This paper presents a new exact series for modal Green's function that arises when solving the problems of electromagnetic scattering from bodies of revolution. The accuracy and computation time of the series are compared with results from numerical integration. The results using the series are in excellent agreement with those from integration. Moreover, results show that the computation time using the series is better compared to that using the integration.

In recent time, considerable interest has been shown to electromagnetic wave propagation in arid regions and the influence of sand and dust storms with emphasis on signal attenuation and cross polarization in dual polarized systems for frequency spectrum conservation. The realization of a dual-polarized system is, however, usually limited by degree of cross polar discrimination that can be achieved between the two orthogonal channels. Aside from non-sphericity of falling dust particles, cross polarization has been attributed to tendency of the falling particles to align in a particular direction (canting angle). While dust particle shape is yet to be well tackled, the existing works on cross polarization have only succeeded in guessing the canting angle. To this end, this work explicitly deals with modelling and calculations of canting angle as input to the cross polarization discrimination by considering the forces influencing orientation of the falling particle and use of turbulence shear and inertial torque. Results of the proposed model compared with some published results show close agreement.

The performance of the reflector antenna may seriously degrades in terms of the gain reduction and beam shift when a dust/sand is accreted on its surface. This paper investigates the effects on the radiation pattern of a practical parabolic reflector antenna due to the dust accretion on its surface. A modified far-fields expressions for a parabolic reflector that is fed by a rectangular waveguide and partially covered with a uniform layer of dust are formulated. These modified field expressions are used to study the radiation pattern of a reflector antenna of 1 m in diameter which is fed by a rectangular waveguide of sides 0.95λ×0.70λ operating at X-band, assuming an acretion of a uniform layer of dust on the reflector. It is found that the reduction of the maximum signal level at a given frequency changes almost periodically with a period nearly equal to π as t increases, and the thick layers of dust does not necessarily result in more reduction in the signal level or beam phase shift compared to the thin layers. For t=6 mm, the worst signal reduction and phase shift in the main beam occur when the surface of the reflector is half-covered with dust, while the full-covered reflector results in a minimum signal reduction without any phase shift in the main beam.

This paper implicitly discusses method of using dual orthogonal polarizations to optimally conserve frequency spectrum. This has, in the recent time, received considerable interest in the field of electromagnetic wave propagation in sand and dust storms. The realization of a dual-polarized system is thus limited by degree of cross polar discrimination that can be achieved between the two orthogonal channels. Cross polarization discrimination is a parameter widely used to quantify the effects of polarization interference. Apart from non-sphericity of falling dust particles, dust induced microwave cross polarization has been attributed to tendency of the particles to align in a particular direction. This paper investigates and identifies important forces acting on the alignment which are inputs to the cross polarization discrimination evaluation. The method adopted involves the use of reliable measure of turbulence shear, inertial torque and Brownian motion effects. The results showed the influence of the relevant forces on the alignment of the dust particles. Inertial torque becomes a domineering force for systematic alignment at some particle size range.

Microwave communication systems are planned to utilize orthogonal polarization. Two independent information channels of the same frequency band sent over a single link to make an optimum use of the frequency spectrum. However, above 10 GHz, the amount of rain on the transmission line can severely degrade the performance of both satellite and terrestrial links, especially in tropical regions, at millimetre wave bands. This paper evaluates the differential attenuation and differential phase shift for the prediction of cross polarization discrimination using a 10-year rain data recorded in Conakry, Guinea. The drop size distribution (DSD) was computed using Marshall and Palmer (MP) model.

The recent rapid evolution of new satellite services, including VSAT for internet access, LAN interconnection and multimedia applications, has triggered an increasing demand for bandwidth usage by satellite communications. However, these systems are susceptible to propagation effects that become significant as the frequency increases. Scintillation is the rapid signal fluctuation of the amplitude and phase of a radio wave, which is significant in tropical climates. This paper presents the analysis of the tropospheric scintillation data for satellite to Earth links at the Ku-band. Twelve months of data (January–December 2011) were collected and analyzed to evaluate the effect of tropospheric scintillation. Statistics were then further analyzed to inspect the seasonal, worst-month, diurnal and rain-induced scintillation effects. By employing the measured scintillation data, a modification of the Karasawa model for scintillation fades and enhancements is proposed based on data measured in Malaysia.

