摘要:

In this paper antenna measurements in the presence of multipath waves are discussed. Methods are proposed to diagnose the degradation sources of a compact range measurement system operating at a frequency much lower than the designed frequency range. A range-gating technique is employed to improve the capability of the compact range measurement system. With this technique, the field responses over a bandwidth for each rotation angle are coherently measured, and the fast Fourier transform is applied to obtain the range profile. A suitable window function is applied to extract the desired path and eliminate all other undesired paths. The antenna pattern is plotted according to the filtered response of the desired path. We have expressed the receiving voltage of a testing antenna in terms of its gain pattern and input impedance of the testing antenna operating in multipath environment. If the bandwidth of the testing antenna is very narrow and the mismatch problem is very serious over the required bandwidth, an algorithm is proposed to compensate the mismatch effect so that the obtained radiation pattern and the antenna gain are more accurate. Numerical and experimental results have verified the effectiveness of our method.

ISSN:0920-5071    

DOI:10.1163/156939300X01210

出版商:Taylor & Francis Group    

年代:2000

数据来源: Taylor

摘要:

The dispersion relation and the wave polarisation coefficients of the electromagnetic waves in the ionospheric plasma have been obtained by considering the magnetic declination. If the magnetic declination is taken into account, the polarisation coefficients have real and imaginary parts. It is pointed out that the peculiarity of the real parts of the wave polarisation coefficients become more obvious in the vicinity of the frequency ω(= ωpe+ ωpi) in the ionospheric plasma, while has no effect on the imaginary parts. This result is different from as in the absence of the magnetic declination.

ISSN:0920-5071    

DOI:10.1163/156939300X01229

出版商:Taylor & Francis Group    

年代:2000

数据来源: Taylor

摘要:

In this paper, a general ray-theoretical representation of the X-wave pulse is introduced. This representation is based on pulsed plane waves having wave vectors lying on a conic surface. The pulsed plane wave representation is used to study the behavior of X-waves in the presence of planarly layered media. In the case of an interface separating two semi-infinite media, the pulsed plane wave representation provides a clear explanation for the disintegration of the transmitted field. It shows that unlike the incident and reflected fields, the wave vectors associated with the transmitted field do not lie on a cone. Furthermore, this approach allows one to estimate several characteristic properties of the layered media (e.g., dielectric constant and thickness of each layer) from the time of arrival of the reflected X-waves.

ISSN:0920-5071    

DOI:10.1163/156939300X01238

出版商:Taylor & Francis Group    

年代:2000

数据来源: Taylor

摘要:

The spectral structure of the reflected and transmitted fields due to a three dimensional electromagnetic X-wave incident on a planar air-dielectric interface is examined. Using a novel superposition of azimuthally dependent pulsed plane waves, it is shown that for oblique incidence the reflected pulse has a localized wave structure. On the other hand, the transmitted field maintains its localization up to a certain distance from the interface beyond which it starts disintegrating. A study of the effects of polarization on the amplitudes of the reflected and transmitted wave fields is presented.

ISSN:0920-5071    

DOI:10.1163/156939300X01247

出版商:Taylor & Francis Group    

年代:2000

数据来源: Taylor

摘要:

This paper presents a method of moments (MoM) analysis, obtains the non-uniform current distribution in closed form, and computes the resulted radiated patterns in both near and far zones, of regular hexagonal loop antennas with electrically large perimeter. An oblique incident field in its general form is considered in the formulation of the non-uniform current distributions. In the Galerkin's MoM analysis, the Fourier exponential series is considered as the full-domain basis function series. As a result, the current distributions along the hexagonal loops are expressed analytically in terms of the azimuth angle for various sizes of large loops. Finally, an alternative vector analysis of the electromagnetic (EM) fields radiated from thin hexagonal loop antennas of arbitrary length a is introduced. This method which employs the dyadic Green's function (DGF) in the derivation of the EM radiated fields makes the analysis general, compact and straightforward in both near- and far-zones. The EM radiated fields are expressed in terms of the vector wave eigenfunctions. Not only the exact solution of the EM fields in the near and far zones outside a are derived by use of the spherical Bessel and Hankel functions of the first kind respectively, but also the inner regions between √3a2 and a are characterized by both the spherical Bessel and Hankel functions of the first kind. Validity of the numerical results is discussed and clarified.

ISSN:0920-5071    

DOI:10.1163/156939300X01256

出版商:Taylor & Francis Group    

年代:2000

数据来源: Taylor

摘要:

A plane wave impinges on a perfectly conducting thick screen perforated with a periodic array of apertures with arbitrary cross-section. The transmission and reflection coefficients are determined by a mode matching technique, earlier used by other authors for the case where the apertures have the geometry of a classical waveguide. The fields outside the screen are expanded in Floquet modes, while the fields inside the apertures are expanded in waveguide modes, obtained by the finite element method (FEM). Excellent agreement between computed and measured transmission is found for a thick screen at the X-band (8.2-12.4 GHz). Moreover, the present method and the spectral Galerkin method (which assumes that the thickness of the screen is zero) is compared for very thin screens, e.g., for a frequency selective surface (FSS). Investigation of the thickness dependence is also done, and it is found that the bandwidth decreases when the thickness dependence is also done, and it is found that the bandwidth decreases when the thickness of the screen is increased. Finally two complex geometries, hexagonal and tripole apertures, are studied.

ISSN:0920-5071    

DOI:10.1163/156939300X01265

出版商:Taylor & Francis Group    

年代:2000

数据来源: Taylor

ISSN:0920-5071    

DOI:10.1163/156939300X01274

出版商:Taylor & Francis Group    

年代:2000

数据来源: Taylor