# IEEE Transactions on Antennas and Propagation

Published by Institute of Electrical and Electronics Engineers

Print ISSN: 0018-926X

Published by Institute of Electrical and Electronics Engineers

Print ISSN: 0018-926X

Publications

Retinitis pigmentosa and age-related macular degeneration lead to blindness through progressive loss of retinal photoreceptors. Attempts are under way to construct a visual prosthesis to recover a limited sense of vision for these patients with the aid of implantable electronic devices. The function of these microchips is to provide electrical stimulation to existing viable retinal tissues - living ganglion and bipolar cells - using an array of on-chip stimulus circuits, while the dominant mechanism for power and data communication for these implanted devices has been wireless inductive telemetry using coils. This paper describes methods and models used to estimate the heating induced in the human eye and surrounding head tissues subject to the operation of this retinal prosthesis. A two-dimensional 0.25-mm high-resolution human head model has been developed with the aid of a new semiautomatic graphical segmentation algorithm. Finite-difference-based numerical methods for both electromagnetic and thermal modeling have been used to determine the influence of the specific absorption rate (associated with 2-MHz inductive coupling to the implant) and of stimulator integrated circuit (IC) power on tissue heating under different operational conditions and different hypothesis on choroidal blood flow and properties of the complex implanted circuitry. Results, provided in Part II of this paper, show that temperature increases of approximately 0.6 and 0.4°C are induced in the midvitreous of the human eye in the absence and presence of choroidal blood flow, respectively, for a 60-electrode retinal prosthesis chip. Correspondent temperature rises of approximately 0.19 and 0.004°C on the retina are obtained for these cases. Comparison with in vivo experimental measurements on intraocular heating in dog eyes shows good agreement.

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This is the second of a series of two papers on the thermal increase in the human eye and head in response to an implanted retinal stimulator. This paper provides specific absorption rates induced in the human head by the extraocular unit and the temperature increases associated with induced electromagnetic fields and power dissipation of the implanted microchip. Results are provided for different assumptions about choroid blood flow. It is shown that computed results associated with the power dissipation of the implanted microchip, corresponding to temperature increases of approximately 0.6°C in the midvitreous of the eye and 0.2°C in the retina, closely parallel in-vivo experimental results in animals.

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The validity of the volume integral equation formulation (VIEF) is
validated for the purpose of estimating the extinction cross section
(ECS) of raindrops. The validations are performed against two
well-established models. The validated volume integral equation model is
employed to calculate the extinction cross sections of raindrops
specified by a modified Pruppacher and Pitter (MPP) model. Data are
given for drops with mean radii between 0.25 to 3.5 mm in the frequency
range 0.6 to 100 GHz. Our study has showed that results of the
extinction cross section for vertical and horizontal polarizations can
be used to predict rain attenuation in wireless communications

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An empirical formula for calculating the extinction cross section
(ECS) by raindrops over a broad frequency range is first derived based
on extensive calculations made on a widely varying in mean radius of
modified Pruppacher and Pitter (MPP) raindrop models ranging from 0.25
to 3.5 mm. The expansion coefficients in the empirical formula are
determined by least-squares curve fitting of numerical data obtained by
the volume integral equation formulation (VIEF). The formula satisfies
the frequency and raindrop size dependence. Numerical results obtained
from the empirical formula for calculating the ECS are generally in good
agreement with those calculated by the VIEF for raindrops with mean
radius varying from 0.25 to 3.5 mm in the frequency range from 0.6 to
100 GHz. The average error in the ECS is less than 10%. The formula thus
provides a simple and inexpensive method for calculating the ECS of
raindrops, which otherwise requires complicated and expensive methods of
calculation. By implementing this empirical formula of ECS into the rain
attenuation equation, a new numerically empirical formula for
calculating the specific rain attenuation is also proposed. The validity
of the empirical formula for calculating the specific rain attenuation
is also checked by comparing the obtained results of specific rain
attenuation with those obtained from Li et al.'s (1995) solution, Yeo et
al.'s (1993) measurement, and Olsen et al.'s (1978) power-law equation

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Plane-wave scattering by strip-loaded circular, multilayered dielectric cylinders is investigated in the most general case of oblique incidence and arbitrary polarization. The problem is formulated via systems of singular integral-integrodifferential equations of the first kind which are most efficiently discretized on the basis of previously developed analytical algorithms. Several internal tests along with extended comparisons with available results have been completed to validate the numerical codes. Plotted results for both the induced surface current densities and the total radar cross section reveal how the scattering properties may be controlled by changing several physical and geometrical parameters of the structure.

