Rain-induced crosspolarisation and raindrop canting

A discussion is given on the effect of wind on raindrops. It is shown that wind in itself cannot be a cause of raindrop canting. Vertical wind gradients are shown to produce a horizontal force on the drops, resulting in a canting angle. Calculations, based on a general model for wind gradients between 10 and 200 m for strong winds, show that canting angles of several degrees are possible.     

Effect of raindrop size distribution and rain rate inhomogeneity on the relationship between attenuation and depolarization

In addition to attenuation, depolarization due to rain is another factor that degrades satellite propagation signals, especially in the higher frequency bands and in places that have high rates of rainfall. A formula to predict cross‐polarization as a function of attenuation has been proposed, and it is derived by a theoretical calculation using frequency, the forward scattering amplitude of raindrops, rainfall rates, the raindrop size distribution (DSD), and various other propagation parameters. In this paper, a formula for predicting cross‐polarization is derived on the basis of the assumption of a gamma‐type DSD up to 100 GHz. These results are compared with conventional exponential‐type DSDs, such as the Marshall‐and‐Palmer DSD. Moreover, for a more realistic propagation situation, we consider the effect on the aforementioned relationship of rainfall rate inhomogeneity along the propagation path. It is shown that, for practical purposes, this inhomogeneity does not have a significant effect on satellite propagation. Copyright © 2017 John Wiley & Sons, Ltd.     

Inference of raindrop size distribution from attenuation and rain rate measurements

A method to infer the raindrop size distribution from the measurements of point rain rate and rain attenuation at a frequency in centimeter or millimeter wave bands is proposed. This method is applied to the results of field propagation experiments at 81.8, 34.5, and 11.5 GHz through natural rain. It is shown that, if an appropriate frequency is employed, this method is effective as a frequency-scaling method for short terrestrial propagation paths.     

Effect of surface distortions on the crosspolarisation performance of paraboloidal reflectors

The effects on the crosspolarisation and the radiation patterns of circular paraboloidal reflectors produced by (a) reflector distortions which are periodic in the circumferential direction and (b) a lateral offset of the feed have been evaluated for a wide range of reflector and feed parameters. It is shown that the use of a balanced hybrid-mode feed minimises crosspolarisation introduced by reflector distortions.     

Prediction of dual-shaped-reflector antenna crosspolarisation

An efficient computer program for the analysis of dual-shaped-reflector antennas is described. This is used to predict the crosspolarisation of a typical earth station antenna. Employing a feed with intrinsic crosspolarisation, it is shown that the crosspolar performance of the antenna may be reflector depolarisation limited. The effect of subreflector beam steering on the crosspolar response is also discussed.     

Variations in raindrop size distribution and specific attenuation due to rain in Nigeria

The raindrop size distributions had been measured at three locations in Nigeria - Calabar, Ile-Ife and Zaria. The data collected over a period of three years have been utilized to obtain models of drop size distributions for drizzle, shower, widespread and thunderstorm rain. The lognormal distribution provides a good fit for all the rain types at these tropical locations except drizzle, where the exponential model is adequate. The implications of the models for specific attenuation calculations for tropical rain are discussed.     

Effect of profile errors with large correlation diameter on axisymmetric antenna crosspolarisation

The effect of profile errors with large correlation diameter on the boresight crosspolarisation of axisymmetric antennas is investigated. An expression for boresight isolation as a function of the correlation diameter, the r.m.s. surface error and the polarisation efficiency of the antenna is given. Curves of isolation as a function of the r.m.s. surface error and the correlation diameter are given for polarisation efficiencies of 0.99 and 0.999 (i.e. for antennas with peak off-axis cross-polarisation of ?25.3 dB and ?35.4 dB, respectively).     

Investigation of crosspolarisation in offset Cassegrain antennas

The geometric optics condition for zero crosspolarisation in offset Cassegrain antennas is discussed. It is shown that with a feed having perfect pattern symmetry the crosspolar performance of the offset Cassegrain is limited by diffraction. As a result minimum crosspolarisation is only achieved at the geometric optics condition when the subreflector diameter is at least 40 wavelengths.     

Scattering amplitudes and crosspolarisation of ice particles

The Fredholm-integral method is used to calculate accurate forward and backward scattering amplitudes of spheroidal ice particles for several frequencies in the range 4?30 GHz. Using a valuable oblique-angle rule, crosspolarisation at any elevation angle may be deduced. The implications of crosspolarisation on satellite paths are discussed.     

Rigorous analysis of crosspolarisation in flange-mounted rectangular-waveguide radiators

A rigorous Nh-order Rayleigh-Ritz variational solution to the problem of a flange-mounted rectangular waveguide radiating into a halfspace is formulated. The analysis shows the importance of including higher-order modes whose electric fields are polarised orthogonally to the dominant mode. The mechanism of coupling from the dominant mode to the higher-order modes is discussed and numerical results for the aperture admittance are given.     

