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A novel type of short monopole antenna is proposed. Its structure consists of a periodically slotted coaxial line radiating in linear polarization (LP). The structure can be easily modified by the addition of a thin layer of dipoles in order to generate circular polarization (CP). The number of slots along the structure, their periodicity and the tuning of the terminating load allow control of the antenna main beam' angle and width. One advantage of the proposed design over conventional monopoles is that the antenna does not require a ground plane. Both left-hand and right-hand CP are possible with the same design, simply by inverting the symmetry of the printed dipoles. Design details for the annular slotted coaxial line and for the LP and CP mast antennas are given. Prototypes in both LP and CP were fabricated and their characteristics were measured. Simulations and measurement are in good agreement. The proposed CP mast antenna is compliant with the requirements of the XM digital satellite radio system, with a frequency bandwidth of more than 1.7% in which VSWR < 2, and an AR < 4 dB in the elevation range of [+ or -]30[degrees] < [theta] < [+ or -]70[degrees]. Index Terms--Coaxial transmission line discontinuities, mobile antenna, monopole antenna, printed dipoles, slot arrays.

A new design of two-dimensional cylindrical Luneberg lens is introduced based on T[E.sub.10] mode propagation between parallel plates, with special focus on ease of manufacturing. The parallel plates are partially filled with low cost polymer material (Rexolite [[epsilon].sub.r] = 2.54) to match Luneberg's law. A planar linear tapered slot antenna (LTSA) is inserted into the air region between the parallel plates at the edge of the Luneberg lens as a feed antenna, with fine positioning to the focal point of the Luneberg lens to optimize the antenna system performance. A combined ray-optics/diffraction method is used to obtain the radiation pattern of the system and results are compared with predictions of a time domain numerical solver. Measurements done on a 10-cm Luneberg lens designed for operation at 30 GHz agree very well with predictions. For this prototype, 3-dB E- and H-plane beamwidths of 6.6[degrees] and 54[degrees] respectively were obtained, and the sidelobe level in the E-plane was -17.7-dB. Although the parallel plate configuration should lead to a narrow band design due to the dispersion characteristics of the T[E.sub.10] mode, the measurement results demonstrate broadband characteristics with radiation efficiencies varying between 43% and 72% over the tested frequency band of 26.5-37 GHz. The designed cylindrical Luneberg lens can be used to launch multiple beams by implementing an arc array of planar LTSA elements at the periphery of the lens, and can be easily extended to higher mm-wave frequencies. Index Terms--Cylindrical lens, fan beam, Luneberg Lens, millimeter waves, multiple beams, tapered slot antenna, wideband.

The design of a C-band electronically beam scanning reflectarray is presented. The reconfigurable reflectarray element consists of a microstrip patch, printed on a flexible membrane substrate, aperture-coupled to a transmission line loaded with two varactor diodes. The designed element allows continuous tuning of the reflected signal's phase over a 360[degrees] range with a maximum loss of 2.4 dB at 5.4 GHz. The measured results on a 30-element reflectarray breadboard show that by varying the bias voltages on each element the main beam can be steered to large angles, up to 40[degrees] from broadside in the H-plane. A loss analysis of the proposed reflectarray is also presented. Index Terms--Antenna arrays, microstrip antennas, reflectarray antennas, scanning antennas.

A novel approach is proposed for reflectarray antenna design which makes use of planar near field measurements to characterize the scatterers and the incident field on the structure. This method is based on an equivalent transmission line model of a unit cell in an infinite periodic array. A monopole probe is used to sense a scattered signal in the near field of a reflectarray illuminated by a primary source. These measurements are introduced in the model and provide all the necessary data to accomplish the design. The method has been validated experimentally with four prototypes based on different levels of approximation, at a central frequency of 15 GHz. The results indicate that the proposed near-field based technique allows the design of reflectarrays having similar or better performance than those obtained when numerical modeling is used to obtain the phase response of the unit cells. Index Terms--Design methodology, near-field characterization, reflectarrays.

The equivalent magnetic currents method can be used to characterize radiating structures and to obtain information about the current distribution on the radiating device under test (DUT). In order to do so, the system is assumed to be linear and inversion from near-field measurements can be carried out using numerical techniques such as the singular value decomposition. However this approach is very sensitive to modeling and measurement errors, which is a consequence of the ill-conditioned nature of the inversion process. Here, we propose to account for the ill-conditioning by using a Tikhonov regularization technique to perform the inversion. This technique requires a regularization parameter to be set to an appropriate value, and we show that a generalized cross-validation approach yields adequate determination of the regularization parameter directly from the measured data. The proposed method can be tailored to account for specific characteristics of the DUT. This point is illustrated by the estimation of the current distribution of a bidimensional radiating structure for which prior information about the orientation of radiating edges is incorporated into the inversion method. Index Terms--Antenna measurements, equivalent magnetic currents (EqMC), generalized cross validation, ill-posed problems, inverse problems, near field (NF), Tikhonov regularization technique (TRT).

Direction-finding systems for radio signals are mostly used in mobile communications and avionics applications for antenna tracking or navigation purposes. In general, such systems require accurate calibration and may be sensitive to noise and external interference. In this paper, we will investigate the performance of a neural network-based direction-finding system under such conditions. The proposed topology is a hybrid one, combining a simple RF signal beamformer with a neural network. The training of the neural network is accomplished experimentally with a three-element antenna array by varying the beam's direction and the carrier frequency. The error on the estimated direction of arrival caused by the environment and training limitations are investigated. Index Terms - Mobile communication, navigation, neural network applications.

Coplanar waveguide (CPW) fed slot antennas are attractive due to fabrication simplicity and ease of integration with active devices. In this paper, a new concept for exciting slots with a CPW line based on inductive coupling is presented. It is described how this coupling structure can be designed to tune the impedance of the antenna over a wide range. Single elements and arrays designed for 5- and 25-GHz operation are described. Simulated and measured characteristics are presented. This new coupling topology is particularly suitable for series-fed array configurations and broad-band design. Index Terms - Coplanar waveguide, slot antenna.

A near-field to far-field transformation based on the antenna representation by equivalent magnetic current (EMC) sources has been proposed and validated experimentally on large high-directivity antenna arrays. In this paper, the use of EMC is extended to the diagnostics of low-directivity printed antennas. The limitation of the near-field to far-field transformation applied to EMC models of low-directivity antennas, caused by the finite dimensions of the antenna ground plane, is demonstrated. A method to partially overcome this limitation by including the contribution of diffracted rays is implemented, and its effectiveness is demonstrated with antenna prototypes. It is shown that the agreement between the far-field patterns measured in an anechoic chamber and the patterns computed from the EMC model obtained from the near-field measurements is significantly improved upon, within a sector of [+ or -] 90 [degrees] with respect to the antenna boresight in the E plane. The influence of the near-field sampling density and topology of the EMC model on the accuracy of the predicted far-field pattern is examined. Index Terms--Near field, printed antennas.