Symmetrical Feeding Research Articles

Periodic and irregular wave patterns in an open tubular gel reactor

In an open quasi-one-dimensional Belousov-Zhabotinsky reaction-diffusion system we investigated simple periodic, complex and irregular spatiotemporal concentration patterns emerging under oscillatory boundary conditions. Symmetric (identical) feed concentrations were used at each boundary of a tabular gel reactor and the length of the system was varied. For tubes shorter than 11 mm the intrinsic wavelength is larger than half the length of the tube. In this case regular pairs of pulse waves are generated. When the length of the tube was increased we found more complex spatiotemporal behavior. Small inevitable differences in the oscillation frequencies at the boundaries finally lead to irregular patterns. The Karhunen-Loeve decomposition technique was applied for an analysis of the observed patterns.

Sustained patterns in chlorite–iodide reactions in a one-dimensional reactor

We describe an open one-dimensional reaction–diffusion system, the Couette reactor, which provides a permanent feed of fresh reactants and simultaneously preserves the diffusion process. Different compositions of variants of chlorite–iodide reactions are fed at both ends, but there is no net axial mass flux in the reactor. The effective diffusion coefficient is the same for all chemical species, and can easily be varied from 10−2 to 1 cm2/s, permitting large size reaction–diffusion structures. Spatial bistability was obtained with the minimal chlorite–iodide reaction in symmetric feed conditions. A larger variety of patterns was observed with the chlorite–iodide–malonic acid reaction and asymmetric feed conditions: a single steady chemical front, two steady fronts, three steady fronts, a single oscillating front, two oscillating fronts, three oscillating fronts, simple colliding fronts, and a bursting pattern. The patterns were studied as a function of several parameters to determine the sequences of bifurcations. Our observations demonstrate the existence of steady chemical patterns other than Turing patterns.

Cross polarization in beam waveguide-fed Cassegrain reflector antennas

The sensitivity of the cross-polarization level to a deviation from the geometrical condition derived by M. Mizusawa and T. Kitsuregawa (1973) for different geometrical configurations is studied. This condition restricts the number of possible beam waveguide configurations for beam waveguide-fed Cassegrain reflector (BFCR) antennas. For a symmetrical feed, this condition results in a symmetrical aperture distribution with no cross-polarized component. By examining a number of beam waveguide configurations satisfying the condition, it was observed that for a linearly polarized feed, the cross-polarization level is very sensitive to a deviation from this condition. For a circularly polarized feed, deviation from this condition does not increase the cross-polarization level; however, it results in the squinting of the beam for BFCRs.< <ETX xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">&gt;</ETX>

A shaped offset-fed dual-reflector antenna

A shaping scheme based on geometric optics for offset-fed dual-reflector antennas is presented. A ray tube emerging from a symmetric feed horn is transformed, after reflections, into a circular beam with a uniform phase and a prescribed radial power distribution on the aperture. In this scheme, Snell's law was not imposed on the main reflector. Based on this approximate solution, computer runs were taken for a 5.5-m dish baseline system, and very satisfactory results were obtained. The system so designed not only gives very low sidelobes but also provides a very high aperture efficiency. At 12 GHz an estimated 84 percent of aperture efficiency was achieved in spite of the severe constraint that the ray intersecting the edge of the main reflector meet a -10-dBi criterion.

Dual offset reflectors shaped for zero cross-polarisation and prescribed aperture illumination

The problem of shaping dual offset reflectors, fed by a linearly polarised rotationally symmetric feed horn, to produce a parallel beam over a non-circular aperture with zero cross-polarisation, is considered under the assumptions of geometrical optics. The condition for zero cross-polarisation leads to a partial differential equation, which hitherto has been solved numerically, but is shown here to possess an analytical solution. For uniqueness an additional constraint is put on systems with a plane of symmetry by prescribing the power distribution along the aperture line of symmetry. A suitable choice yields a uniform aperture edge illumination.

High-efficiency microwave reflector antennas - A review

The paper provides a review of current research on microwave reflector antennas with particular regard to those antennas which produce pencil-beam radiation patterns. After a preliminary examination of microwave antennas of different types attention is focused on circularly symmetric antennas with axially symmetric feed systems. This class of antenna accounts for the largest number of applications which include microwave point-to-point communication, satellite communication, and radio astronomy. The choice of optimum feed is considered in more detail and it is demonstrated that metallic feeds with corrugated walls represent the best choice in most cases. Brief reference is also made to dielectric cone feeds which may find a place in future systems. The design of dual-reflector systems is considered in depth and different computer optimisation techniques are reviewed. These include the latest methods of diffraction optimization in which more than one of the antenna specifications is involved. An example is given where efficiency and VSWR are simultaneously optimized.

Maximum Power Transmission Between Two Reflector Antennas in the Fresnel Zone

Power transfer between two ellipsoidal reflector antennas with common focal points and dual-mode feeds has been investigated. Assuming a circularly symmetric feed pattern, the H-plane pattern of an open-end circular waveguide excited by the TE 11 mode, the maximum transmission coefficient between reflector apertures is found to be within 0.5 percent of the value computed for transmission between two circular apertures with optimum illumination described by the generalized prolate spheroidal function. Transmission loss between two reflector antennas versus illumination taper is computed for various values of the parameter [p = (ka 1 a 2 )/R]. The minimum transmission loss is obtained as a compromise between feed spill-over and aperture transmission efficiency. In view of the inconvenience of building different ellipsoidal reflectors for different antenna spacings in order to achieve maximum power transfer, we examine the feasibility of using a defocused ellipsoid to simulate ellipsoids of different focal lengths. The deviation of the aperture phase distribution of a defocused ellipsoid from a required spherical phase front is approximately given by an explicit expression. A simple upper bound of the transmission loss due to small phase deviation is obtained for a given maximum phase deviation.

On the optimum illumination taper for the objective of a microwave aerial

The Fourier-transform description of the radiation pattern from an aperture is used to obtain curves relating the overall gain of an idealized aerial system, comprising an objective and waveguide flare, with the size and illumination of the aperture of the latter. For rectangular objectives the dimensions of the aperture of the flare may be varied independently in the two planes of symmetry and the overall gain obtained by multiplying together the gain factors for the distributions across the objective in these two planes. In the case of the circular objective this no longer applies, but curves are drawn showing the variation of the gain of the system as the size of a symmetrical feed is varied by equal amounts in the two planes of symmetry. It is shown that maximum overall gain is obtained with a rectangular objective when the size of the feed is adjusted to give a primary pattern such that the illumination intensity at the edges of the objective is 8½ dB below that at the centre. The corresponding optimum taper for circular objectives is 11½ dB.Primary radiation-patterns are drawn for “constant,” “cosine” and “double-cosine” illuminations of the feed aperture. The variations of secondary beam-widths and first side-lobe levels with feed size for rectangular and circular objectives are discussed.