Isotropic Radiation Pattern Research Articles

Axial ratio improvement in aperture antennas using high-impedance ground plane

High-impedance surfaces are studied as a means of improving the radiation pattern symmetry in aperture antennas. Compared to a solid metal ground plane, the results suggest that high-impedance surfaces can significantly improve the axial ratio of circularly polarised antennas. This can be used to reduce interference between left and right polarisation components.

Cavity-backed antenna integrating a self-oscillating mixer

An active cavity-backed antenna with self-mixing capabilities is demonstrated. The FET oscillator operates at the resonance of the cavity in order to reduce the LO signal radiation. Experimental results show good performance in terms of isotropic conversion gain and radiation patterns. ©1999 John Wiley & Sons, Inc. Microwave Opt Technol Lett 21: 156–158, 1999.

Seismic radiation from explosively loaded cavities in isotropic and transversely isotropic media

Abstract We analyze the influence of cavity shape on far-field seismic-wave radiation from small explosion sources in isotropic and transversely isotropic media using the indirect boundary-element method (BEM). The analysis utilizes two possible mathematical models for the pressure fields generated by the explosion in the acoustic medium inside the cavity. One simple model for the source is a volume injection (explosion) point source inside the cavity, which generates a pressure field that arrives approximately instantaneously at each point on the cavity wall. The amplitude of the field decays at a rate inversely proportional to propagation distance. We use this model to compare results with finite-difference solutions, and the test confirms the accuracy of the BEM implementation. Strong compressional and shear signals propagate into the far-field from the source cavity, and the compressional-wave amplitude depends strongly on direction of propagation. The behavior is similar at frequencies up to 200 Hz, including those typical of regional seismic-wave propagation (1 to 10 Hz). A second model of the source, one that has been applied in previous analytical and numerical work, is to assume that the explosion will instantaneously and uniformly pressurize the source cavity. This approach yields a radiation pattern that is very different from the point source model. The compressional-wave radiation patterns have a large variation in signal strength and very strong shear waves when frequencies up to 200 Hz are included, but seismograms computed for frequency ranges of interest in regional wave propagation (around 1 Hz) have a nearly isotropic compressional-wave radiation pattern and a much smaller shear wave. This shear wave is largest for long, tunnel-like cavities or short, disklike cavities. When the cavity is located in a transversely isotropic medium, quasi-shear waves are generated, but, for the media we consider, their magnitude decreases for long, tunnel-like cavities and is larger for more equidimensional cavities. These somewhat counterintuitive results show that anisotropy can actually cause a radiation pattern to appear more isotropic than the corresponding wave fields in an isotropic medium.

Source models of the Harzer explosion from regional observations of fundamental-mode and higher mode surface waves.

Abstract For shallow source depths, the moment-tensor elements with isotropic radiation patterns, M zz and M xx + M yy , are not resolvable in moment-tensor inversions of long-period surface-wave data because their respective Green9s functions have similar frequency dependence. Examination of Green9s functions at higher frequencies shows that, while resolution for some momenttensor elements improves, it is still not possible to resolve the isotropic-radiating moments from fundamental-mode data recorded at regional distances. On the other hand, the addition of higher mode Rayleigh-wave amplitudes can improve the resolution of these moments significantly, since there are appreciable differences between the higher mode Green9s functions. An extension of Romanowicz9s algorithm is presented to invert fundamental-mode and higher mode data for the second-order moment tensor. A source of “noise” in the estimation of the moment tensor for underground explosions is the spallation of geologic strata at ground zero, since this too can be a significant source of higher modes. The inversion proposed in this paper accounts for possible higher mode excitation due to spall using the source model of Day et al . The inversion is applied to regional recordings of the Harzer explosion of 6 June 1981 at the Nevada Test Site. The results of the inversion demonstrate a trade-off between the second-order moment tensor representation and the single, directed force used to represent spall. Observations and numerical simulations of spall provide estimates of spall source parameters, which in turn constrain the moment-tensor representation of Harzer. The results of this study demonstrate the clear need for observations to characterize the spall source associated with underground explosions. Furthermore, they demonstrate the potential benefit of using higher modes to refine estimates of the explosion moment with reduced uncertainties compared to moment determinations from fundamental-mode data alone.

Improving the symmetry of radiation patterns for offset reflectors illuminated by matched feeds

The large difference between the dominant sidelobe levels in the asymmetric and symmetric plane radiation patterns of the short-focal-length offset paraboloids is minimized by illuminating the reflector with a prime-focus tilted matched feed. The amplitude of the asymmetric mode (TE/sub 21/) component in the matched feed is numerically evaluated for achieving an optimum peak cross-polar level. Effect of the feed tilt on the reflector efficiency is also indicated.< <ETX xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">&gt;</ETX>

Microstrip feeds for prime focus fed reflector antennas

The use of microstrip radiators as feeds for reflector antennas is of great current interest, as they can result in reduced feed volume and weight, and such aspects are important in a number of applications, including satellite systems. In the paper the performance characteristics, and in particular the radiation pattern symmetry and cross-polarisation, together with bandwidth and efficiency, of such feeds are quantified. Both singlepatch and small-array feeds suitable for prime focus fed circularly symmetric reflectors with a range of F/D ratios are examined. The crosspolarisation level of the reflector is found to be between − 25 and − 30 dB and is limited mainly by unwanted feed radiation and substrate surface wave scattering and, when these are well controlled, by the fact that a thin microstrip radiator is primarily a magnetic source and thus deviates from the balanced magnetic and electric Huygen source used in conventional feeds. This latter limit, together with the reduced bandwidth, stem directly from the great physical advantage of microstrip, namely its small size. Application is thus foreseen when the feed volume or weight are constrained system parameters.

A combination of linear and slot antennas for quasi-isotropic coverage

The combination of a linear antenna and slot antenna can be used to provide a relatively isotropic radiation pattern when the dipole is perpendicular to the plane of the slot. The theoretical far-field radiation pattern of a dipole flanked by a two-element array of slot antennas is formulated. The fields for two different phasings between the elements of the combination antenna are calculated and the degree of approximation to an isotropic source in the sense of the total field noted. An experimental antenna was fabricated and the radiated fields measured with good agreement found with the theoretical formulas.

Thin-wire loop antennas

An experimental investigation on the impedance and radiation characteristics of thin-wire loop antennas is described. Impedance plots and radiation patterns are presented, showing a wide range of characteristic impedances and a capability of producing nearly isotropic radiation patterns.