66,178 research outputs found
A theory of the city as object: or, how spatial laws mediate the social construction of urban space
A series of recent papers (Hillier et al 1993, Hillier 1996b, Hillier 2000) have outlined a generic
process by which spatial configurations, through their effect on movement, first shape, and
then are shaped by, land use patterns and densities. The aim of this paper is to make the
spatial dimension of this process more precise. The paper begins by examining a large
number of axial maps, and finds that although there are strong cultural variations in different
regions of the world, there are also powerful invariants. The problem is to understand how
both cultural variations and invariants can arise from the spatial processes that generate cities.
The answer proposed is that socio-cultural factors generate the differences by imposing a
certain local geometry on the local construction of settlement space, while micro-economic
factors, coming more and more into play as the settlement expands, generate the invariants
A Novel Design Approach to X-Band Minkowski Reflectarray Antennas using the Full-Wave EM Simulation-based Complete Neural Model with a Hybrid GA-NM Algorithm
In this work, a novel multi-objective design optimization procedure is presented for the Minkowski Reflectarray RAs using a complete 3-D CST Microwave Studio MWS-based Multilayer Perceptron Neural Network MLP NN model including the substrate constant εr with a hybrid Genetic GA and Nelder-Mead NM algorithm. The MLP NN model provides an accurate and fast model and establishes the reflection phase of a unit Minkowski RA element as a continuous function within the input domain including the substrate 1 ≤ εr ≤ 6; 0.5mm ≤ h ≤ 3mm in the frequency between 8GHz ≤ f ≤ 12GHz. This design procedure enables a designer to obtain not only the most optimum Minkowski RA design all throughout the X- band, at the same time the optimum Minkowski RAs on the selected substrates. Moreover a design of a fully optimized X-band 15×15 Minkowski RA antenna is given as a worked example with together the tolerance analysis and its performance is also compared with those of the optimized RAs on the selected traditional substrates. Finally it may be concluded that the presented robust and systematic multi-objective design procedure is conveniently applied to the Microstrip Reflectarray RAs constructed from the advanced patches
The Standing Wave Phenomenon in Radio Telescopes; Frequency Modulation of the WSRT Primary Beam
Inadequacies in the knowledge of the primary beam response of current
interferometric arrays often form a limitation to the image fidelity. We hope
to overcome these limitations by constructing a frequency-resolved,
full-polarization empirical model for the primary beam of the Westerbork
Synthesis Radio Telescope (WSRT). Holographic observations, sampling angular
scales between about 5 arcmin and 11 degrees, were obtained of a bright compact
source (3C147). These permitted measurement of voltage response patterns for
seven of the fourteen telescopes in the array and allowed calculation of the
mean cross-correlated power beam. Good sampling of the main-lobe, near-in, and
far-side-lobes out to a radius of more than 5 degrees was obtained. A robust
empirical beam model was detemined in all polarization products and at
frequencies between 1322 and 1457 MHz with 1 MHz resolution. Substantial
departures from axi-symmetry are apparent in the main-lobe as well as
systematic differences between the polarization properties. Surprisingly, many
beam properties are modulated at the 5 to 10% level with changing frequency.
These include: (1) the main beam area, (2) the side-lobe to main-lobe power
ratio, and (3) the effective telescope aperture. These semi-sinusoidsal
modulations have a basic period of about 17 MHz, consistent with the natural
'standing wave' period of a 8.75 m focal distance. The deduced frequency
modulations of the beam pattern were verified in an independent long duration
observation using compact continuum sources at very large off-axis distances.
Application of our frequency-resolved beam model should enable higher dynamic
range and improved image fidelity for interferometric observations in complex
fields. (abridged)Comment: 12 pages, 11 figures, Accepted for publication in A&A, figures
compressed to low resolution; high-resolution version available at:
http://www.astro.rug.nl/~popping/wsrtbeam.pd
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