689 research outputs found
Diffraction and Pattern Perturbation Effects in Offset Gregorian Reflector Antennas with Wideband Feeds
The offset Gregorian reflector system of the Square
Kilometer Array radio telescope is required to operate with wideband feed (WBF) antennas down to very low frequencies, where reflectors become (electrically) relatively small. The considered WBF technologies are the log-periodic (Eleven) antenna and the quadruple-ridged flared horn. This paper investigates some of the performance degradation effects that are specific for the low frequencies and typical for these WBFs, including the feed pattern perturbations and diffraction. Several performance metrics, such as the antenna receiving sensitivity and side lobe levels are considered
Analysis and design of conical transmission line power combiners
Thesis (PhD (Electrical and Electronic Engineering))--University of Stellenbosch, 2007.This dissertation presents a technique for the design of N-way conical line power combiners,
which offers significant advantages over existing axially symmetric combining techniques.
A full analytical study of conical transmission lines is done, and evaluated results are presented.
These include a proof of the cutoff frequency equation, and plots of the field patterns, of higher
order modes which are unavailable in literature.
A coaxial fed conical line combiner for 10 inputs is proposed, designed and evaluated. The
design technique relies on the uniform transmission line characteristics of the conical lines to
eliminate the need for complex full wave optimisation, typically needed in the design of the more
commonly used radial line combiners. Circuit models are instead employed to achieve a wide
matched bandwidth by using optimised stepped impedance coaxial lines to feed the combining
structure. The prototype developed at X-band displays more than an octave bandwidth with
a return loss of better than -14.5 dB. Using tapered line matching sections increases the power
handling capability of the combiner by eliminating sharp edges, and allows for tolerance insensitive
manufacture of the structure by widening conductor spacings. Such a 10-way prototype
is developed at X-band which displays a -18.7 dB return loss bandwidth of 47% with very low
losses.
A study is done to determine the limitations on the design of general N-way combiners, and
the results are incorporated into the design technique. The full process is demonstrated by
the design and simulation of a 30-way combiner at Ku-band which displays a simulated -20 dB
return loss bandwidth of 34%.
The design technique is simple to execute and requires very little full wave analysis. Results
obtained with the manufactured combiners are better than those of any previously published
axially symmetric combiners
Analysis and design of conical transmission line power combiners
Thesis (PhD (Electrical and Electronic Engineering))--University of Stellenbosch, 2007.This dissertation presents a technique for the design of N-way conical line power combiners,
which offers significant advantages over existing axially symmetric combining techniques.
A full analytical study of conical transmission lines is done, and evaluated results are presented.
These include a proof of the cutoff frequency equation, and plots of the field patterns, of higher
order modes which are unavailable in literature.
A coaxial fed conical line combiner for 10 inputs is proposed, designed and evaluated. The
design technique relies on the uniform transmission line characteristics of the conical lines to
eliminate the need for complex full wave optimisation, typically needed in the design of the more
commonly used radial line combiners. Circuit models are instead employed to achieve a wide
matched bandwidth by using optimised stepped impedance coaxial lines to feed the combining
structure. The prototype developed at X-band displays more than an octave bandwidth with
a return loss of better than -14.5 dB. Using tapered line matching sections increases the power
handling capability of the combiner by eliminating sharp edges, and allows for tolerance insensitive
manufacture of the structure by widening conductor spacings. Such a 10-way prototype
is developed at X-band which displays a -18.7 dB return loss bandwidth of 47% with very low
losses.
A study is done to determine the limitations on the design of general N-way combiners, and
the results are incorporated into the design technique. The full process is demonstrated by
the design and simulation of a 30-way combiner at Ku-band which displays a simulated -20 dB
return loss bandwidth of 34%.
