Approximations to the radiation resistance and directivity ofcircular-loop antennas

The purpose of this article is to derive approximate formulas for the radiation resistance (R), and the directivity (D) of circular loop antennas. It is shown that simple approximations to the Bessel functions can be employed, to accurately model the oscillatory behavior of the Bessel function integral for both small and intermediate-sized loop antennas. Furthermore, when these approximations are combined with the usual asymptotic contributions to the integral in the case of a large input parameter ka (a=loop radius, a=2π/λ) accurate and relatively simple results for R and D can be secured for all loop sizes. Numerical results can, if necessary, be obtained using a simple pocket calculator     

A statistical method for obtaining the factors inelectronic-component reliability-prediction models

According to US Mil-Hdbk-217, the failure rate of most electronic components can be predicted as λ_p=λ_b×π₁ ×π₂···π_n. A statistical method of obtaining these parameters is presented. The method is based on large quantities of field data, and the more current the data are, the more accurate the values of each level of λ_b and π_i. Using these values, the models in US Mil-Hdbk-217 can be applied to predict the life of electronic products. The method can also be used to verify the present values of each level of λ_b and π_i, and to provide the basis of further amendment     

Estimation of Path Length Reduction Factor by Using One Year Rain Attenuation Statistics over a Line of Sight Link Operating at 28.75 GHz in Amritsar (INDIA)

The effect of environmental factors in general and rain droplets in particular, on microwave propagation is a very well known fact now. If the rain droplets are present in an inhomogeneous way across the path length of the microwave communication system then, a new concept of path length reduction factor is introduced which accounts for the inhomogeneous nature of the rain droplets along the path length of the microwave signal. The present paper presents results of path length reduction factor using data on attenuation levels obtained on a LOS link operating at 28.75 GHz in Amritsar region and its comparison with Crane’s and ITU-R’s model.     

A prediction model that combines rain attenuation and otherpropagation impairments along Earth-satellite paths

The rapid growth of satellite services using higher frequency bands such as the Ka-band has highlighted a need for estimating the combined effect of different propagation impairments. Many projected Ka-band services will use very small terminals and, for some, rain effects may only form a relatively small part of the total propagation link margin. It is therefore necessary to identify and predict the overall impact of every significant attenuating effect along any given path. A procedure for predicting the combined effect of rain attenuation and several other propagation impairments (at frequencies between 4 and 35 GHz) along Earth-satellite paths is presented. Where an accurate model exist for some phenomena, these have been incorporated into the prediction procedure. New models were developed, however, for rain attenuation, cloud attenuation, and low-angle fading to provide more overall accuracy, particularly at very low elevation angles (<10°). In the absence of a detailed knowledge of the occurrence probabilities of different impairments, an empirical approach is taken in estimating their combined effects. An evaluation of the procedure is made using slant-path attenuation data that have been collected with simultaneous beacon and radiometer measurements which allow a near complete account of different impairments. Results indicate that the rain attenuation element of the model provides the best average accuracy globally between 10 and 30 GHz and that the combined procedure gives prediction accuracies comparable to uncertainties associated with the year-to-year variability of path attenuation     

A simple real‐time frequency scaling technique for rain attenuation and its performance

A new algorithm has been proposed for frequency scaling of rain attenuation. It uses the real‐time estimates of a spatial variation index in the rain rate profile suggested in simple atmospheric model (SAM) to obtain the scaled attenuation. Measured data are used to validate the algorithm which showed a typical accuracy of better than 2.5 dB for 95% confidence. The time profile of a model parameter derived from measured values has also been effectively utilized to envisage the rain cell movement, particularly for high elevation conditions. Simulated data with predefined rain rate profiles were also used for validations. The relative contributions of the model error and the measurement noise to the final error were obtained. The errors are attributed to the modelling mismatch and intermediate parameter estimation error. The estimation accuracy of the algorithm was found to worsen with reducing elevation. The overall error remains within 3 dB for 0.5 dB of measurement noise and up to lowest elevation of 30°. The simple approach of the model in conjunction with the adequate accuracy makes this algorithm a potential candidate to be used for real‐time open‐loop fade mitigation.     

