Calibrated Polarisation Tilt Angle Recovery for Wireless Communications

In this paper, we show how the polarisation state of a linearly polarised antenna can be recovered through the use of a three-term error correction model. The approach adopted is shown to be robust in situations where some multipath exists and where the sampling channels are imperfect with regard to both their amplitude and phase tracking. In particular, it has been shown that error of the measured polarisation tilt angle can be improved from 33% to 3% and below by applying the proposed calibration method. It is described how one can use a rotating dipole antenna as both the calibration standard and as the polarisation encoder, thus simplifying the physical arrangement of the transmitter. Experimental results are provided in order to show the utility of the approach, which could have a variety of applications including bandwidth conservative polarisation sub-modulation in advanced wireless communications systems.

EMI Suppression in an Aircraft Cooling Vent

Digital avionics systems are increasingly under threat from external electromagnetic interference (EMI). The same avionics systems require a thermal cooling mechanism and one method of providing this is to mount an air vent on the body of the aircraft. For the first time, a nacelle-mounted air vent that may expose the flight critical full authority digital engine controller (FADEC) to high intensity radiated fields (HIRF) is examined. The reflection/transmission characteristics of the vent are reported and the current shielding method employed is shown to provide a low shielding level (5 dB at 18 GHz). A new design has been proposed, providing over 100 dB of attenuation at 18 GHz. To the authors' knowledge this is the first time this shielding method has been applied to aircraft air vents.

Full-Wave Analysis of Layered Aperture Arrays

The new rigorous numerical-analytical technique based upon Galerkin method with the entire domain basis functions has been developed and applied to the study of the periodic aperture arrays containing multiple dissimilar apertures of complex shapes in stratified medium. The rapid uniform convergence of the solutions has enabled a comprehensive parametric study of complex array arrangements. The developed theory has revealed new effects of the aperture shape and layout on the array performance. The physical mechanisms underlying the TM wave resonances and Luebbers' anomaly have been explained for the first time.

Channelled Waves in Anisotropic Mesh Metamaterials

Channelled waves in 2-D periodic anisotropic L-C mesh metamaterials have been investigated. Circuit simulation and the newly developed analytical model of a unit cell have demonstrated full qualitative agreement for both lossless and lossy cases. Isofrequencies for a lattice unit cell and the circuit simulations of finite meshes have shown that propagating waves are channelled from a point source as pencil beams which can travel only along specific trajectories. The beam direction varies with frequency, and at the resonance frequency, the phase and group velocities of the travelling wave are orthogonal. The effect of losses was explored, and it was shown that losses cause qualitative changes of the channelled wave type. It was proven that the channelled waves do not follow the laws of geometrical optics (Snell's law, specular reflection, etc.) at the interfaces of L-C meshes but are governed by the conditions of phase synchronism and impedance matching. Only in the special case of dual L-C and C-L meshes with the interface parallel to the axis of rectangular grid excited at the resonance frequency (X=1) do the channels follow the trajectories of optical rays. A planar mesh test cell has been designed and used for retrieving the unit cell L-C parameters from the S-parameter measurements.

Spatial Demultiplexing in the sub-millimeter wave band using multlayer free-standing frequency selective surfaces

In this paper, we show that a multilayer freestanding slot array can be designed to give an insertion loss which is significantly lower than the value obtainable from a conventional dielectric backed printed frequency selective surface (FSS). This increase in filter efficiency is highlighted by comparing the performance of two structures designed to provide frequency selective beamsplitting in the quasioptical feed train of a submillimeter wave space borne radiometer. A two layer substrateless FSS providing more than 20 dB of isolation between the bands 316.5â??325.5 GHz and 349.5â??358.5 GHz, gives an insertion loss of 0.6 dB when the filter is orientated at 45 incidence in the TM plane, whereas the loss exhibited by a conventional printed FSS is in excess of 2 dB. A similar frequency response can be obtained in the TE plane, but here a triple screen structure is required and the conductor loss is shown to be comparable to the absorption loss of a dielectric backed FSS. Experimental devices have been fabricated using a precision micromachining technique. Transmission measurements performed in the range 250â??360 GHz are in good agreement with the simulated spectral performance of the individual periodic screens and the two multilayer freestanding FSS structures.

Quadrifilar Loop Antenna

A new quadrifilar antenna has been developed for generating circularly polarized backfire radiation. The antenna consists of two orthogonal rectangular conducting loops, each incorporating capacitive coupling and fed using either a single or two coaxial cables. Though the geometry is much simpler than a conventional quadrifilar helix antenna, the radiation pattern performance is very similar. Measured and simulated patterns are compared for two antennas with different feed arrangements. It is shown that the resonant structure can produce a cardioid pattern with a directivity of 4.5 dB (120 3-dB beamwidth) and a front-to-back ratio of more than 20 dB at the center operating frequency. A 10% impedance bandwidth (VSWR

Effect of relaxation on the oxygen K-edge electron energy-loss near-edge structure in yttria-stabilized zirconia

The electron energy-loss near-edge structure (ELNES) at the oxygen K-edge has been investigated in a range of yttria-stabilized zirconia (YSZ) materials. The electronic structure of the three polymorphs of pure ZrO2 and of the doped YSZ structure close to the 33 mol %Y2O3 composition have been calculated using a full-potential linear muffin-tin orbital method (NFP-LMTO) as well as a pseudopotential based technique. Calculations of the ELNES dipole transition matrix elements in the framework of the NFP-LMTO scheme and inclusion of core hole screening within Slater's transition state theory enable the ELNES to be computed. Good agreement between the experimental and calculated ELNES is obtained for pure monoclinic ZrO2. The agreement is less good with the ideal tetragonal and cubic structures. This is because the inclusion of defects is essential in the calculation of the YSZ ELNES. If the model used contains ordered defects such as vacancies and metal Y planes, agreement between the calculated and experimental O K-edges is significantly improved. The calculations show how the five different O environments of Zr,Y,O, are connected with the features observed in the experimental spectra and demonstrate clearly the power of using ELNES to probe the stabilization mechanism in doped metal oxides.

