable capacitors for frequency tuning is demonstrated in this paper. The MEMS-based variable capacitors were fabricated in a standard polysilicon surface micromachining technology. The variable capacitors have a nominal capacitance of 1.4 pF and have a Q factor of 23 at 1 GHz and 14 at 2 GHz. The capacitance is variable from 1.4 to 1.9 pF as the tuning voltage is swept from 0 to 5 V. The VCO, fabricated in a 0.5 /spl mu/m CMOS technology, was assembled in a ceramic package where MEMS and CMOS dice were bonded together. The oscillator operates at 2.4 GHz, achieves a phase noise of -122 dBc/Hz at 1 MHz offset from the carrier, and exhibits a tuning range of 3.4%.

Design and development of ferroelectric tunable microwave components for Kuand K-band satellite communication systems

Abstract: Integration of a high-temperature superconductor with a nonlinear dielectric ferroelectric such as strontium titanate, i.e., SrTiO/sub 2/ (STO), has created a new class of electrically tunable low-loss microwave components. We have designed and fabricated frequency and phase agile components using a conductor/ferroelectric/dielectric two-layered microstrip configuration. Some examples of these components are: microstrip ring resonators, local oscillators, edge coupled filters, and phase-shifter circuits. These structures have been fabricated using YBa/sub 2/Cu/sub 3/O/sub 7-/spl delta// or gold conductor-based microstrip lines fabricated on lanthanum aluminate (LaAlO/sub 3/) or magnesium oxide (MgO) substrates coated with an STO thin film. Frequency and phase agility are achieved using the nonlinear dc electric-field dependence of the relative dielectric constant of STO ferroelectric thin him (E/sub rSTO/). In this paper, we will present an assessment of the progress that our group has achieved thus far toward integration of this technology into wireless and satellite communication systems.

Tunable terahertz-wave parametric oscillators using LiNbO/sub 3/ and MgO:LiNbO/sub 3/ crystals

Abstract: Coherent tunable terahertz waves were generated successfully using a terahertz-wave parametric oscillator (TPO) based on laser light scattering from the A/sub 1/-symmetry polariton mode of LiNbO/sub 3/. This method has several advantages, such as continuous and wide tunability (frequency: 0.9-3.1 THz), a relatively high peak power (more than a few milliwatts), and compactness of its system (tabletop size). In addition, the system simply requires a fixed-wavelength pump source and it is easy to tune. This paper deals with the general performance of this terahertz-wave source using the prism output-coupler method as well as the development and applications of the system. Its tunability, coherency, power, and polarization were measured, and this tunable source was used for terahertz spectroscopy to measure the absorption spectra of LiNbO/sub 3/ and water vapor. Also, the use of MgO-doped LiNbO/sub 3/ (MgO:LiNbO/sub 3/) in our terahertz regime, as well as its far-infrared properties, is described. We found that the MgO:LiNbO/sub 3/ TPO is almost five times more efficient than the undoped LiNbO/sub 3/ TPO, and we have proven that the enhancement mechanism originates from the enhanced scattering cross section of the lowest A/sub 1/-symmetry mode in a spontaneous Raman experiment.

A compact dual-polarized 8.51-GHz rectenna for high-voltage (50 V) actuator applications

Abstract: This paper describes a dual-polarized rectenna capable of producing a 50-V output voltage that can be used for driving mechanical actuators. This study demonstrates a circuit topology that allows the output of multiple rectenna elements to be combined in order to step up the output voltage. In this paper, an independent rectifying circuit is used for each of two orthogonal polarizations. By proper combination, the output voltage is doubled over that of the single polarization case. Such panels are being explored for use on the next-generation space telescope to eliminate wiring between actuators and provide for true mechanical isolation.

Effects of frequency, permittivity, and voxel size on predicted specific absorption rate values in biological tissue during electromagnetic-field exposure

Abstract: Current electromagnetic-field (EMF) exposure limits have been based, in part, on the amount of energy absorbed by the whole body. However, it is known that energy is absorbed nonuniformly during EMF exposure. The development and widespread use of sophisticated three-dimensional anatomical models to calculate specific-absorption-rate (SAR) values in biological material has resulted in the need to understand how model parameters affect predicted SAR values. This paper demonstrate the effects of manipulating frequency, permittivity values, and voxel size on SAR values calculated by a finite-difference time-domain program in digital homogenous sphere models and heterogeneous models of rat and man. The predicted SAR values are compared to empirical data from infrared thermography and implanted temperature probes.