Microwave links performance during dust storms has received considerable interest in recent time with emphasis on signal attenuation. However, phase shift and cross polarization have not been tackled enough. This paper investigates the cross polarization discrimination (XPD) induced by dust storms at millimetre wave band. It introduces simple models of wave propagation through dust storms. The models are developed based on the forward scattering amplitude of dust particles using Rayleigh method. Three conditions are set to validate the suitability of the Rayleigh approximation for the model. It is shown that the method is valid for determining the scattering of ellipsoidal dust particles for the particle sizes and frequency range considered. The scattering coefficients are derived and mathematical models for phase shift and attenuation are proposed in terms of relative permittivity and visibility. Results of the proposed model compared with some published results show close agreement. Differential phase shift and attenuation are computed and XPD introduced by dust storms in such links are predicted using the model parameters as inputs. Attenuation in dry dust is only significant when the visibility becomes severe. XPD at such visibility also becomes significant i.e. numerically low. A similar trend is found as the frequency increases.

A technique that is based on spherical wave functions expansion for the free space Green's function is used to derive exact near and far electromagnetic fields expressions for traveling wave circular loop antennas (TW-CLAs) with arbitrary phase change. These expressions describe separately electromagnetic fields inside and outside a sphere that generated when the loop revolves about its diameter. This makes the investigation of fields in the near region and inside the loop antenna simpler. Furthermore, a simplified expressions for the magnetic fields along the axis of the antenna and the known far-fields expressions for TW-CLA are obtained as special cases of the general field expressions presented in this paper.

Exact electromagnetic near-fields expressions of a circular loop antenna (CLA) over a ground plane are presented in this paper. The technique is based on the spherical Bessel functions (SBFs) and associated Legendre polynomials (ALPs) expansion for the Green's function. The known far-field expressions for the loop/ground plane problem are derived from the general expressions by applying the large argument approximations for SBFs. Fields of the uniform CLA on the axis of the loop are obtained as a special cases.

In this paper an exact electromagnetic near-fields expressions for an array of circular loop antennas (CLA) are presented. The technique is based on the spherical wave functions expansion for the free space Green's function. Fields of the uniform current CLAs array and that on the axis of the array, as well as the fields of the so called gate antenna arrangement, are obtained from the general expressions as a special cases. The known far-field expressions for the array of CLAs are derived from the general expressions which check the correctness of these general field expressions.

This paper discusses the effect of propagation of electromagnetic waves through a cloud of very small particles. The size of these particles is normally less than 0.2 mm in radius, much smaller than the microwave wavelength; therefore it is possible to avoid laborious computation by using approximations. In this work we have checked the Rayleigh theory (Rayleigh scatterer) against a full solution of Maxwell's main equations through a Point Matching Technique (PMT). The agreement between Rayleigh approximations and PMT for these very small particles is found to be good. These results have been used in quantifying some telecommunication channel impairments.

Cross polarization in microwave link, method of using dual orthogonal polarizations to optimally conserve the frequency spectrum, has received considerable interest in the recent time. This paper implicitly discusses its prediction in sand and dust storms. Cross polarization in dust storms occurs due to the non-sphericity of the falling dust particles and the tendency of the particles to align in a particular direction at a time (canting angle). It is therefore important to have adequate information on the dust particle sizes, particle shapes, orientation of particles along the propagation path etc. for the cross polarization due to dust storms. This work deals with the latter two points by making use of a more reliable measure of turbulence shear and inertial torque to tackle orientation of sand and dust storms suspended particles calculations as input to cross polarization discrimination magnitudes. The paper explicitly analyses the systematic alignment of particles due to torque of fluid flow round dust particles as a main parameter causing cross polarization. The results obtained show that there exists some form of systematic alignment of particles in the most relevant size range that was considered.