The near-edge structure in energy-loss spectroscopy: many-electron and magnetic effects in transition metal nitrides and carbides

We investigate the ability of the local density approximation (LDA) in density functional theory to predict the near-edge structure in electron energy-loss spectroscopy in the dipole approximation. We include screening of the core hole within the LDA using Slater's transition state theory. We find that anion K-edge threshold energies are systematically overestimated by 4.22 +/- 0.44 eV in twelve transition metal carbides and nitrides in the rock-salt (B1) structure. When we apply this 'universal' many-electron correction to energy-loss spectra calculated within the transition state approximation to LDA, we find quantitative agreement with experiment to within one or two eV for TiC, TiN and VN. We compare our calculations to a simpler approach using a projected Mulliken density which honours the dipole selection rule, in place of the dipole matrix element itself. We find remarkably close agreement between these two approaches. Finally, we show an anomaly in the near-edge structure in CrN to be due to magnetic structure. In particular, we find that the N K edge in fact probes the magnetic moments and alignments of ther sublattice.

Electron impact excitation of Ar XVII

Energies for the lowest 49 levels among the 1s(2) and 1snl (n = 2-5) configurations of Ar XVII have been calculated using the GRASP code of Dyall et al. (1989, Comput. Phys. Comm., 55, 424). Additionally, radiative rates, oscillator strengths, and line strengths are calculated for all electric dipole (E1), magnetic dipole (M1), electric quadrupole (E2), and magnetic quadrupole (M2) transitions among these levels. Furthermore, collision strengths have also been calculated for all the 1176 transitions among the above 49 levels using the Dirac Atomic R-matrix Code (DARC) of Norrington & Grant (2005, Comput. Phys. Commun., in preparation), over a wide energy range up to 580 Ryd. Resonances have been resolved in the threshold region, and effective collision strengths have been obtained over a wide temperature range up to log T-e = 7.2 K. Comparisons are made with the limited results available in the literature, and the accuracy of the data is assessed. Our energy levels are estimated to be accurate to better than 0.1%, whereas results for other parameters are probably accurate to better than 20%.

Radiative rates, collision strengths and effective collision strengths for transitions in Mo XXXIV

Radiative rates for electric dipole (E I), electric quadrupole (E2), magnetic dipole (M1), and magnetic quadrupole (M2) transitions among the lowest 60 fine-structure levels of the (1s(2)) 2S(2)2p(5), 2s2p(6), and 2S(2)2p(4)3l configurations of F-like Mo XXXIV have been calculated using the fully relativistic GRASP code. Additionally, collision strengths for transitions among these levels have been computed over a wide energy range below 3200Ry, using the Dirac Atomic R-matrix Code. Resonances have been resolved in a fine energy mesh in order to calculate the effective collision strengths. Results for radiative rates and excitation rates are presented for all transitions, and for collision strengths for transitions from the lowest three levels to the higher lying levels. The accuracy of the present data is assessed to be similar to 20%.

Radiative rates for E1, E2, M1 and M2 transitions in Fe X

Energies of the 54 levels belonging to the (1s(2)2s(2)2p(6)) 3s(2)3p(5), 3s3p(6), 3s(2)3p(4)3d and 3s3p(5)3d configurations of Fe X have been calculated using the GRASP code of Dyall et al. (1989). Additionally, radiative rates, oscillator strengths, and line strengths are calculated for all electric dipole (E1), magnetic dipole (M1), electric quadrupole (E2), and magnetic quadrupole (M2) transitions among these levels. Comparisons are made with results available in the literature, and the accuracy of the data is assessed. Our energy levels are estimated to be accurate to better than 3%, whereas results for other parameters are probably accurate to better than 20%. Additionally, the agreement between measured and calculated lifetimes is better than 10%.

Radiative rates for transitions in Fe XVII

Energies of the lowest 157 levels belonging to the (1s(2)) 2s(2)2p(6), 2s(2)p(5)3l, 2s(2)2p(5)4l, 2s(2)2p(5)4l, 2s2p(5)5l, 2s2p(6)4l and 2s2p(6)5l configurations of Fe XVII have been calculated using the GRASP code of Dyall et al. (1989). Additionally, radiative rates, oscillator strengths, and line strengths are calculated for all electric dipole (E I), magnetic dipole (M I), electric quadrupole (E2), and magnetic quadrupole (M2) transitions among these levels. Comparisons are made with the results already available in the literature, and the accuracy of the data is assessed. Our energy levels are expected to be accurate to better than M whereas results for other parameters are probably accurate to better than 20%.

Energy levels and radiative rates for transitions in Ar XIII, Ar XIV and Ar XV

Energies for 524 levels of Ar XIII, 460 levels of Ar XIV and 156 levels of Ar XV have been calculated using the GRASP code of Dyall et al. (1989). Additionally, radiative rates, oscillator strengths, and line strengths are calculated for all electric dipole (E1), magnetic dipole (M1), electric quadrupole (E2), and magnetic quadrupole (M2) transitions among these levels. Comparisons are made with the limited results available in the literature, and the accuracy of the data is assessed. Our energy levels are estimated to be accurate to better than 1%, whereas results for other parameters are probably accurate to better than 20%. Additionally, the level lifetimes derived from our radiative rates are in excellent agreement with measured values.