Comments on "Effect of wet antenna attenuation on propagation data statistics"

For original paper by Kharadly and Ross, see IEEE Trans. Antennas Propag., vol.49, p.1183-91 (2001). They presented two ad hoc corrections for the effects of water on the receiver antenna for Ka-band beacon attenuation measurements made at the University of British Columbia (UBC), Vancouver, British Columbia. While it is widely recognized that the attenuation statistics measured with the ACTS beacons require correction Crane and Rogers suggest that the corrections proposed in that paper appear to be based on unfounded assumptions. In a reply Kharadly and Ross address the comments made.     

平面光波导膜层厚度对传播损耗的影响

根据平面光波导损耗的波动理论，通过分析计算，对于传播损耗随波膜厚度的变化关系提出了新的看法，文中给出了一组以波导膜厚度为参变量的传播损耗曲线，说明在某些波导膜厚度范围内，不能有效地进行光波导传播。     

The prediction of rain attenuation statistics

The prediction of rain attenuation statistics is a question of meteorology and probability. Therefore, the possible accuracy of these predictions depends on the knowledge of meteorological mechanisms and data as well as on the statistical uncertainty due to the stochastic nature of meteorological events. Attenuations exceeded for less than 0.1 % of the time within the worst-month are caused by meteorological phenomena which are not well known. Experimental and theoretical investigations show that the inaccuracy is of the same order as the unavoidable statistical uncertainty.     

Application of synthetic storm data to evaluate simpler techniques for predicting rain attenuation statistics

The concept of employing synthetic storm data to evaluate simpler attenuation prediction techniques based on cumulative distributions of point rain rate is introduced. As examples, several versions of the «path average rain rate technique» and a new method developed by Crane are tested using 10-year rain rate data bases from several Canadian locations. The results indicate that there is no universal relationship for the ratio of the path average to the point rain rate in terms of either rain rate or percentage of time. However, Crane’s new method appears to be an improvement over earlier versions of the path average rain rate technique.     

Higher order statistics of antenna subset diversity

We use a virtual-branch technique to derive higher order statistics of the output signal-to-noise ratio (SNR) of antenna subset diversity in a multipath fading environment. In particular, closed-form expressions of the cumulants, central moments, skewness, and kurtosis for various antenna subset diversity schemes are derived. These measures characterize the statistical behavior of the output SNR distribution.     

Worst month statistics of attenuation and XPD on Earth-space path

Using the attenuation and cross-polarization discrimination (XPD) statistics measured in the CS (20/30 GHz) and BSE (12/14 GHz) centimeter wave propagation experiments performed for over four and three years, the worst month statistics of attenuation and those of XPD on earth-space path at 11.7 and 19.5 GHz are derived. The probability relation between the annual and worst month statistics is approximated by a power-law relation, and the constants for the relation are derived by a least squares procedure. It is found that the relation is independent of both frequency and polarization. The cumulative probability ratios of the worst month to annual statistics at the same attenuation or XPD values are derived as a function of annual cumulative probability. The ratios are well described by the Brussaard model of the worst month statistics. Attenuation ratio and XPD difference among the annual and worst month statistics are also derived and compared with the predicted results.     

A new method of estimating microwave attenuation over a slant propagation path based on rain gauge data

A basic problem in estimating the microwave attenuation over a satellite-earth propagation path on the basis of rain gauge data is that, for a given precipitation event, the attenuation and the rainfall records very often do not have consistent detailed correlation. Such inconsistencies can be greatly reduced if the falling speed of the rain drops, i.e., 2 to 9 m/s for drop sizes of 0.05 to 0.7 cm, is taken into account. Adjustments in time and drop size spectrum are needed for data collected from field rain gauges to realize the actual rainfall along the slant path for correlating rain gauge data with measured satellite signals.     

