A new design of horizontally polarizedand dual-polarized uniplanar conical beam antennas for HIPERLAN

It is shown that a conical beam 5.2-GHz antenna suitable for HIPERLAN application, but working in horizontal polarization, can be realized as a group of microstrip patch radiators in a ring formation. Layouts with three and four patches are described, and radiation patterns are found to agree well with predictions from a simple array model. The three-patch form is smaller and gives a closer approximation to an azimuth-independent pattern. Patterns are very similar to those achieved in vertical polarization with previously reported disk antenna realizations, giving peak radiation at about 50/spl deg/ elevation. Two methods of impedance matching are found to give satisfactory results. A dual-polarized conical-beam microstrip antenna, with a strictly uniplanar conductor pattern, is also presented and realized as an array of three square patches whose corners meet a central feed point. For the second polarization, the antenna functions as a series fed array. Fairly good conical beam patterns have been obtained, though only moderate polarization purity appears to be obtainable from three-element arrays.     

High-power synchronously pumped AgGaS(2) optical parametric oscillator

Greater than 2 W of average power was generated in the infrared region by a AgGaS(2) optical parametric oscillator (OPO). A Q -switched mode-locked laser was used to pump the OPO synchronously. Tunability from 1.4 to 1.9mum and a maximum output power of 750 mW at 1.44mum were achieved with a standing-wave cavity. Redesigning the cavity into a ring configuration allowed the depleted pump, signal, and idler beams to be extracted efficiently through separate mirrors. This design generated signal and idler beams of high spatial quality at respective power levels of 1.5 and 620 mW at a pulse repetition rate of 2 kHz.     

Dynamics of Inflationary Universes with Positive Spatial Curvature

If the spatial curvature of the universe is positive, then the curvature term will always dominate at early enough times in a slow-rolling inflationary epoch. This enhances inflationary effects and hence puts limits on the possible number of e-foldings that can have occurred, independently of what happened before inflation began and in particular without regard for what may have happened in the Planck era. We use a simple multi-stage model to examine this limit as a function of the present density parameter ₀ and the epoch when inflation ends.     

Crosspolar cancellation for partial frequency reuse Earth stations

In a dual-polarised frequency reuse satellite system, an earth station may only be required to demodulate one of the two incident polarisations, or to transmit a single polarisation. In such cases rain crosspolarisation can be substantially reduced by a lossless network with one variable parameter.     

Analytical calculation of bistatic radar cross-sections for rain scatter problems

A formula is derived for the effective bistatic radar cross-section of a rain cell with Gaussian reflectivity profile assuming antennas with Gaussian radiation patterns. The size and location of the rain cell are arbitrary, as is the bistatic scattering geometry. The expression is analytical, avoiding the need for numerical integration, and general, allowing treatment of cases with only partial beam intersection. Using a recently proposed model, sidelobe contributions and multiple cells embedded within widespread rain debris can be incorporated.     

Effect of substrate thickness and metallization thickness on dispersion characteristics of CPW

Approximate conformal mapping techniques have been used for analysing the effect of finite substrate thickness on coplanar wave guide (CPW). Calculations for impedance and effective dielectric constant are presented for CPW's with finite substrate thicknesses. Analytical formulation are presented for calculations. Network analytical methods of electromagnetic fields are employed to evaluate the effect of thick metal coating on CPW. Dispersion characteristics of CPW have been plotted for various metallization thicknesses. Effect of thick metal coating on guide wavelength is also plotted. Increase in metallization thickness of CPW causes an increase in wavelength. Due to this fact characteristic impedance and effective dielectric constant decreases.     

Spectral analysis of the received signal on a 20 GHz bistatic scatter link

The paper describes spectral measurements on a 20 GHz shallow-angle bistatic scatter link using antennas directed off-axis in the vertical plane. This configuration retains the property of a line-of-sight microwave link of that the Doppler and vibration spectra being separated. Theoretical Doppler and vibration spectra are presented. In some cases the observed Doppler spectra agree with theory, and show qualitatively the expected shifts associated with drop size distribution variations. Other interesting spectra show unexpectedly high Doppler frequencies, suggesting that downdraughts are being observed. In some cases a high frequency tail on the spectra is regarded as a possible, though not conclusive, detection of the vibration component.     

Attenuation and cross-polarization measurements at 20 GHz using the ATS-6 satellite with simultaneous radar observations

Measurements of cross polarization using the 20-GHz beacon on the NASA-ATS6 satellite as received at a groundstation on Oxenhope Moor, Yorkshire, U. K. are reported. A co-sited range-gated radar was used to locate hydrometeors causing cross polarization. High cross polarization was frequently measured with little rainfall on the satellite radio path. Radar evidence shows that such "anomalous" cross polarization is associated with high-altitude particles well above the radar bright band. A pronounced correlation between atmospheric electricity, as measured with a point-discharge current probe and cross polarization is reported. In addition, abrupt changes in cross polarization were seen, coincident with lightning strokes. These results are interpreted in terms of scattering from ice needles lying in the horizontal plane and suffering orientation under the influence of static electric fields.     

Mechanism of enzymatic fluorination in Streptomyces cattleya

Recently a fluorination enzyme was identified and isolated from Streptomyces cattleya, as the first committed step on the metabolic pathway to the fluorinated metabolites, fluoroacetate and 4-fluorothreonine. This enzyme, 5'-fluoro-5'-deoxy adenosine synthetase (FDAS), has been shown to catalyze C-F bond formation by nucleophilic attack of fluoride ion to S-adenosyl-l-methionine (SAM) with the concomitant displacement of l-methionine to generate 5'-fluoro-5'-deoxy adenosine (5'-FDA). Although the structures of FDAS bound to both SAM and products have been solved, the molecular mechanism remained to be elucidated. We now report site-directed mutagenesis studies, structural analyses, and isothermal calorimetry (ITC) experiments. The data establish the key residues required for catalysis and the order of substrate binding. Fluoride ion is not readily distinguished from water by protein X-ray crystallography; however, using chloride ion (also a substrate) with a mutant of low activity has enabled the halide ion to be located in nonproductive co-complexes with SAH and SAM. The kinetic data suggest the positively charged sulfur of SAM is a key requirement in stabilizing the transition state. We propose a molecular mechanism for FDAS in which fluoride weakly associates with the enzyme exchanging two water molecules for protein ligation. The binding of SAM expels remaining water associated with fluoride ion and traps the ion in a pocket positioned to react with SAM, generating l-methionine and 5'-FDA. l-methionine then dissociates from the enzyme followed by 5'-FDA.