M. Paquay

Thin AMC Structure for Radar Cross-Section Reduction

A thin artificial magnetic conductor (AMC) structure is designed and breadboarded for radar cross-section (RCS) Reduction applications. The design presented in this paper shows the advantage of geometrical simplicity while simultaneously reducing the overall thickness (for the current design ). The design is very pragmatic and is based on a combination of AMC and perfect electric conductor (PEC) cells in a chessboard like configuration. An array of Sievenpipers mushrooms constitutes the AMC part, while the PEC part is formed by full metallic patches. Around the operational frequency of the AMC-elements, the reflection of the AMC and PEC have opposite phase, so for any normal incident plane wave the reflections cancel out, thus reducing the RCS. The same applies to specular reflections for off-normal incidence angles. A simple basic model has been implemented in order to verify the behavior of this structure, while Ansoft-HFSS software has been used to provide a more thorough analysis. Both bistatic and monostatic measurements have been performed to validate the approach.

An electromagnetic bandgap curl antenna for phased array applications

A promising way to reduce the problems created by surface waves in printed phased array antennas is to use an electromagnetic bandgap (EBG) substrate instead of a standard dielectric. Two metallo-dielectric EBG substrates well suited for phased array applications have been designed and tested. One of them has a compact unit cell and exhibits a broad bandwidth performance. The other EBG structure has a narrower bandwidth but is simple to manufacture and has a low mechanical profile. Single radiating elements and a small linear array have been manufactured on a standard dielectric substrate and on the EBG substrates. The measurements show a significant improvement in radiation efficiency as well as of the front to back ratio for the antennas manufactured on the EBG substrates compared to the ones obtained for the antennas manufactured on the standard dielectric substrate. These results clearly demonstrate the effectiveness of the EBG substrates in reducing the surface and guided waves.

Terahertz Antenna Technology and Verification: Herschel and Planck - A Review

One of the aspects that the space and terrestrial terahertz imaging systems have in common is that they require state-of-the-art technology to achieve their ambitious goals. Although technology is advancing at a rapid pace in this frequency range, the requirements for these systems go well beyond what is currently available such that there are also no standards or calibration reference sources in this field. This paper describes the novel procedures that have been implemented to assess the “in-orbit” RF performance of two European Space Agency (ESA) satellites using on-ground verification procedures. These consisted of using several different measurement techniques at both ambient and cryo-temperatures and software model correlation to be able to predict the final performance. ESAs Herschel and Planck observatories are used as an example to highlight some of the hurdles that had to be overcome for the challenging task of flight-performance verification at (sub)millimeter-wave frequencies. Significant advances have been achieved despite the lack of internationally agreed procedures and practices pushing terahertz reflector and instrument technologies to new limits. This is a review paper and has been written on behalf of the large scientific, engineering, and management teams that were involved over many years in the development, production, testing, and operation of the two spacecraft.

Radiation-Pattern Measurements and Predictions of the PLANCK RF Qualification Model [AMTA Corner]

PLANCK is one of the scientific missions of the European Space Agency, devoted to observing the cosmic microwave background radiation with unprecedented accuracy. One of the key factors for the performance is the radiation pattern of the telescope, especially the sidelobe performance in the direction of hot celestial bodies such as the sun, Earth, and moon. The satellite will operate around the L2 Lagrangian point in deep space under cryogenic conditions. These conditions cannot be realized in an antenna test range for a payload of this size. The predictions for the performance under flight conditions therefore depend strongly on numerical simulations. The model to be used had never before been verified to this level of confidence. The challenge was to conduct a test campaign at frequencies up to 320 GHz (far beyond the normal range of the used CATR) with a very large object (the PLANCK RF Qualification Model with an aperture size of 1.5 m, i.e., more than 1500 wavelengths at 320 GHz) to demonstrate sidelobe levels down to -90 dB. A selection of the measurement results and comparisons with predictions are presented.

Radiation Pattern Measurements and Predictions of the PLANCK RF Qualification Model

PLANCK is one of the scientific missions of the European Space Agency, devoted to observe the cosmic microwave background radiation with unprecedented accuracy. One of the key factors for the performance is the radiation pattern of the telescope, especially the sidelobe performance in the direction of hot celestial bodies like Sun, Earth and Moon. The satellite will operate around the L2 Lagrangian point in deep space under cryogenic conditions. These conditions can not be realized in an antenna test range for a payload of this size. Therefore, the predictions for the performance under flight conditions depend highly on numerical simulations. The model to be used had never before been verified to this level of confidentiality. The challenge was to conduct a test campaign at frequencies up to 320 GHz (far beyond the normal range of the used CATR) with a very large object (the PLANCK RF qualification model with an aperture size of 1.5 m, i.e. more than 1500 wavelength at 320 GHz) to demonstrate sidelobe levels down to -90 dB. A selection of the measurement results and comparison with predictions will be presented.

Combination of AMC and PEC cells for RCS applications

A combination of perfect electric conductor (PEC) and artificial magnetic conductor (AMC) cells with a periodicity around lambda can be applied to improve the RCS, by reducing the power scattered in the specular direction. A 180deg phase difference is created between the contribution from the PEC and AMC cells, obtaining destructive interference. A theoretical model is presented based on array theory which agrees with the simulated results using the commercial software Ansoft HFSS. The structure has been breadboarded and tested. Results match the predictions and will be presented at the conference.

Ka-band measurement results of the irregular near-field scanning system PAMS

The portable antenna measurement system PAMS was developed for arbitrary and irregular near-field scanning. The system utilizes a crane for positioning of the near-field probe. Inherent positioning inaccuracies of the crane mechanics are handled with precise knowledge of the probe location within the transformation algorithm. The probe position and orientation is tracked by a laser while the near-field is being sampled. Far-field patterns are obtained by applying modern multi-level fast multipole techniques. The measurement process includes full probe pattern correction of both polarizations and takes into account channel imbalances. Because the system is designed for measuring large antennas the RF setup utilizes fiber optic links for all signals from the ground instrumentation up to the gondola, at which the probe is mounted. This paper presents results of the Ka-band test campaign in the scope of an ESA/ESTEC project. First, the new versatile approach of characterizing antennas in the near-field without precise positioning mechanics is briefly summarized. The setup inside the anechoic chamber at Airbus Ottobrunn, Germany is shown. Test object was a linearly polarized parabolic antenna with 33 dBi gain at 33 GHz. The near-fields were scanned on a plane with irregular variations of over a wavelength in wave propagation. Allowing these phase variations in combination with a non-equidistant sampling grid gives more degree of freedom in scanning with less demanding mechanics at the cost of more complex data processing. The setup and the way of on-the-fly scanning are explained with respect to the crane speed and the receiver measurement time. Far-fields contours are compared to compact range measurements for both polarizations to verify the test results. The methodology of gain determination is also described under the uncommon near-field constraint of coarse positioning accuracy. Finally, the error level assessment is outlined on the basis of the classic 18-term near-field budgets. The assessment differs in the way the impact of the field transformation on the far-field pattern is evaluated. Evaluation is done by testing the sensitivity of the transformation with a combination of measured and synthetic data.