Frequency and attenuation dependent fade slope statistics

Results obtained from slant path propagation experiments carried out with the OLYMPUS satellite are reported. Concurrent attenuation measurements at 12.5, 20 and 30 GHz have been analyzed with regard to the rate of change of rain attenuation (fade slope). The results indicate that pronounced fade slopes occur mostly in the high attenuation range.<>     

Statistics of differential rain attenuation on adjacent earth-space propagation paths

Forty hours of observations of the three-dimensional radar reflectivity structure of rain in the Montreal area were used to simulate the centimetre-wavelength attenuations occurring simultaneously over a multitude of earth-space propagation paths. The same data base had been used in a previous study which confirmed that the radar estimates of attenuation statistics are in good agreement with attenuation distributions measured independently by passive radiometers. Differences between the attenuations occurring simultaneously over pairs of paths to a common point at the ground were tabulated as a function of angular path separation, viewing direction, radio frequency, and system margin. These data are required for evaluating the possibility of interference at an earth station caused by a signal from an adjacent satellite. In this study, values of angular separation from 2 to 9 degrees, radio frequencies from 11 to 30 GHz, elevation angles from 5 to 30 degrees, and system margins from 2 to 10 dB were considered. It was found that the cumulative distributions of differential attenuation could be characterized effectively by the attenuation difference occurring during 1 % of the time that the carrier signal suffered some attenuation, but not an attenuation exceeding the system margin. It was also found that this characteristic differential attenuation could be expressed to good approximation as a simple analytical function of the system parameters.     

Tropospheric emission and attenuation statistics at 110 GHz

The probability distribution of atmospheric emission noise from the zenith at 110 GHz has been measured by a radiometer at Slough, UK. The derived attenuations in the vertical direction exceed 13 and 6 dB for 0·5 and 5% of the time, respectively.     

Path attenuation statistics influenced by orientation of rain cells

The influence of path azimuth on fade and space diversity statistics associated with propagation along earth-satellite paths at a frequency of 18 GHz is examined. The approach utilizes the methods employed by Goldhirsh and Robison [1], and Goldhirsh [2], in which a radar rain reflectivity data base obtained during the summer of 1973 is injected into a modeling program and the attenuation along parallel earth-satellite paths are obtained for a conglomeration of azimuths. In this work the statistics are separated into two groupings: one pertaining to earth-satellite paths oriented in the northwest-southeast and the other in the northeast-southwest quadrants using a fixed elevation angle of45/deg. The latter case shows fading to be greater with a degraded space diversity suggesting rain cells to be elongated along this direction. Great circle distance intervals along which the path attenuations are greater than a fixed threshold level (i.e., 2 dB) are defined here as "attenuation cell sizes." These cell dimensions are analyzed for both sets of quadrants and are found to have average values which are larger by 2 km in the northeast-southwest quadrants; a result consistent with the fade and space diversity results. Examination of the wind direction for the 14 rain days of data analyzed shows good correlation of the average or median wind directions with the directions of maximum fading and degraded space diversity.     

Calculations of wave propagation on a helix in the attenuation region

It is a well-known fact that the attenuation region of the traveling-wave tube is very important to the performance of the tube, particularly with respect to gain and efficiency of the tube which is relatively short in terms of wavelengths. Characteristics of the attenuator in the passive tube have been measured frequently and variations of attenuation and wave velocity with respect to frequency and thickness of coating are known. In the interests of trying to understand the effect of the attenuator on efficiency, we have attempted to make certain calculations of performance of a theoretical attenuator such as is frequently used on certain spirals and the results of such calculations are presented here.     

Slant-path attenuation and depolarisation statistics at the Ka-bandin Belgium

Experimental results of long term attenuation and depolarisation statistics at 20 and 30 GHz using ESA's Olympus satellite are discussed and compared with current prediction methods. While copolar attenuation (CPA) is accurately estimated, crosspolar attenuation (XPD) is poorly predicted, mainly owing to an inadequate account of the ice depolarisation effect     

A data-gathering antenna array for transhorizon propagation research

A versatile vertical antenna array and receiving system have been developed for measuring relative phase and amplitude along the wavefront of anS-band transhorizon signal. The array, which operates at 3.200 GHz, consists of 12 parabolic antennas 1.22 meters (4 feet) in diameter which are spaced 1.39 meters apart on a tower for a total effective vertical aperture of 15.24 meters (162 lambda). The raw data outputs from the array are amplitude and differential phase successively sampled from the 12 elements. Data can be taken at variable rates of up to 100 passes per second over the 12 elements. The data are processed in a computer to form conventional uniformly illuminated array beams 0.3 degree wide with a grating lobe spacing of 3.9 degrees or to produce many combinations of correlation functions. A reference signal is used to cancel the effects of relative motion of the array support structure from the phase data. The array and receiving system and actual performance characteristics are described.     