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In the above titled paper (ibid., vol. 52, no. 11, pp. 3012-3018, Nov 04), there were several typographical errors and mistakes. Corrections are presented here.

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In the above titled paper (ibid., vol 53, no. 1, pp. 560-563, Jan 05), there were two typographical errors. Corrections are presented here.

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Plane wave scattering by single or double slots loaded with semicircular dielectric cylinders is investigated in the most general case of oblique incidence and arbitrary polarization. To this end, systems of singular integral-integrodifferential equations of the first kind are constructed and discretized on the basis of recently developed algorithms. Several internal tests and extensive comparisons with available results were made in order to validate the numerical codes. Plotted results both for the surface magnetic current densities and the radar cross sections reveal how the scattering properties may be controlled by changing several physical and geometrical parameters of the structure.

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The paper discussed the correction to the article ldquoOptimal Interpolation Of Translation Operator In Multilevel Fast Multipole Algorithmrdquo.

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In the above titled paper (ibid., vol. 54, no. 7, pp. 2169-2173, Jul. 06), there was an error in production for the flowchart (Fig. 2). A corrected figure is presented here.

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In the above titled paper (ibid., vol. 55, pt. 2, pp. 892-906, Mar 07), several errors occurred. The corrections are presented here.

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In the above titled paper (ibid., vol. 55, pp. 2402-2408, Aug 07), Equation (10) is incorrect. The correction is presented here.

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In the above titled paper (ibid., vol. 56, no. 6, pp. 1624-1632, Jun. 08), there were mistakes in equations (16) and (17). The corrections are presented here.

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In the above titled paper (ibid., vol. 56, no. 4, pp. 927-932, Apr. 08), two errors occurred in equation (30). The correct equation is presented here.

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Errors were identified in the above titled paper (ibid., vol. 56, no. 1, pp. 151-157, Jan. 08), including several numerical errors on Table I. The errors do not affect the discussions. Corrections are presented here.

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In the above titled paper (ibid., vol. 56, no. 2, pp. 416-424, Feb 08), Fig. 1 appeared with a typographical error. The correct version is presented here.

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Studies relating to transients in cylindrical transmitting and receiving antennas generally require knowledge of both the steady-state effective length and impedance of these structures over a wide frequency range. A table for the complex radian effective half-length of cylindrical antennas less than 1.3 lambda long, when the incident electric field is polarized parallel to the axis of the conductor, is provided in this paper.

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The development of a 1.4-mm heterodyne radiometric system and the initial measurements taken with it are described. The Delta T of the system at the antenna terminals was approximately 18.5deg K for a 0.25-Hz post-correlation noise bandwidth. Measurements show that the attenuation through the atmosphere at this wavelength is primarily due to water vapor, and an estimate of the zenith attenuation is given by A (dB)= 2.8w , where w is the precipitable water in centimeters. Measurements of the antenna half-power beamwidths using the sun as a source show that at 1.4 mm the atmospheric turbulence effects are not appreciable for observations through the atmosphere at zenith angles less than 45deg with a 15-foot antenna.

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The optical design of the 1.4-m telescope for the Q/U Imaging ExperimenT (QUIET) is described. The telescope is intended to image the polarization of the cosmic microwave background (CMB) with detectors at two frequencies (Q- and W-band) with unprecedented sensitivity. The RF optics uses a classical Dragonian side-fed geometry that employs a paraboloidal main reflector illuminated by a concave hyperboloidal subreflector. The relatively large offset distance and focal length of the main reflector, and the avoidance of caustics between the two reflectors, yield relatively flat reflectors with very low cross-polarization and a wide field of view capability. The antenna is fed by either a 91-element W-band or a 19-element Q-band corrugated platelet feed horn array. The telescope is installed on the CBI platform in Chile, and data taking is underway.