Determination of crosspolarisation by the use of focal-region fields

Focal-region-field techniques can be used to determine the crosspolarisation performance of a front-fed paraboloidal antenna. Expressions are derived for the conditions that the aperture fields of a feed must satisfy to give low crosspolarisation. This extends the concept of a Huygens source to finite-aperture feeds.     

Radome crosspolarisation on Intelsat standard 'A' communication satellite antennas

The reduction in size of Intelsat standard 'A' satellite communication antennas from 30.5 m to 15 m in diameter has necessitated the computation of depolarisation effects when the smaller antennas employ metal and dielectric space frame radomes for hardening and protection. Using the standard 15 m antennas enclosed by appropriately scaled-down metal and dielectric space frame radomes, the crosspolar discrimination (XPD) and the boresight shift (BS) have been computed close to antenna axis at 4 and 6 GHz. Comparison with the larger 30.5 m antennas enclosed by 45 m radomes have been made. It is reported that within 0.1 degrees of antenna boresight, the XPD introduced by the radomes does not exceed 33.0 dB as the worst case at 4 GHz. In particular, the Intelsat Specification of 30.7 dB for the XPD within the antenna's tracking beamwidth in clear weather conditions for frequency re-use satellite communication systems is satisfied. The BS is negligibly low.<>     

Comment on raindrop size distribution model

The parameter set for the exponential raindrop size distribution model presented by Yeo, Kooi and Leong (see ibid. vol.41, P.709, 1993) was derived using a data set measured in Singapore. A limitation to the model is that it is accurate only for small rain rate or when the rain medium consists predominantly of small droplets with diameter D is < λ/π . A new parameter set that enables accurate computation of rain attenuation for droplets with diameter D = λ/π has since been derived using the same data set. This new parameter set has been proven to be more accurate in the computation of rain attenuation     

Parametric study of the crosspolarisation efficiency of parabolic reflectors

The crosspolarisation efficiency of a reflector antenna is the ratio between the peak crosspolarisation from the complete antenna to that from the feed. The results of a parametric study into this factor for parabolic reflectors fed by a mathematical model of a common class of feeds are presented, and it is concluded that the efficiency is typically ?4 to ?6 dB.     

Effect of rain structure on rain induced attenuation

The effect of rain structure on rain induced attenuation is examined. The important role of raindrop-size distribution is evident from the comparison between experimental and calculated data. Commonly adopted parameters for calculating specific attenuation versus rainfall intensity are acceptable for system design at 18 GHz but tend to underestimate attenuation at 11 GHz.     

Characteristics of rain induced attenuation and phase shift at cm and mm waves using a tropical raindrop size distribution model

The tropical raindrop size distribution model developed by Ajayi and Olsen has been employed to study some characteristics of rain induced attenuation and phase shift for a tropical location for spherical, oblate spheroidal and Pruppacher-Pitter drop shapes. Parameters such as the a and b values for the power law relation between the specific attenuation and rainfall rate as well as differential attenuation and phase shift and their normalized values, were computed. A single power law between the specific phase shift and the rain rate was found to be adequate for vertical polarization, whilst a two-segment power law fitting is required for horizontal polarization between 1GHz and about 100GHz. The results were compared in many cases with those obtained with the Laws and Parsons drop size distribution, currently adopted by the CCIR for scattering applications.     

Use of cancellation techniques in the measurement of atmospheric crosspolarisation

Some of the errors implicit in atmospheric crosspolarisation-measurement systems are given, together with their limitations on the accuracy of the resultant measurements. A cancellation technique is described that allows the `clear-weather? performance of measurement links to be considerably improved. The limitations due to rainfall of such a cancellation system on the accuracy of crosspolarisation measurements are investigated, and the system is shown to be preferable to a conventional one.     

Calculation of forward scatter and crosspolarisation from ice spheroids

Forward-scatter complex amplitudes for ice particles with the same eccentricities as raindrops are presented from a field point-matching calculation to a greater accuracy than in a previous letter. However, it is noted that significantly higher crosspolarisation is expected from small ice needles or plates. Arguments for the orientation of ice needles or plates in electric fields are emphasised.     

基于BP神经网络的雨滴谱仪设计

为了减小传统雨滴谱观测仪器的体积、重量、成本,并提高雨滴谱的测量精度,提出一种基于等距导电圆环结构的新型雨滴谱探头、Cortex-M3 ARM处理器及高精度低噪声测量电路的雨滴谱仪。雨滴谱仪利用不同直径雨滴引起输出电压的差异进行测量。以Sigmoid函数作为传递函数,利用梯度下降法建立雨滴谱仪的输出电压和雨滴直径的BP神经网络模型,并将基于该模型的算法嵌入ARM处理器,求出雨滴直径。该雨滴谱仪实现了对直径为2.67～5.56 mm雨滴的观测,测量误差小于±0.1 mm。