The Input Admittance of Thin Prolate Spheroidal Dipole Antennas with Finite Gap Widths

The input admittance of a thin prolate spheroidal dipole antenna of major and minor axes a, b and fed by a gap of finite width 2d operating at the wavelength λ (k=2π/λ) is expressed as a very slowly convergent sum of symmetric prolate spheroidal angle and radial modal functions whose computation in this paper is facilitated by a novel method for evaluating radial functions of the second kind for small and large order with high accuracy. Then, by using Infeld's (1947) method of replacing a sum by an integral, the slowly convergent part of the admittance is evaluated. Three different types of distributions for the gap electric field are considered, and numerical results for the admittance are given as a function of the spheroidal dipole's length (0⩽2a⩽λ) with its thickness (2b⩾2×10^-5 λ), as well as the gap ratio d/b, as parameters     

降雨对雷达探测性能的影响

文中拟合得到了由降雨的雷达体反射率反演雨衰减率的公式,适用频率1~100 GHz,并且给出了降雨存在时的雷达方程、最大作用距离以及雷达接收信号信噪比与信杂比的变化。文中的结果对降雨存在时雷达的目标检测具有指导意义。     

Microwave Signal Attenuation Over Terrestrial Link at 26?GHz in Malaysia

The effect of rain on the microwave systems is more critical especially for countries located in tropical and equatorial region that experience high rainfall rate throughout the year. In order to predict a reliable and an accurate rain prediction model, it is required to determine the one-minute integration time of rainfall rate together with direct measurement of rain attenuation. In order to counter the current trend of employing higher frequencies especially in tropical and equatorial regions, there is an urgent need to carry out studies related to the effect of rain in order to get a better rain attenuation prediction model. Therefore, the purpose of this study is to investigate the effect of rain on terrestrial microwave system operating at 26?GHz in Malaysia. The rain intensity with one minute integration time is measured at Universiti Teknologi Malaysia-Skudai (UTM-Skudai) and 99 rain gauges located throughout the Peninsular Malaysia. This study explains the detailed experimental set up and analyses of both rain rate and rain attenuation measurements. The analysis on large-scale study area includes the comparisons between the measured rainfall data and the Drainage and Irrigation Department (DID) rainfall data and also with the Malaysia Meteorological Services (MMS) rainfall data. This study has successfully proposed a new rain rate and rain attenuation prediction model and the obtained results show satisfactory performance and good agreement.     

Diversity improvement estimation from rain Radar databases using maximum likelihood estimation

This research examines route diversity as a fade mitigation technique in the presence of rain, for terrestrial microwave links. The improvement in availability due to diversity depends upon the complex spatio-temporal properties of rainfall. To produce a general model to predict the advantage due to route diversity it is necessary to be able to predict the correlation of rain attenuation on arbitrary pairs of microwave links. This is achieved by examination of a database of radar derived rain rate fields. Given a representative sample of rain field images, the joint rain attenuation statistics of arbitrary configurations of terrestrial links can be estimated. Existing rain field databases often yield very small numbers of high joint attenuation events. Consequently, estimates of the probability of joint high attenuation events derived from ratios of the number of occurrences can be highly inaccurate. This paper assumes that pairs of terrestrial microwave links have joint rain attenuation distributions that are bi-lognormally distributed. Four of the five distribution parameters can be estimated from ITU-R models. A maximum likelihood estimation (MLE) method is used to estimate the fifth parameter, i.e., the covariance or correlation. The predicted diversity statistics vary smoothly and yield plausible extrapolations into low probability situations.     

The Study of Rain Specific Attenuation for the Prediction of Satellite Propagation in Malaysia

Specific attenuation is the fundamental quantity in the calculation of rain attenuation for terrestrial path and slant paths representing as rain attenuation per unit distance (dB/km). Specific attenuation is an important element in developing the predicted rain attenuation model. This paper deals with the empirical determination of the power law coefficients which allow calculating the specific attenuation in dB/km from the knowledge of the rain rate in mm/h. The main purpose of the paper is to obtain the coefficients of k and α of power law relationship between specific attenuation. Three years (from 1st January 2006 until 31st December 2008) rain gauge and beacon data taken from USM, Nibong Tebal have been used to do the empirical procedure analysis of rain specific attenuation. The data presented are semi-empirical in nature. A year-to-year variation of the coefficients has been indicated and the empirical measured data was compared with ITU-R provided regression coefficient. The result indicated that the USM empirical measured data was significantly vary from ITU-R predicted value. Hence, ITU-R recommendation for regression coefficients of rain specific attenuation is not suitable for predicting rain attenuation at Malaysia.     