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Hologram-based compact antenna test range (CATR) is a potential method for testing large antennas at submillimeter wavelengths. This paper describes testing of a 1.5-m single offset parabolic reflector antenna with a 3-m-diameter hologram-based CATR. This is the first time such a measurement is carried out at submillimeter wavelengths. The antenna tests were done in a CATR that was specifically designed and constructed for these tests. The measured radiation pattern at the frequency of 322 GHz is presented. The measured pattern corresponds reasonably well to the simulated pattern of the antenna. The effect of the quiet-zone field nonidealities on the measurement results and the reasons for the discrepancies in the measured antenna beam are discussed.

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The current maritime L-band (1.5-GHz) mobile satellite communications service (MARISAT) is not available for communications paths below 10deg elevation angles because fading and scintillation at low elevation angles introduce signal degradations. These are attributable to scattering, diffraction, and multipath effects arising from ionospheric, tropospheric, and/or sea surface irregularities. To obtain a better understanding of the above-mentioned signal degradations, propagation measurements were made in the Gulf of Mexico with a MARISAT terminal on board the SS Mobil Aero for elevation angles from ship to satellite in the 15deg to 0deg range. Results indicated that mean signal strength was severely attenuated with remarkably increased fluctuations at elevation angles below 5deg . Characterizations of signal strength and peak-to-peak fluctuations of the L-band signal as a function of elevation angles are presented.

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This paper investigates relative changes in specific absorption rates due to perturbing metallic spectacles in proximity to the face. A representative electrical property biological matter model with 25 distinct tissue types based on magnetic resonance imaging data is used with the finite-difference time-domain method. Both plane wave and dipole stimuli are investigated and are used to represent an excitation from the front of the head. The frequency range investigated is 1.5 to 3.0 GHz. Results show that metallic spectacles may significantly alter SAR level distributions within the head. Specific attention is given to energy interactions with the eyes. Results are given for several common spectacle frame shapes.

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Two independent sets of experimental results are presented on the back-scatter from plexiglass spheres. These are compared with theoretical computations for refractive indices m = 1.60, 1.61 and 1.61-0.0025i. For alpha (ratio of perimeter to wavelength) leq 7.5 , the upper limit of the experimental data, the theoretical values are practically identical to one another and agreement with experiment is excellent. For alpha >7.5 , the theoretical values diverge from one another. The small absorption coefficient has an increasingly important effect as the spheres grow larger, but even at alpha simeq 100 , the cross section remains above its asymptotic value. Except for the interval 30< alpha <40 , plexiglass spheres have larger cross sections than metal ones between 6 < alpha < 60 . This is due to the focusing action of the dielectric sphere and the back surface reflection.

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A simple technique is proposed for modeling short normal-mode
helical antennas using a commercial finite-difference time-domain (FDTD)
code with a rectangular grid and a nominal extension of the wire. The
approach allows affects on the input impedance and radiation performance
of the helix to be examined and importantly does not require
modification of the excitation subroutines. Normal-mode helical antennas
for mobile communications use at 900 and 1800 MHz were designed using
the proposed method and good agreement with measurements of impedance
and near-magnetic field strength was found. The radiating performance of
the helix was compared to that of a λ/4 monopole and generally
found to be inferior at 900 MHz due to only 19.2 % efficiency in the
presence of the head. At 1.8 GHz the two antenna types showed similar
characteristics except in regard to bandwidth, 36.1 % for the monopole
and 7.8 % for the helix, in the presence of the head. The modeled helix
antennas produce spatial peak specific absorption rate (SAR) figures
that are up to 27 % greater at 900 MHz and up to 49 % greater at 1.8 GHz
than the corresponding monopole values due to the shorter antenna

