Germán León

Measurement of Low-Gain Antennas in Non-Anechoic Test Sites through Wideband Channel Characterization and Echo Cancellation [Measurements Corner]

The measurement of an antenna in a reverberant or semi-anechoic chamber, or even in an anechoic chamber with non-ideal absorbing material, will include the effects of multipath propagation, which will be evident in a distorted radiation pattern. In those situations, an improvement of the measurement can be achieved through removal or compensation of the undesired contributions. A simple technique to eliminate those contributions, based on the well-known Fourier transform algorithm, is used in this paper. It starts from measurements of the systems response in the frequency domain, followed by Fourier transformation of data to the time domain, detection and gating of undesired echoes in the time domain, back to the frequency domain, and retrieval of the antenna radiation pattern at the frequency of interest. The method is evaluated under different measurement conditions.

Novel polarization agile microstrip antenna

A novel agile antenna based on an injection-locked third harmonic self-oscillating mixer is presented in this paper. The phase-shift can be tuned, by varying the varactor control voltage, in range of 450deg. The active circuit allows changing the axial ratio of a dual-polarization antenna. The antenna can switch between radiating RHCP, LHCP and linear polarization with good purity.

A Simple Model for Analyzing Transmitarray Lenses

A new model for characterizing a complex system formed by a feeding horn and a transmitarray lens is presented in this paper. This model characterizes the spillover radiation and is capable of analyzing its effect on the radiation pattern. Both the contribution of the lens and the spillover radiation are modeled as planar arrays. The proposed model shows good agreement with the results obtained in simulations using a commercial full-wave software. In addition, some features, such as the characterization of the feeding element, are included in this model, in order to improve its accuracy. Finally, the limits of applicability are also discussed throughout this paper. Thus, it is possible to set up the boundaries, in terms of F/D ratio and number of elements, within which the model is reliable.

Optimization framework on antenna arrays for near field multifocusing

This paper presents a new method for focusing on multiple targets simultaneously using antenna arrays. An optimization framework based on the Levenberg-Marquardt algorithm is used to concentrate the radiated field at certain positions. The proposed synthesis technique is a flexible method that can optimize different sets of parameters in the array, i.e., the weights (phase and/or magnitude) of the elements, their positions or all these features, in order to achieve this goal. Some illustrative examples using simulated and implemented antennas are addressed to evaluate this framework.

Fresnel-Zone-Based Focused Planar Array

This letter presents a new class of focused array, based on Fresnel zone lenses. A punctual isotropic sources model is used to compare this novel array to a periodic one, both with the same size. The Fresnel-zone-based array (FZBA) can reduce the number of radiators without losing focusing characteristics. An FZBA has been implemented in a realistic array in order to demonstrate its simplicity. It has been simulated with a commercial tool, showing the same spot size but lower secondary lobes level than the punctual model.

Receiving Polarization Agile Active Antenna Based on Injection Locked Harmonic Self Oscillating Mixers

A polarization agile active antenna with phase shifter elements based on injection locked third harmonic self oscillating mixers is presented. This phase shifting topology provides the double functionality of continuous range phase shifter and downconverter. The phase shift value introduced by each circuit can be easily tuned through a DC voltage within a theoretical continuous range of 450° . The behavior of the isolated phase shifter circuit is studied, both as a function of the control voltage and versus frequency, through harmonic balance and envelope transient simulations. The polarization tuning performance of the complete active antenna is simulated, analyzing the impact of the operating parameters of the phase shifter on the overall behavior. A receiving polarization agile antenna with an input frequency band centered at 11.25 GHz and an output frequency band centered at 1.5 GHz has been manufactured for the experimental validation of the simulated results. A continuous range of polarization tuning has been observed, including two orthogonal linear polarizations along with left hand and right hand circular polarization.

Transmitting Polarisation Agile Microstrip Antenna Based on Injection Locked Oscillators

In this work, a transmitting two port aperture coupled microstrip antenna with variable polarisation is presented using variable phase shifters based on Injection Locked Oscillators (ILO). The polarisation of the antenna can be tuned in a continuous range, enabling two orthogonal linear polarisations, and both right hand and left hand circular polarisation. The signals applied to the two antenna ports are generated by the ILO circuits and can be phase shifted in a continuous theoretical range of 180 degrees. The phase-shift versus the DC varactor control signal of the individual ILO circuits and the axial ratio of the ILO based polarisation agile antenna is studied through harmonic balance and envelope transient simulations in combination with electromagnetic simulations. A polarisation agile antenna at 10.735 GHz has been manufactured obtaining a good agreement between the simulated and experimentally obtained behaviour.

Millimeter-Wave Offset Fresnel Zone Plate Lenses Characterization

Fresnel Zone Plate Lenses (FZPLs) are transparent-opaque lenses that filter the desirable phase. The centred Fresnel lenses have a strong back radiation towards the feed. In order to solve this drawback, offset feeding or offset pointing lenses are used. In this work, both offset FZPLs are studied using an optical physics method and experimentally characterized in the millimeter band. Two prototypes have been manufactured and measured, presenting a narrow beamwidth. The characteristics of pointing of this beam are studied depending on the feed gain. This work shows the pointing characteristics of the lenses, simply moving the lens in a plane.

Echo Characterization Based on Maximum-Likelihood Estimation for Antenna-Measurement Correction [Measurements Corner]

Antenna measurement systems could be affected by multipath effects resulting in reflected beams that might modify the measured values. In this paper, the use of the Maximum Likelihood estimator provides the basis of several post-processing methods able to estimate the echo parameters in imperfect antenna measurements. Once the reflections are characterized, these parameters are used to correct the measured values and to retrieve the actual radiation pattern corresponding to the antenna under test. Some different techniques are proposed in order to perform such cancellation. Experimental validation is also presented to show the accuracy of the described methods.

Linear aperiodic array of microstrip patch antennas with grating lobes reduction

Aperiodic arrays are very interesting when room for active devices could be achievable. In this paper, a deterministic strategy to generate the aperiodicity, based on three geometric taper distributions, is studied and validated. The method is applied to arrays with inter-elements spacing greater than a wavelength. The arrays exhibit a reduction of the grating lobe level. One of these arrays has been implemented in a microstrip linear array that has been simulated, manufactured and tested. Good agreement between simulated and measured radiation patterns is found. This manufactured antenna demonstrates the reduction of the grating lobes with a similar level to the side lobe.