The performance of the circular loop antenna (CLA) largely improves in terms of impedance matching and directive gain when a body of revolution (BOR)-type of reflector is placed symmetrically relative to the loop. The input impedance, reflection coefficient, VSWR, directivity, and radiation patterns of a loop antenna for cases of a reflector-hemisphere and a reflector-cone are investigated in this letter. It is found that for a resonant loop, directivity over 9 dB and VSWR less than 2 over a bandwidth of 48.5 MHz can be obtained. The study also shows that the characteristics of the CLA depend mainly on the height of the loop over the reflector and the total surface area of the reflector-body irrespective of the body shape.

In order to compute the scattering properties of individual sand/dust particles dispersed in air, as in a true sand/dust storm, we need to derive a permittivity value which is representative of sand/dust particle with its layer of water. As a first step in this, we applied Looyenga’s formula to extract the permittivity of the dry sand/dust component from the mixture which is measured. Measured results could be used to estimate electromagnetic wave propagation impairments in communication systems.

Sand and Dust storms have the potentiality of causing microwave degradation for substantial percentage of time which can lead to drop in the availability and quality of services rendered by communication systems. This paper investigates induced cross-polarization by dust storms at MMW bands. It introduces a model factor to calculate phase shift and attenuation which are to be used as inputs to cross-polarization discrimination (XPD) computation. A parametric analysis of sand and dust storm in northern Nigeria was carried out and its effect on MMW band propagation.

Scintillation is the rapid signal fluctuations of amplitude and phase of a radiowave which can cause signal loss in the transmission path with time. Other propagation factors that contribute to the signal fluctuations must be omitted before using the raw data for scintillation studies. Scintillation occurs continuously whether it is under clear sky condition or raining. Therefore, during raining event, the scintillation data will be accompanied by the signal level attenuation caused by the rain. Hence, attention must be given when analyzing scintillation data during raining event. This paper presents the data analysis of the tropospheric scintillation for earth to satellite link at Ku-band. Eight months (May 2011 till December 2011) data were collected and were analyzed to see the effect of tropospheric scintillation during morning, midday, evening and midnight. In this paper, the scintillation data were analyzed during clear sky condition. Any spurious signals caused by other propagation factors were eliminated accordingly. The experimental data were collected using 2.4 m dish antenna through MEASAT 3 at 10.982 GHz. The elevation angle of the dish antenna is stationed at 77.5°. The findings show that scintillation amplitude during midday is highest if compared to morning, evening and midnight. During midnight, the scintillation amplitudes drop significantly.

Theoretical results show that dust/sand accretion on reflector antennas is a real threat for slant-path communication systems, this includes satellite-to-Earth as well as high-low ground-to-ground microwave systems. Although little information on the accretion process is available it is guessed that under dry conditions these antennas will not form dust/sand layers more than a fraction of a millimeter thick, which will induce an attenuation of a fraction of a decibel; however, this will induce a beam shift of a fraction of a degree, which is significant to satellite communication systems. These threats are particularly significant in severe dust/sand storm countries. The moisture content of the dust is discussed.

A review is given of the literature covering the effects of dust storms on microwave links. Most authors have calculated the effects of absorption of energy, revealing that it is not very significant unless very high suspended dust densities are assumed. A few papers indicate the possibility of more significant cross polarisation. Air refractive effects associated with dust storms have been relatively neglected but are strongly indicated by observations.

A model describing the effect of a sand/dust accretion on the aperture field and gain of a reflector antenna is checked experimentally and extended to predict enhanced crosspolarisation. The largest effects theoretically occur for accretion layers that are uniformly thick but only cover part of the reflector.

9 GHz measurements of effects of dust accretion on the surface of a reflector antenna confirm an earlier prediction of a larger gain reduction when the dust layer covers half the area. Crosspolarisation levels were higher than expected theoretically. Deposits on the primary feed also cause high crosspolarisation.