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This paper presents an experimental investigation of the wideband MIMO channel characteristics in an outdoor nonline-of-sight environment. Our wideband MIMO testbed is a true array system with four transmitting antennas and eight receiving antennas, which operates at a carrier frequency of 1.8 GHz with a bandwidth of 2.5 MHz. The measurements are conducted at the J. J. Pickle Research Campus at the University of Texas at Austin. From the measured data, the matrix channel complex impulse responses and multipath delay profiles are extracted. Statistical descriptions of the elements in the channel matrices and the frequency correlation functions are also presented. Based on the measured data, the channel capacities are computed and the Kronecker model results are evaluated for arrays of different sizes. It shows that the Kronecker model underestimates the channel capacities due to higher correlation matrices for arrays of larger sizes in outdoor environments

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A shallow (0.025 wavelengths) microstrip-fed cavity-backed-slot (CBS) antenna has been demonstrated that tunes from 1 to 1.9 GHz with better than -20 dB reflection coefficient using a single varactor diode (0.45-2.5 pF). This is possible because the slot and the cavity combine to form a single resonance, and therefore, do not need to be tuned independently. The cavity is 72 ?? 72 ?? 3.18 mm<sup>3</sup>, and is dielectrically loaded (??<sub>r</sub> = 2.1). The impedance match over the tuning range has been achieved by searching for combinations of feed and varactor locations for which the impedance is matched to 50 ?? by a single series inductor, and the input impedance is nearly identical whether the antenna is in free-space or conformally mounted on a 1.2 ?? 1.2 m<sup>2</sup> ground plane. The cross-polarization is better than -25 dB at 1.0, 1.5, and 1.9 GHz. To the authors' knowledge, this is the first demonstration of a varactor-tuned CBS antenna.

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A Markov model is constructed for the slow-fade characteristics of
a wireless mobile channel in a city environment at 1.9 GHz. A signal
trace is first generated based on a prescribed driving pattern and a
measurement database. This trace is then divided into states according
to its magnitude and a state transition matrix is constructed by
examining the frequency of state transitions. Our results show that the
statistical properties including cumulative density function, level
crossing rate, and power spectrum of the sequences generated from such
Markov model are in good agreement with those of the original signal
trace

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It is demonstrated that the insertion loss between pairs of thin, linear antennas may be calculated using fairly simple equations that are generally considered to be good engineering approximations. Although the insertion loss calculation does not involve antenna gain directly (some measurements are actually made in the near-field where gain is not defined), the result is precisely the quantity obtained using the antenna gains in Friis's transmission formula, assuming the mismatch losses are zero. Therefore, the antenna gain product is implicit in the more general insertion loss equations. The particular measurement of insertion loss used here yields a quantity called site attenuation by electromagnetic compatibility engineers. A close agreement between measured and calculated data provides confidence in the site attenuation calculations when the site is essentially perfect, and provides confidence in the gain product of the antenna pair calculated using basically the same equations as those used for insertion loss. It is assumed that one-half of the mean value of the difference between the calculated and measured data is a good estimate of individual antenna performance. For the antennas described here, this measure of performance is typically lE0.05 dB and on the outside, iE0.42 dB.

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The finite-difference time-domain technique for simulation of electromagnetic and low-density plasma phenomena is computationally expensive and can require tens of thousands of computer hours to produce one solution. Substantial gains can be made through memory streamlining (factors of 2.3times faster), efficient cache usage (factors of 3times improvement), and through better parallel design (improving scalability to four times the number of CPUs). These improvements are documented and tested across five different supercomputing hardware platforms for idealized problems designed to highlight the effect of the changes. Then, the cumulative effect of these changes are tested across the five different systems for a typical problem of interest, a relativistic magnetron, on 48 CPUs which shows a factor of two to seven reduction in run-time, or best case, from 21 h to only 3 h.

