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A novel technique for the elimination of the open stopband in one-dimensional periodic printed leaky-wave antennas is presented. A quarter-wave transformer, or alternatively a matching stub, is introduced into the unit cell of the antenna, which forces the Bloch-wave impedance of the structure to remain real and non-zero at broadside. The effectiveness of the proposed technique is first demonstrated on a printed periodic microstrip leaky-wave antenna consisting of a single radiating stub per unit cell, which exhibits a significant stopband at broadside. The technique is then also applied to a structure consisting of two radiating stubs per unit cell, which is capable of mitigating, but not eliminating, the open stopband. In both cases the open stopband at broadside is completely suppressed. Index Terms--broadside scanning, leaky-wave antennas, microstrip, one-dimensional (1-D) periodicity, open stopband, printed antennas.

A design of a linear array antenna having two radiation nulls and otherwise omnidirectional radiation pattern is reported. A new configuration of a waveguide broad wall longitudinal slot doublet with positive and negative offsets with respect to the waveguide center line is used to design the array. This type of doublet produces two radiation IF nulls. Each of the null directions can be shifted at least up to [+ or -]25[degrees] by proper choice of slot offset. Resonance and radiation characteristics of the proposed doublet are also presented. Finally, one linear array antenna consisting of seven doublets has been fabricated. Measurements show that the 15 dB null widths are 14.2[degrees] and the null depths are at least 24.8 dB. Maximum gain ripple in the omnidirectional region is 1.75 dB. Index Terms--Array, doublet, null, omnidirectional, slot, waveguide.

The possibility is explored for exploiting the properties of AMC surfaces for controlling the radiation pattern of simple dipole antennas for mobile phone street furniture applications, where the main lobe of radiation is significantly off boresight. Predictions are shown for a halfwave dipole above a square patch AMC, which illustrate the technique operating in the GSM 880-960 MHz frequency band. It will be shown that a profile reduction of 37.4% can be achieved. Index Terms--Antennas radiation patterns, artificial magnetic conductors (AMC).

A varactor-loaded H-shaped antenna with radiation pattern control is described. The antenna has a varactor in each of the four arms. A cylindrical wire model with four varactors is analyzed by the method of moments. The analysis, using the 'FEKO' simulator, shows the figure-of-eight radiation pattern is rotated by 360[degrees] in the plane of the antenna with variation of capacitances of the four varactors. An optimum set of capacitances is derived, considering the beamwidth, null level of the radiation pattern, and voltage standing wave ratio (VSWR). A prototype antenna implemented on FR4 substrate has dimensions of 60 mm by 200 mm with a line width of 2 mm. The dc voltages were supplied to the four varactors via the antenna arms where slits were inserted. A 110 [OMEGA] feed line and 110 [OMEGA]-to-50 [OMEGA] balun were designed for the minimum VSWR, whilst maintaining good radiation properties. The antenna exhibits rotation of a figure-of-eight radiation pattern with a VSWR less than 2.7, and the maximum gain from -2.9 dBi to 1.4 dBi at 750 MHz. Index Terms--Active antennas, method of moments, road vehicles, UHF antennas, varactors.

A new method is presented, which allows the recovery of the cumulative distribution function (CDF) of point rain rate with 1-min integration time (P[(R).sub.1]) from rain rate CDFs with longer integration time (in this case, 1-hour time average rain rate statistics P[(R).sub.60]). The method generates the scaling factors between P[(R).sub.60] and P[(R).sub.1] as a function of the probability level and of the site geographical coordinates. From the scaling factors, a scaling law of the form proposed by Chebil and Raman is devised. The method is applicable worldwide and produces predictions far more accurate than the ones provided by general P(R) estimation techniques. Index Terms--Electromagnetic propagation, prediction methods, rainfall effects.

The radiation properties of an electrically small antenna attached to an infinite conducting ground plane covered by a thin sheet of a metamaterial medium are considered. Using analytical field expressions, the radiation of an electric point dipole with arbitrary constitutive parameters of the material coating is studied. Attention is paid especially to permittivity values close to zero. Finite element simulations are employed to confirm the radiation enhancement and low quality factor Q characteristic of a zero-index coating as compared with regular 'air-filling.' The Q is evaluated partly through integration of the energy of the FEM-simulated fields within the antenna region and partly from the nonradiating component of the multipole fields outside the antenna region. Additionally, the influence of surface wave power is analyzed through simulations. Index Terms--Conducting surface, dipole antenna, Q factor, radiated power, zero-index materials.

A small printed antenna structure is presented featuring directive radiation patterns over ultrawide frequency band without re. lying on large antenna dimensions. The antenna comprises virtually two antennas--one electric-type antenna and one magnetic-type antenna--excited simultaneously in close proximity via one feed point. This combination of radiated fields from different types of sources result in directive radiation patterns without increasing antenna sizes as typically observed in antennas using only one type of sources. Two printed prototypes of the combined antenna are fabricated and their measured VSWR and radiation gain pattern characteristics are discussed. Index Terms--Directive antennas, printed antennas, small antennas, ultrawideband antennas.

Electromagnetic wave propagation from electric currents within the Earth's crust is investigated using a three-dimensional finite-difference time-domain (FDTD) full-vector Maxwell's equations model of the global Earth-ionosphere cavity. The FDTD model employed extends from ~100 km below sea level to an altitude of ~100 km, and can account for arbitrary horizontal as well as vertical geometrical and electrical inhomogeneities and anisotropies of the ionosphere, lithosphere, and oceans. Using this model, the surface horizontal magnetic field is calculated for different depths and orientations of an electric current occurring below the epicenter of the 1989 Loma Prieta earthquake. Results show that the alignment and depth of the electric current within the Earth's crust yields significant differences for the calculated surface magnetic field time-waveforms and spectra. Further, it is found that EM wave phenomena measured at the Earth's surface due to electric currents buried in the Earth's crust will only have significant spectra below ~ 1 Hz. Index Terms--Earth, earth-ionosphere waveguide, earthquake, earthquake precursors, electric current, finite-difference time-domain (FDTD), Loma Prieta earthquake.

A total-field/scattered-field (TF/SF) plane-wave source is developed for the finite-difference time-domain analysis of general (possibly lossy) planar layered media. A 1-D auxiliary grid is created to generate the incident field in the presence of the layered medium. Inhomogeneous plane waves are also allowed for lossless layers and narrowband excitation. Index Terms--Electromagnetic propagation in nonhomogeneous media, finite-difference time-domain (FDTD) methods.

The particle swarm optimization (PSO) method has been successfully applied to different electromagnetic optimization problems. Because of the complexity of this kind of problems, the associated cost function is in general computationally expensive. A fast convergence of the optimization algorithm is hence required to attain results in short time. Here few variations over the standard algorithm, referred to as differentiated meta-PSO, aimed to enhance the global search capability, and to improve the algorithm convergence, are introduced. In order to verify their effectiveness the different techniques have been first applied to benchmark test functions and then used for the optimization of a planar array. Index Terms--Arrays, ontimization, particle swarm.