The design technique is simple to execute and requires very little full wave analysis. Results
obtained with the manufactured combiners are better than those of any previously published
axially symmetric combiners
Regular sparse array direction of arrival estimation in one dimension
Traditionally regularly spaced antenna arrays follow the spatial Nyquist criterion to guarantee an unambiguous analysis. We present a novel technique that makes use of two sparse non-Nyquist regularly spaced antenna arrays, where one of the arrays is just a shifted version of the other. The method offers several advantages over the use of traditional dense Nyquist spaced arrays, while maintaining a comparable algorithmic complexity for the analysis. Among the advantages we mention: an improved resolution for the same number of receivers and reduced mutual coupling effects between the receivers, both due to the increased separation between the antennas. Because of a shared structured linear system of equations between the two arrays, as a consequence of the shift between the two, the analysis of both is automatically paired, thereby avoiding a computationally expensive matching step as is required in the use of so-called co-prime arrays. In addition, an easy validation step allows to automatically detect the precise number of incoming signals, which is usually considered a difficult issue. At the same time, the validation step improves the accuracy of the retrieved results and eliminates unreliable results in the case of noisy data. The performance of the proposed method is illustrated with respect to the influence of noise as well to the effect of mutual coupling
Primary beam effects of radio astronomy antennas -- II. Modelling the MeerKAT L-band beam
After a decade of design and construction, South Africa's SKA-MID precursor
MeerKAT has begun its science operations. To make full use of the widefield
capability of the array, it is imperative that we have an accurate model of the
primary beam of its antennas. We have taken available L-band full-polarization
'astro-holographic' observations of three antennas and a generic
electromagnetic simulation and created sparse representations of the beams
using principal components and Zernike polynomials. The spectral behaviour of
the spatial coefficients has been modelled using discrete cosine transform. We
have provided the Zernike-based model over a diameter of 10 deg averaged over
the beams of three antennas in an associated software tool (EIDOS) that can be
useful in direction-dependent calibration and imaging. The model is more
accurate for the diagonal elements of the beam Jones matrix and at lower
frequencies. As we get more accurate beam measurements and simulations in the
future, especially for the cross-polarization patterns, our pipeline can be
used to create more accurate sparse representations of MeerKAT beams.Comment: 16 pages, 18 figures. This is a pre-copyedited, author-produced PDF
of an article accepted for publication in MNRAS following peer review. The
version of record [K. M. B. Asad et al., 2021] is available online at:
https://doi.org/10.1093/mnras/stab10
Presence of papillomavirus sequences in condylomatous lesions of the mamillae and in invasive carcinoma of the breast
BACKGROUND: Viruses including EpsteināBarr virus (EBV), a human equivalent of murine mammary tumour virus (MMTV) and human papillomavirus (HPV) have been implicated in the aetiology of human breast cancer. We report the presence of HPV DNA sequences in areolar tissue and tumour tissue samples from female patients with breast carcinoma. The presence of virus in the areolarānipple complex suggests to us a potential pathogenic mechanism. METHODS: Polymerase chain reaction (PCR) was undertaken to amplify HPV types in areolar and tumour tissue from breast cancer cases. In situ hybridisation supported the PCR findings and localised the virus in nipple, areolar and tumour tissue. RESULTS: Papillomavirus DNA was present in 25 of 29 samples of breast carcinoma and in 20 of 29 samples from the corresponding mamilla. The most prevalent type in both carcinomas and nipples was HPV 11, followed by HPV 6. Other types detected were HPV 16, 23, 27 and 57 (nipples and carcinomas), HPV 20, 21, 32, 37, 38, 66 and GA3-1 (nipples only) and HPV 3, 15, 24, 87 and DL473 (carcinomas only). Multiple types were demonstrated in seven carcinomas and ten nipple samples. CONCLUSIONS: The data demonstrate the occurrence of HPV in nipple and areolar tissues in patients with breast carcinoma. The authors postulate a retrograde ductular pattern of viral spread that may have pathogenic significance
Magnetically Driven Accretion Flows in the Kerr Metric II: Structure of the Magnetic Field
We present a detailed analysis of the magnetic field structure found in
general relativistic 3D MHD simulations of accreting tori in the Kerr metric
with different black hole spins. Among the properties analyzed are the field
strength as a function of position and black hole spin, the shapes of field
lines, the degree to which they connect different regions, and their degree of
tangling. We investigate prior speculations about the structure of the magnetic
fields and discuss how frequently certain configurations are seen in the
simulations. We also analyze the distribution of current density, with a view
toward identifying possible locations for magnetic energy dissipation.Comment: Submitted to ApJ. PDF and PostScript files with high-resolution
figures are available at http://www.pha.jhu.edu/~shirose/GRMHD/PaperII
Antenna Position Estimation Through Subsampled Exponential Analysis of Signals in the Near Field
In a previous article we explored the use of a subsampled exponential analysis algorithm to find the antenna-element positions in a large irregular planar array after the installation phase. The application requires an unmanned aerial vehicle to be flown over the antenna array while transmitting several odd harmonic signals. The received signal samples at a chosen reference antenna element are then compared to those at every other element in the array in order to find its position. Previously, the far-field approximation was used to calculate the time delay between received signals. In this article the method is reconsidered for the more realistic case of when the source is in the near field of the array. A number of problems that arise are addressed, and results from a controlled simulation are presented to illustrate that the computational method works
Revival of the magnetar PSR J1622-4950: observations with MeerKAT, Parkes, XMM-Newton, Swift, Chandra, and NuSTAR
New radio (MeerKAT and Parkes) and X-ray (XMM-Newton, Swift, Chandra, and
NuSTAR) observations of PSR J1622-4950 indicate that the magnetar, in a
quiescent state since at least early 2015, reactivated between 2017 March 19
and April 5. The radio flux density, while variable, is approximately 100x
larger than during its dormant state. The X-ray flux one month after
reactivation was at least 800x larger than during quiescence, and has been
decaying exponentially on a 111+/-19 day timescale. This high-flux state,
together with a radio-derived rotational ephemeris, enabled for the first time
the detection of X-ray pulsations for this magnetar. At 5%, the 0.3-6 keV
pulsed fraction is comparable to the smallest observed for magnetars. The
overall pulsar geometry inferred from polarized radio emission appears to be
broadly consistent with that determined 6-8 years earlier. However, rotating
vector model fits suggest that we are now seeing radio emission from a
different location in the magnetosphere than previously. This indicates a novel
way in which radio emission from magnetars can differ from that of ordinary
pulsars. The torque on the neutron star is varying rapidly and unsteadily, as
is common for magnetars following outburst, having changed by a factor of 7
within six months of reactivation.Comment: Published in ApJ (2018 April 5); 13 pages, 4 figure
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