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Three methods for predicting attenuation due to diffraction are tested against a large database of 115 614 measurement points, representing 115 614 different path profiles of 100-m horizontal resolution, and vertical root mean square error of about 6 m. The signal level at each geographic measurement point is calculated as the median of about 40 basic measurements, and the measurement database is thus reduced from about four million basic measurements. The mobile measurements are taken from main FM (88-108 MHz) broadcasting emitters located in southern Norway, and the corresponding broadcasting antenna diagrams have been measured by helicopter. Path profiles are categorized by number of terrain obstructions between emitter and receiver, in order to study their effect on each propagation loss method. The current ITU method and Picquenard's construction, with a variable number of included terrain obstructions, are compared with the measurements, and difference statistics are calculated. A particular version of Picquenard's construction is shown to be better than the current ITU method in terrain of Norwegian type. This new method is, in contrast to the ITU method, within the estimated expected errors resulting from using Norwegian digital terrain elevation data.

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Radio waves with frequencies above 10 GHz at low-elevation-angle
paths are significantly affected not only by rain but also by
atmospheric irregularities in the troposphere. The tropospheric
scintillation due to irregularities of the refractive index in the
troposphere is investigated using 14/11-GHz low-elevation measurements
made during 1983 at Yamaguchi, Japan. The diurnal and seasonal
variations, frequency dependence derived from a comparison of the 11-
and 14-GHz signals, and elevation-angle dependence of the scintillation
data are presented. A comparison between the propagation data and
ground-level meteorological measurements indicated a high correlation
between the scintillation characteristics and the water vapor
contribution to the radio refractive index inferred from local humidity
and temperature data. This suggests a method for predicting the severity
of scintillation fading using local measurements of meteorological
parameters

…

Rain attenuation measurements over New Delhi carried out with a microwave radiometer installed at the National Physical Laboratory (NPL), New Delhi and operating on 11 GHz for a period of more than three years are presented. For 0.01 percent of time for the period June 1977-April 1978, the attenuation exceeded for the monsoon period is 14.0 dB whereas for the whole year, it exceeds 10.4 dB. During the winter for the same percentage of time, the attenuation exceeded 1.5 dB, whereas for March-April it exceeds 0.5 dB. For the period May 1978-June 1980, it is observed that for 0.01 percent of time the attenuation for the whole year exceeds 9.0 dB. During the winter for the same percentage of time, the attenuation exceeds 1.4 dB whereas for March-April it exceeds 0.4 dB. A comparison of attenuation over New Delhi and those reported elsewhere are discussed. Yearly and worst month time ratio over New Delhi are given also as the values reported for the European region. Comparison of the attenuation distribution and the rate of surface rainfall measured with a rapid-response rain gauge are also given. The comparison shows that for the monsoon period and for 0.01 percent of time, the attenuation value exceeded for 14 dB corresponds to the surface rainfall rate of 140 mm/h. For the monsoon of 1978, 1979, comparison shows that for 0.1 percent of time, the attenuation value exceeded for 9.0 dB corresponds to the surface rainfall rate of 90 mm/h. Variation of attenuation and effective path length for various rainfall rates and elevation angles are also given.

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The designer of a digital radio system (DRS) requires quantitative data on flat and frequency-selective fading in order to assess the need for diversity and equalization and also the feasibility of using the same repeater sites developed for systems in the lower bands. Preliminary results from an experiment which measured the statistical distribution of the complex channel response across a 1 GHz bandwidth at 11 GHz on a 36 km path are described. On this test path the cumulative distribution function of signal attenuation due to rain was found to be about an order of magnitude greater than that due to multipath fading. Incuded are typical records of the complex frequency response during frequency selective fading (showing large variations of group delay), joint cumulative distributions of amplitude, amplitude slope, and notch speed.

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Measurements of attenuation at 11 and 18 GHz from precipitation have been made on a slant path over a three-year period in the Tokyo area using a sun tracker and a radiometer. Simultaneous rain attenuation measurements of a 2.9-km terrestrial path at 19 GHz were used to clarify the correlation characteristics between the terrestrial and the slant paths. The following results are presented: frequency, seasonal, annual, and elevation angle dependence of rain attenuation, rain rate distribution, effective distance, correlation characteristics of attenuation between slant and terrestrial paths, attenuation due to snowfall, and site diversity effect for a separation of 14.3 km at 18 GHz.