Enhanced skin effect for partial-element equivalent-circuit (PEEC) models

Abstract: In this paper, a skin-effect modeling approach is presented that is suitable for all frequency regimes of interest and therefore is most appropriate for transient interconnect analysis. Yet, the new formulation lends itself to a model that can be abstracted for use in conjunction with surface integral and finite difference-based electromagnetic tools for interconnect modeling. While a volume filament technique is not computationally feasible at high frequencies, where a fine discretization is necessary, the formulation that is presented avoids this difficulty by carefully casting the behavior of a conductor into the form of a global surface impedance, thus requiring fewer unknowns. Several examples illustrating the ability of the proposed model to accurately capture proximity and skin-effect behaviors will be shown, Interconnect resistance and inductance per-unit-length results are given and compared with those obtained using different models.

Laguerre-SVD reduced-order modeling

Abstract: A reduced-order modeling method based on a system description in terms of orthonormal Laguerre functions, together with a Krylov subspace decomposition technique is presented. The link with Pade approximation, the block Arnoldi process and singular value decomposition (SVD) leads to a simple and stable implementation of the algorithm. Novel features of the approach include the determination of the Laguerre parameter as a function of bandwidth and testing the accuracy of the results in terms of both amplitude and phase.

Electrooptic mapping and finite-element modeling of the near-field pattern of a microstrip patch antenna

Abstract: A comprehensive electrooptic field-mapping technique is applied to the characterization of near-field radiation patterns above a microstrip patch antenna. The amplitude and phase maps of three orthogonal electric-field components, measured using electrooptic crystals above the patch, also have revealed the transition from the near field to the far field of the radiation pattern. In addition, experimental results have been compared with a finite-element method (FEM) simulation. The measurememts show superior results to the FEM simulation, especially in terms of spatial resolution and data acquisition times. Furthermore, the scattering parameter S/sub 11/ for the patch antenna has been calculated from the electrooptic measurement results of standing waves on the feeding line and compared with results from a conventional network analyzer.

An efficient algorithm for analyzing large-scale microstrip structures using adaptive integral method combined with discrete complex-image method

Abstract: An efficient algorithm combining the adaptive integral method and the discrete complex-image method (DCIM) is presented in this paper for analyzing large-scale microstrip structures. The arbitrarily shaped microstrips are discretized using triangular elements with Rao-Wilton-Glisson basis functions. These basis functions are then projected onto a rectangular grid, which enables the calculation of the resultant matrix-vector product using the fast Fourier transform. The method retains the advantages of the well-known conjugate-gradient fast-Fourier-transform method, as well as the excellent modeling capability offered by triangular elements. The resulting algorithm has the memory requirement proportional to O(N) and the operation count for the matrix-vector multiplication proportional to O(N log N), where N denotes the number of unknowns. The required spatial Green's functions are computed efficiently using the DCIM, which further speeds up the algorithm. Numerical results for some microstrip circuits and a microstrip antenna array are presented to demonstrate the efficiency and accuracy of this method.

Reduction of numerical dispersion in FDTD method through artificial anisotropy

Abstract: In this paper, a simple and computationally low-cost modification of the standard finite-difference time-domain (FDTD) algorithm is presented to reduce numerical dispersion in the algorithm. Both two- and three-dimensional cases are considered. It is shown that the maximum error in phase velocity can be reduced by a factor of 2-7, depending on the shape of the FDTD cell. Although the reduction procedure is optimal for only single frequency, numerical examples show that the proposed method can also improve the accuracy significantly in wide-band inhomogeneous problems.

Recent advances in the performance of InP Gunn devices and GaAs TUNNETT diodes for the 100-300-GHz frequency range and above

Abstract: Improved heat dissipation in InP Gunn devices resulted in RF power levels exceeding 200, 130, 80, and 25 mW at oscillation frequencies of around 103, 132, 152, and 162 GHz, respectively. Corresponding dc-to-RF conversion efficiencies exceeded 2.3% from 102 to 132 GHz. Power combining increased the available RF power levels to over 300 mW at 106 GHz, around 130 mW at 136 GHz, and more than 125 mW at 152 GHz with corresponding combining efficiencies from 80% to over 100%. Operation in a second harmonic mode yielded RF power levels of more than 3.5 mW at 214 GHz, over 2 mW around 220 GHz as well as over 1 mW around 280, 300, and 315 GHz. RF power levels exceeding 10 mW at 202 GHz, 9 mW around 210 GHz, and 4 mW around 235 GHz were obtained from GaAs TUNNETT diodes in a second harmonic mode as well. Corresponding dc-to-RF conversion efficiencies were around 1% at 202 and 210 GHz.