106 research outputs found
Leaky wave antenna with amplitude controlled beam steering based on composite right/left-handed transmission lines
An antenna comprising two different composite right/left-handed transmission
line structures is proposed which enables easy beam steering at an operation
frequency of 10 GHz. The composite right/left-handed transmission
lines are based on planar, periodically arranged via free unit cells,
implemented in microstrip technology. Both transmission lines exhibit the
infinite wavelength phenomenon which occurs at 9.72 GHz and
9.89 GHz, respectively. Thus, operating the different leaky wave
structures at 10 GHz, radiation with azimuth angles of ±8°
and ±17° can be achieved depending on the selected input port. In
order to obtain a tunable main beam direction, the radiation patterns of both
structures are superimposed by feeding them simultaneously. The influence of
each guiding structure, and hence the direction of the main beam, can be
controlled via the feeding amplitude. As a result of this, the beam can be
steered between ±17° with a gain of up to 10 dBi. The guiding
structures are arranged in parallel with a clearance of <i>a</i>=12.2 mm
which is less than half of the wavelength in free space. This allows in a
further step the attachment of additional guiding structures in order to
increase the tunable angle range or creating an antenna array with a small
beamwidth in the elevation plane without the occurrence of grating lobes. An
antenna prototype was fabricated and validated by measurements
Design and analysis of an isotropic two-dimensional planar Composite Right/Left-Handed waveguide structure
A two-dimensional isotropic Composite Right/Left-Handed (CRLH) waveguide
structure is proposed which is designed for operation in <i>X</i>-band. The
balanced structure possesses left-handed behaviour over a large bandwidth
from 7.5 GHz up to its transition frequency at 10 GHz. Above
this region, the unit cell behaves in a right-handed manner up to
13.5 GHz. Operating the structure within these bands yields a
frequency dependent index of refraction ranging from −2.5 ≤ <i>n </i> ≤ 0.8.
Isotropic characteristics are obtained between 8.5 GHz ≤ <i>f </i> ≤ 12 GHz resulting in −1.5 ≤ <i>n</i> ≤ 0.8. The planar CRLH
structure is designed based on transmission line theory, implemented in
microstrip technology and optimized using full-wave simulation software. An
equivalent circuit model is determined describing the electromagnetic
behaviour of the structure whose element values are obtained by even and odd
mode analysis. The design of the unit cell requires an appropriate
de-embedding process in order to enable an analysis in terms of dispersion
characteristics and Bloch impedance, which are performed both
A fully probe corrected near-field far-field transformation technique employing plane-wave synthesis
The far-field behavior of an antenna under test (AUT) can be obtained by
exciting the AUT with a plane wave. In a measurement, it is sufficient if the
plane wave is artificially generated in the vicinity of the AUT. This can be
achieved by using a virtual antenna array formed by a probe antenna which is
sequentially sampling the radiating near-field of the AUT at different
positions. For this purpose, an optimal filter for the virtual antenna array
is computed in a preprocessing step. Applying this filter to the near-field
measurements, the far-field of the AUT is obtained according to the
propagation direction and polarization of the synthesized plane wave. This
means that the near-field far-field transformation (NFFFT) is achieved simply
by filtering the near-field measurement data. Taking the radiation
characteristic of the probe antenna into account during the synthesis
process, its influence on the NFFFT is compensated.
The principle of the
plane-wave synthesis and its application to the NFFFT is presented in detail
in this paper. Furthermore, the method is verified by performing
transformations of simulated near-field measurement data and of near-field
data measured in an anechoic chamber
Interdigital Resonators in Wideband Ridged-Waveguide Filters
An interdigital resonator approach for wideband filter applications in ridged-waveguide technology is presented. The interdigital arrangement of the ridged-waveguide resonators ensures stronger coupling between the resonators. As the coupling sections are consequently enlarged by the interdigital arrangement of the resonators, more feasible filter structures are possible at increasing frequencies.
The approach itself can be easily implemented with conventional filter synthesis formulas, which is demonstrated by two 20 GHz examples with a bandwidth of 2 GHz and 100 MHz, respectively. The designed filters are subsequently compared to the standard implementation of ridged-waveguide filters.</p
Accuracy and Conditioning of Surface-Source Based Near-Field to Far-Field Transformations
The conditioning and accuracy of various inverse surface-source formulations
are investigated. First, the normal systems of equations are discussed. Second,
different implementations of the zero-field condition are analyzed regarding
their effect on solution accuracy, conditioning, and source ambiguity. The
weighting of the Love-current side constraint is investigated in order to
provide an accurate problem-independent methodology.