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Forward scattering from rain was measured at 11 GHz over a 9-km line-of-sight path. For rain rates of 100-150 mm/h, the scattered signal was observed to be about 40 dB below the direct signal. Good overall agreement was obtained between theory and experiment.

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The diffractor grating is a new version of the microwave passive repeater developed to improve the transmission qualities of links utilizing mountain diffraction. It has special advantages in the high-frequency region, as well as in the 11-Gc/s common carrier band, where the construction of a large-aperture flat reflector encounters difficulties and becomes uneconomical. Principles and characteristics of diffractor grating are given with test results. Various factors affecting the diffractor grating behavior are investigated, and comparisons are made with the double-plane reflector system. The diffractor grating is shown to be applicable to short-haul microwave systems and possibly reduce construction costs.

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Generalized rational Krylov model-order reduction techniques are
applied to the spectral Galerkin system describing frequency selective
surfaces, resulting in surface reflection coefficient models that depend
on both the frequency and the incident angle of the exciting wave. The
procedure is composed of three steps: construction of the spectral
Galerkin system, linearization of that system, and reduction of the
linearized system. The linearization of the spectral Galerkin matrix is
carried out using two-dimensional (2-D) polynomial interpolation and the
generation of a “two-parameter” companion form of the
polynomial system. The subsequent model-order reduction is based on the
concept of generalized Krylov subspaces, which are defined in the text.
It results in a small system with a frequency and incident angle
dependent output that matches the two-parameter polynomial interpolant
system transfer function and its derivatives at many points in the
frequency/incident angle plane. The technique is applied to the
characterization of several frequency selective surfaces, and numerical
results that demonstrate the accuracy of the techniques over a broad
band of frequencies and range of incident angles are presented

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The TE_{11} mode coaxial waveguide or horn radiator provides a natural geometry for multiple frequency confocal feeding of a paraboloid reflector antenna system. Its radiation patterns, however, are generally characterized by unequal E - and H -plane beamwidths and high E -plane sidelobe amplitudes. These characteristics become increasingly more obvious for coaxial radiators designed to illuminate reflector systems requiring feeds with reduced fields of view, such as the symmetrical paraboloid reflector antenna with large focal-length-to-diameter ratio or the offset paraboloid reflector antenna. Left uncorrected, these feed characteristics severely limit the available antenna performance. Incorporation of a radial reactive iris in the aperture plane of a TE_{11} mode coaxial radiator has been found to produce rotationally symmetric pattern characteristics with reduced sidelobe levels. This paper describes an approximate technique for determining the radiation patterns of a TE_{11} mode coaxial radiator with a radial aperture reactance. Experimental results are shown that reveal the pattern improvements predicted by the analysis. A multiple frequency coaxial feed that employs radial aperture reactances is capable of providing the aforementioned reflector systems with a high performance capability.

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Observations with a phase-stable radio-link interferometer are described. Measurements of phase stability have been made at 2595 MHz with a baseline of 11.3 km. The phase behavior is worse in summer than in winter and this is attributed to changes in the atmospheric water content. Phase fluctuations are not large enough to affect seriously aperture-synthesis measurements.

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Two-site diversity gain measurements made with a pair of 11.4 GHz
satellite beacon receivers are compared to predictions based on
dual-polarized radar reflectivity data and to values calculated from the
Hodge model. All three agree closely. Examinations of data for a third
path synthesized from radar data alone indicate that baseline
orientation has a negligible effect on diversity gain

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A study of atmospheric-induced fading on 23 km links at 9.6, 11.4, and 28.8 GHz near Boulder, CO, is described. Height-gain observations and refractivity profile measurements were obtained using a 300 m tower as one end of the links. Observations were obtained when the vertical refractive index profile was smooth and when it displayed significant variations in gradient. Narrow beam and wide beam receiving antennas were used for comparison of relative performance during fading. Fading types which result from atmospheric ray bending are listed and characterized. Examples of the different fading types observed, e.g., atmospheric multipath and radio holes, are presented.