A dual-polarization rain profiling algorithm

A new attenuation correction algorithm based on profiles of reflectivity, differential reflectivity, and differential propagation phase shift is presented. A solution for specific attenuation retrieval in rain medium is proposed, which solves the integral equations for reflectivity and differential reflectivity with cumulative differential propagation phase shift constraint. The conventional rain profiling algorithms that connect reflectivity and specific attenuation can retrieve specific attenuation values along the radar path assuming a constant intercept parameter of the normalized drop size distribution. However, in convective storms, the drop size distribution parameters can have significant variation along the path. This paper presents a dual-polarization rain profiling algorithm for horizontal looking radars incorporating reflectivity as well as differential reflectivity profiles. The dual-polarization rain profiling algorithm has been evaluated with X-band radar observations simulated from drop size distribution derived from high-resolution S-band measurements collected by the Colorado Statue University CHILL radar. The analysis shows that the retrieved specific attenuation, differential attenuation, reflectivity, and differential reflectivity from the dual-polarization rain profiling algorithm provide significant improvement over the current algorithms.     

A Stochastic Dynamic Model of Rain Attenuation

A dynamic mathematical model of rain attenuation has been developed and is presented in this paper. This model permits the expression of analytic relationships between parameters commonly used to describe the properties of interest for communication. The dynamic model is based on the lognormal distribution of rain attenuation and utilizes a memoryless nonlinear device to transform attenuation and rain intensity into a one-dimensional Gaussian stationary Markov process. Hence, only one parameter is required to introduce the dynamic properties of rain attenuation into the model. Experimental results and the known properties of rain have been used to derive and to verify the model; comparative results are presented and demonstrate good correspondence. The application of the model to the statistical analysis of the performance of communications systems is illustrated in the paper. The use of a dynamic rain attenuation model is necessary in order to analyze radio communication systems with transmit power control to offset the effects of rain attenuation, and where the finite response time of the control system affects the performance. An advantage of the model is the simplicity with which it allows simulation of communication link performance under the influence of rain attenuation. Such simulations are of great interest for complex models of adaptive networks where several deteriorating effects, including finite response times, are present.     

基于BP网络的卫星通信雨衰预测模型

电磁波的降雨衰减对卫星通信链路具有很大的影响。在分析基于BP算法的多层前馈网络具有逼近任意连续非线性函数的能力和降雨率与降雨衰减的非线性关系的基础上,提出并建立了基于BP神经网络的卫星通信降雨衰减预测模型,根据某地区的实测数据进行训练和预测,并与ITU-R模型进行了对比。结果显示神经网络模型具有较小的误差,为降雨衰减预测提供了一种更加准确有效的方法。     

Results from the Virginia Tech propagation experiment using theOlympus satellite 12, 20 and 30 GHz beacons

A comprehensive set of propagation experiments was performed using the Olympus satellite 12, 20, and 30 GHz beacons. This set of experiments is unique in North America because of simultaneous reception of signals spanning the Ku- and Ka-bands from the same orbital slot, which permits direct inference of the frequency behavior of signal variations. The elevation angle from the receiving site in Blacksburg, VA, to the satellite was 14 degrees. Beacon, radiometric, and weather data for one year were analyzed. The statistical results for rain rate, beacon attenuation, attenuation ratios, radiometrically derived attenuation, fade duration and fade slope are presented. They are important to the design of Ku- and Ka-band satellite communication systems. The beacon attenuation results include cumulative statistics for attenuation with respect to free space and with respect to clear air. Attenuation ratio data are presented using attenuation with respect to clear air to focus on rain effects. Instantaneous attenuation ratios computed from instantaneous beacon levels were found to be nearly identical to statistical attenuation ratios obtained from cumulative attenuation statistics at each frequency     

On the Space-Time Variations of Rain Attenuation

Rain attenuation shows a considerable temporal and spatial variability. To simulate fade mitigation techniques such as route diversity, a space-time channel model which accounts for the spatial and temporal variation of rain attenuation is needed. In this paper we investigate the space-time correlation of rain attenuation utilizing 42 GHz star-like network measurements. By combining the spatial and temporal correlation properties of rain attenuation, a simulation model for generating multiple correlated rain attenuation time series based on the Maseng–Bakken model is developed. The model is validated by comparing the statistical and angular diversity properties of the model with those of measurements and theoretical diversity gain models. Furthermore, parameters for the Maseng–Bakken dynamic rain attenuation model were extracted from the star-like network measurements. In addition, using a systematic multivariable technique a model for the parameter $beta _{s}$ which controls the dynamics of rain attenuation in the Maseng–Bakken model was developed. Moreover, using available rain attenuation measurements the advantage of route diversity with selection combining is investigated.      