The transformation results for simulated and measured near-field data show a
comparable behavior regarding accuracy and conditioning for most of the
formulations. Advantages of the Love-current solutions are found only in
diagnostic capabilities. Regardless of this, the Love side constraint is a
computationally costly way to influence the iterative solver threshold, which
is more conveniently controlled with the appropriate type of normal equation.
The solution behavior of the inverse surface-source formulations is mostly
influenced by the choice of the reconstruction surface. A spherical Huygens
surface leads to the best conditioning, whereas the most accurate solutions are
found with a tight, possibly convex hull around the antenna under test.Comment: 15 pages, 13 figures, 4 tables, accepted for publication in IEEE
Transactions on Antennas and Propagatio
Patient risk profiles and practice variation in nonadherence to antidepressants, antihypertensives and oral hypoglycemics
BACKGROUND: Many patients experience difficulties in following treatment recommendations. This study's objective is to identify nonadherence risk profiles regarding medication (antidepressants, antihypertensives, and oral hypoglycemics) from a combination of patients' socio-demographic characteristics, morbidity presented within general practice and medication characteristics. An additional objective is to explore differences in nonadherence among patients from different general practices. METHODS: Data were obtained by linkage of a Dutch general practice registration database to a dispensing registration database from the year 2001. Subjects included in the analyses were users of antidepressants (n = 4,877), antihypertensives (n = 14,219), or oral hypoglycemics (n = 2,428) and their GPs. Outcome variables were: 1) early dropout i.e., a maximum of two prescriptions and 2) refill nonadherence (in patients with 3+ prescriptions); refill adherence < 80% was considered as nonadherence. Multilevel modeling was used for analyses. RESULTS: Both early dropout and refill nonadherence were highest for antidepressants, followed by antihypertensives. Risk factors appeared medication specific and included: 1) non-western immigrants being more vulnerable for nonadherence to antihypertensives and antidepressants; 2) type of medication influencing nonadherence in both antihypertensives and antidepressants, 3) GP consultations contributing positively to adherence to antihypertensives and 4) somatic co-morbidity influencing adherence to antidepressants negatively. There was a considerable range between general practices in the proportion of patients who were nonadherent. CONCLUSION: No clear risk profiles for nonadherence could be constructed. Characteristics that are correlated with nonadherence vary across different types of medication. Moreover, both patient and prescriber influence adherence. Especially non-western immigrants need more attention with regard to nonadherence, for example by better monitoring or communication. Since it is not clear which prescriber characteristics influence adherence levels of their patients, there is need for further research into the role of the prescriber
Is new drug prescribing in primary care specialist induced?
<p>Abstract</p> <p>Background</p> <p>Medical specialists are often seen as the first prescribers of new drugs. However, the extent to which specialists influence new drug prescribing in primary care is largely unknown.</p> <p>Methods</p> <p>This study estimates the influence of medical specialists on new drug prescribing in primary care shortly after market introduction. The influence of medical specialists on prescribing of five new drugs was measured in a cohort of 103 GPs, working in 59 practices, over the period 1999 until 2003. The influence of medical specialists on new drug prescribing in primary care was assessed using three outcome measures. Firstly, the proportion of patients receiving their first prescription for a new or reference drug from a specialist. Secondly, the proportion of GPs prescribing new drugs before any specialist prescribes to their patients. Thirdly, we compared the time until the GP's first own prescribing between GPs who waited for prescriptions from specialists and those who did not.</p> <p>Results</p> <p>The influence of specialists showed considerable differences among the new drugs studied. The proportion of patients receiving their first prescription from a specialist was greatest for the combination salmeterol/fluticasone (60.2%), and lowest for rofecoxib (23.0%). The proportion of GPs prescribing new drugs before waiting for prescriptions from medical specialists ranged from 21.1% in the case of esomeprazole to 32.9% for rofecoxib. Prescribing new drugs by specialists did not shorten the GP's own time to prescribing.</p> <p>Conclusion</p> <p>This study shows that the influence of medical specialists is clearly visible for all new drugs and often greater than for the existing older drugs, but the rapid uptake of new drugs in primary care does not seem specialist induced in all cases. GPs are responsible for a substantial amount of all early prescriptions for new drugs and for a subpopulation specialist endorsement is not a requisite to initiate in new drug prescribing. This contradicts with the idea that the diffusion of newly marketed drugs always follows a two-step model, with medical specialists as the innovators and GPs as the followers.</p
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