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Depolarization (XPD) data at 11.6 GHz in circular polarization collected during the SIRIO experiment on the Italian stations of Fucino and Lario are presented and analyzed. The obtained results are presented both in conditional form (i.e., XPD statistics conditioned to the co-polar attenuation (CPA)) and in "equiprobable" form (i.e., by comparing XPD-CPA values exceeded for the same fraction of the total observation time). The conditional XPD appears spread in an interval of pm5 dB around the conditional median value. This latter appears to be very close to the "equiprobable" value. The International Radio Consultative Committee (CCIR) procedure giving the "equiprobable" XPD appears fairly well supported by the Fucino data, while the Lario data give higher depolarization than predicted by CCIR.

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The results of three years of attenuation and depolarization measurements at 11.6 GHz on a 10.7deg elevation angle downlink from the SHRIO spacecraft are summarized. Cumulative distributions and worst-month statistics for rain rate, attenuation, and cross-polarization discrimination (XPD) are presented. Beacon attenuation and radiometric attenuation are compared, and the statistics of fade durations and of rapid XPD changes are discussed.

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The ratio of simultaneous attenuation measurements performed at 11.6 and 17.8 (or 17.0 GHz) with the Italian satellite SIRIO is discussed. The main application of this information is in the field of the "up-path power control" in earth-space telecommunication systems for which the requirement of estimating the uplink attenuation form measurements on the downlink will probably be taken into consideration as a tool to keep constant the total power received on board. The analysis shows that the average ratio between attenuations at 17.8 and 11.6 GHz, conditioned to the downlink attenuation (11.6 GHz) tends to decrease slightly with attenuation; it turns out to be very close to the ratio between equiprobable values of attenuation computed from the concurrent cumulative distributions (long-term frequency scaling ratio); the standard deviation is of the order of 10 percent of the average value and also exhibits some tendency to decrease; both the overall cumulative distributions and the conditional ones are well fitted by log-normal probability models with standard deviation (of the natural logarithm) constant and equal to 0.13-0.14 up to 5.5 dB in the downlink attenuation.

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Expanded aluminum honeycomb with a nominal cell width of 0.156 in was used to fabricate a 24-in diameter unzoned lens for operation at 55 GHz. The lens exhibits good focusing properties but its efficiency of only 18 percent reflects significant random phase errors. These errors are shown to result mainly from variations of the cell width. From the measured radiation characteristics the average width of the cells is deduced to be 0.161 in with a standard deviation of 0.003 in. Since phase errors associated with variations of the cell width are proportional to the lens thickness, it is deduced that the much thinner zoned lenses fabricated from this material would exhibit efficiencies approaching 50 percent.

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Experimental results of the ratio of the atmospheric amplitude scintillation spectra at 110 GHz and 36 GHz and the amplitude coherences at these two frequencies are presented. Both results agree well with the theoretical predictions.

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Results are presented of the ratios of the rainfall attenuation measured simultaneously along a common path at 36 GHz and 110 GHz. These ratios agree well with the theoretically derived ratios assuming the Laws-Parsons distribution.

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The design of rectangular dielectric resonator antennas (DRA) with ultrawide bandwidths, in the range of 60-110%, is described. The DRA exploits multiple low-Q modes with overlapping bandwidths to achieve a wide contiguous bandwidth. This is achieved using a full-length, low-permittivity inset between a higher permittivity dielectric volume and a ground plane. With the proposed dielectric arrangement and a feed inside DR, it is possible to efficiently couple a sufficient number of such overlapping modes to a 50 Ω feedline. The volume of such DRAs is also reduced by means of a finite planar conducting wall. These advantages led to an example design with a bandwidth that is significantly wide and at the same time has a smaller DR volume than conventional DRAs. A prototype antenna designed to operate in the FCC UWB band from 3.1 to 10.6 GHz has a dielectric volume of 12 × 8 × 15.2 mm<sup>3</sup> (or 1.7 × 10<sup>-3</sup> λ<sub>o</sub><sup>3</sup> at 3.1 GHz), and an average measured gain of 5 dB over the band.

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