Cloud and Rain Effects on AltiKa/SARAL Ka-Band Radar Altimeter—Part I: Modeling and Mean Annual Data Availability

The AltiKa project, developed by the French Centre National d'Etudes Spatiales, is based on a wideband Ka-band altimeter (35.75 GHz). The technical characteristic of the instrument will offer higher performance both in terms of spatial and vertical resolutions that will lead to the improved observation of ice, coastal areas, inland waters, and wave height. An Indian Space Research Organization satellite, called Satellite with ARgos and AltiKa, will embark the AltiKa altimeter. The launch is scheduled at the end of 2010. The major drawback of Ka-band use is the attenuation of the radar signal by atmospheric liquid water. Clouds and rain effects will thus be a strong constraining factor, because the altimeter link budget imposes an attenuation of less than 3 dB. The impact of rain and clouds on Ka-band altimeter data is analyzed and quantified using an analytical model that computes AltiKa waveforms in the presence of rain or clouds. The results are then used to quantify the waveform attenuation and distortion, as well as the error induced on the altimeter geophysical parameter estimates. Because of the nonlinearity of attenuation relations, the impact of clouds/rain depends more on the cloud/rain variability within the altimeter footprint than on the mean characteristics, which makes correction using coincident rain or cloud data difficult. Small rain cell and small dense clouds can thus strongly distort the waveforms and lead to erroneous geophysical parameter estimates. The probability of 20 Hz and 1-s averaged data loss are computed from the model results and from cloud and rain climatologies. On a global scale, about 3.5% of the 20-Hz data will be lost because of rain and clouds and 2.5% of the 1-s averaged data. However, the probability strongly varies over the global ocean and can exceed 10% in the Tropics.      

Radiometric rain attenuation measurements at 11.6 GHz in Peru

Rain attenuation is one of the key design parameters when setting up satellite communications systems at frequencies above 10 GHz. Reliable and economical system design requires fairly accurate estimates of this parameter. Although rain attenuation prediction models are available for temperate climates, little has been reported for tropical regions where rain impairments can be very much more severe. This letter presents radiometrically determined rain attenuation results at 11.6 GHz in a tropical climate.<>     

Electromagnetic Waves Attenuation due to Rain: A Prediction Model for Terrestrial or L.O.S SHF and EHF Radio Communication Links

Because of the interest raised for SHF and EHF radio communications, the attenuation of electromagnetic waves by rain will always constitute a major concern for telecommunication engineers and scientists. The rain attenuation prediction models exposed in literature calculate the attenuation related to a given rain rate or else to a given percentage of time. The new model proposed in this paper, predicts with a good accuracy the percentage of time for which any given rain attenuation will be exceeded on terrestrial SHF, EHF radiowaves links, provided the rain rate R₀₀₁ (mm/h) that represents rain rate value exceeded for 0.01% of time in the locality of interest is available. R₀₀₁ (mm/h) data being available for most of the localities across the world in ITU-R data base, we may conclude that this new model proposed here, can be broadly and successfully used.     

General N-State Markov Model for Rain Attenuation Time Series Generation

Nowadays and mainly in the near future the wireless point-to-point or point-to-multipoint connections are operating in high frequency. These systems are applied in feeder network for future cellular mobile communication systems or BFWA (Broadband Fixed Wireless Access) networks. Besides the obvious benefits of the applied high carrier frequency there is a significant disadvantage, the considerable attenuation caused by precipitation, especially by rain. For accurate planning of the proposed microwave links the statistics of the expectable rain attenuation is highly important. Applying our previous research results the this work provides a general N-state Markov Chain model to generate rain attenuation time series on a proposed microwave link according to the link parameters. The first and second order rain attenuation statistics of the generated time series can be derived directly from the Markov model parameters, so the N-state Markov model can be applied for prediction of rain attenuation on the proposed link even in the early planning phase. With our proposed model very accurate realisation of the physical fade process can be achieved.     

Estimation of soil microwave effective temperature at L and C bands

The soil microwave effective temperature (T_e) is an important parameter which improves the accuracy of the soil surface moisture derived from low frequency microwave radiometric observations. A new semi-empirical model of T_e at L- and C-bands is proposed. The model is based on the following inputs: the air temperature (T_a), a deep soil temperature (T_d), and the microwave brightness temperature measured at X-band (λ≃3 cm) and V polarization (T_BXV). Unlike other approaches based on the surface temperature (T_s), the proposed model can be implemented without being dependent on the clear sky conditions required to measure T_s with a spaceborne infrared radiometer. However, the proposed model may also use T_s when available. The model was designed from a large data set simulated by a physical model for smooth bare soil. The model of T_e was then successfully validated with experimental data acquired during a ground based experiment with the multifrequency PORTOS radiometer. This model designed from smooth soil data was successfully tested on rough bare soil using experimental data