Eccentric Circumferential Cracks in Cylindrical Specimens

The effect of eccentricity on the stress intensity factors for circumferential cracks in cylindrical specimens subjected to torsional and axial loading is studied. BEM solutions are compared with proposed approximate methods and some agreement is found for the axial load cases. A simple method of how the eccentricity develops under axial loading as the crack grows is presented. It shows that the eccentricity accelerates as the crack grows and that the problem thereby can be characterized as unstable.     

Experimental study of a microstrip array on high permittivity substrate

The results of an experimental study of microstrip patch antennas on moderately thick (0.02-0.03lambda_{0}) substrates with relative permittivities of 10 and 13 are reported. Measured input impedances are qualitatively similar to those of antennas on thinner substrates with lowerepsilon_{r}, but significant differences in resonant resistance are observed for probe-fed and microstripline-fed antennas. Radiation patterns of individual elements exhibit deep scallops that appear to be caused by surface wave diffraction at the edges of the substrate, but four-element monopulse patterns exhibit a usable main beam and tracking null.     

Characterization of tapered slot antenna feeds and feed arrays

A class of feed antennas and feed antenna arrays used in the focal plane of paraboloid reflectors and exhibiting higher than normal levels of cross-polarized radiation in the diagonal planes is addressed. A model which allows prediction of element gain and aperture efficiency of the feed/reflector system is presented. The predictions are in good agreement with experimental results. Tapered slot antenna (TSA) elements are used as an example of an element of this type. It is shown that TSA arrays used in multibeam systems with small beam spacings are competitive in terms of aperture efficiency with other, more standard types of arrays incorporating waveguide type elements     

Quantum-noise theory for terahertz hot electron bolometer mixers

In this paper, we first review general quantum mechanical limits on the sensitivity of heterodyne receivers. The main aim of the paper is to explore the quantum-noise (QN) properties of hot electron bolometric (HEB) mixers. HEB mixers have a characteristic feature not found in other mixers: based on the "hot-spot" model, the conversion loss varies along the length dimension of the bolometer, and some sections of the bolometer are essentially passive, in which little frequency conversion occurs. We analyze a quantitative distributed quantum-noise model of the HEB mixer, making use of simulated hot-spot model data, that takes into account the continuous variation of the sensitivity along the bolometer bridge. An expression for the HEB receiver noise temperature, including optical input loss, is derived. We find that the predicted double-sideband receiver noise temperature agrees well with the available measured data (up to 5.3 THz). The results of our analysis suggest that QN and classical HEB noise contribute about equally at 3 THz, while at higher terahertz frequencies QN dominates. QN thus appears to show measurable effects in existing HEB mixers and will be even more important to take into account as HEB mixers continue to be developed for higher terahertz frequencies.     

Highly Packaged Terahertz Down-Converter Modules Using 3-D Integration

This letter describes the design of highly packaged heterodyne receivers for terahertz applications. The 3-D integration of a terahertz mixer with a low-noise intermediate frequency amplifier is implemented for the first time using off-the-shelf components. Thereby, an-order-of-magnitude volume and weight reduction are accomplished. We validate our packaging approach experimentally, demonstrating performance comparable to that of a similar receiver assembled with planar interconnects. These 3-D receivers can in principle constitute the basis of close-fitting, densely populated focal plane arrays.     

Cr-TiO2as an L-band maser material

The inverted susceptibility is used as a figure of merit to discuss the maser properties of Cr-TiO2atL-band frequencies. This material has the advantage of a high inversion ratio and narrow linewidth resulting in a gain per unit length 10 to 25 times larger than for ruby in the liquid helium temperature range. The optimum concentration of Cr³⁺is found to be around 0. 016 percent from measured data at 1.4 Gc/s. The gain calculated from the figure of merit agrees with measurements on an actual 1.4 Gc/s traveling-wave maser. The decrease in inversion ratio with increasing Cr³⁺concentration is an important factor which determines the behavior of the figure of merit. This variation is shown to be consistent with calculations based on a simplified spin-lattice relaxation model.     

A new integrated slot element feed array for multibeam systems

A feed array consisting of constant width slot antennas (CWSA), fed from a block containing fin-line transitions, has been developed. The array has a two-dimensional configuration, with five elements each on five parallel substrates. Beamwidths are compatible with use inf/D = 1.0multibeam systems, with optimum taper. Array element spacings are close to a factor of two smaller than for other typical arrays, and spillover efficiency is about 65 percent.     

Endfire tapered slot antennas on dielectric substrates

Endfire tapered slot antennas are suitable for many integrated circuit applications, imaging and phased arrays. We report on an investigation of single elements of such antennas, including slots which are exponentially tapered (Vivaldi), linearly tapered (LTSA) and constant width (CWSA). For antennas of all types, a good general agreement is obtained for curves of beamwidth versus length, normalized to wavelength, when one compares the data with that for traveling-wave antennas published by Zucker. An important condition for this agreement is that the effective dielectric thickness, defined in the text, is in a certain optimum range. This condition is qualitatively explained in terms of the theory for traveling-wave antennas.     

Monolithic integration of a 94 GHz AlGaAs/GaAs 2-DEG mixer onquartz substrate by epitaxial lift-off

In this paper, we report the integration of an AlGaAs/GaAs two-dimensional electron gas (2-DEG) bolometric mixer and a quartz-based microstrip circuit using the epitaxial lift-off (ELO) technique. The 1 μm thick high-mobility 2-DEG device transplanted on quartz showed no sign of degradation resulting from the ELO process. The 2-DEG mixer fabrication procedure demonstrated here is advantageous for its simplicity and uncritical choice of substrate. We obtained a minimum intrinsic conversion loss of 17 dB at 94 GHz at liquid nitrogen temperature. The measured IF bandwidth of the mixer was greater than 3 GHz