Analysis of the effects of water on the ACTS propagation terminal antenna

The NASA advanced communications technology satellite (ACTS) propagation experiment was designed to observe the attenuation produced by rain on Earth-satellite paths operating in the Ka-band. Unwanted effects of water on the antenna reflector surface were noted. Wet-antenna attenuation could be attributed to the combined effect of a water layer on the reflector surface and water wetting the feed window surface. A model was developed to calculate the antenna reflector and feed surface water layer thickness values as a function of position on each surface. The thickness values were used to calculate the additional attenuation produced by the water layers as a function of rain rate on the antenna. The wet-antenna-attenuation prediction model was verified by sprayer tests. The goal of the ACTS propagation experiment was to obtain path attenuation statistics, statistics that represent the effects of rain on the Earth-satellite path but not on the antenna itself. The wet-antenna attenuation prediction model was used to remove the effects of water on the antenna from the combined antenna-plus-path attenuation statistics produced by the experiment. The overall efficacy of the model was demonstrated by comparing the corrected path loss statistics from two ACTS propagation experiment sites with earlier COMSTAR path loss measurements made at or near those sites. The empirical distribution functions from both the ACTS and COMSTAR experiments were identical within the expected uncertainty of an empirical annual distribution of attenuation by rain.     

A satellite data transmission antenna

A retrodirective satellite antenna for a broad bandwidth, satellite-to-ground data transmission link, is described. An information-collecting satellite which transmits its data in the direction of the ground receiver upon command, is considered. The satellite antenna described for this system is an active retrodirective antenna which utilizes the beam-forming characteristics of a phasing matrix. A signal from the ground station is received on a particular beam port of the matrix which corresponds to a particular beam-pointing direction. Control circuitry in the satellite activates a switching matrix and a transmitter so that the data-carrying signal is transmitted in the same direction from which the satellite is interrogated. Other retrodirective techniques in addition to the phasing-matrix antenna are evaluated in terms of their relative merit. The systems considered include the active Van Atta array, the automatic three-dimensional electronic scanning, antenna (ATHESA), the Luneberg lens and various other antenna array techniques.     

Ice depolarization statistics for 19-GHz satellite-to-earth propagation

Observations were made at 19 GHz of depolarization due to ice crystals along a satellite-earth path with a38.6degelevation angle. The one-year data base included sufficient information to determine depolarization for any incident polarization angle. Depolarization was often observed in the absence of significant rain-produced copolarized signal attenuation. This depolarization is caused by ice crystals whose symmetry axes, as observed in the plane perpendicular to the direction of propagation, were usually within5degof vertical and horizontal. Maximum depolarization was observed for45deglinear or circular polarizations, and never exceeded -16 dB. For vertical and horizontal incident polarizations 8-10-dB lower maximum depolarization values were observed. Depolarization due to ice was also observed during most rain attenuation events. The unknown differential phase characteristics of rain-produced depolarization prevent further exact analysis of this ice depolarization.     

Prediction of slant path rain propagation statistics usingdual-polarized radar

The authors conducted a year-long experiment in which a dual-polarized S-band radar probed the volume surrounding two 11.45-GHz satellite downlink paths during rain. Accuracy was assessed by comparison to directly measured link attenuation with two 11-GHz beacon receivers 7.3-km apart at an 18.5° elevation angle, one colocated with the radar. Drop size distributions calculated from the radar horizontal reflectivity (Z_H) and differential reflectivity (ZDR) measurements were used to predict 11.45-GHz satellite beacon attenuations. The radar-predicted attenuations and those measured on the radio links agree, both on an event basis and in terms of annual cumulative distributions     

相对湿度对0.4THz电磁波大气传输衰减的影响

太赫兹波大气衰减系数的测量精确度对太赫兹技术的工程应用有着重要的影响。介绍了一种用于测量大气衰减系数的高Q值准光谐振系统,通过测量不同相对湿度下的水分子吸收谱线,得到对应环境下的品质因数,进而计算出对应的大气衰减系数。将0.4 THz电磁波的大气衰减系数与毫米波传输模型分析结果进行了比较与分析。结果表明,在相对湿度较低的情况下,测量值与模型值相差较大;当相对湿度增加到17.79%以后,测量值与模型值吻合较好。