Päivi Koivisto

Quality factor of an electrically small antenna radiating close to a conducting plane

Expressions are derived for the smallest achievable radiation quality factor (Q) of an electrically small antenna in front of a conducting plane. Applying the low-frequency approximation to the source region involving an electric or a magnetic point dipole plus their images behind the plane, an expression is formed for the field in the radiation zone. The contribution of non-propagating energy in the near field is obtained explicitly using a spherical harmonics decomposition. The radiation Q is found to depend on the radius (relative to wavelength) of the smallest sphere that encloses the antenna and its image, the ratio of the vertical and horizontal dipole moments, as well as the positions of the dipoles relative to each other and to the plane. A number of simple wire structures are analysed with NEC (based on the method of moments), and the approximate Q obtained from their fractional bandwidth are compared to the corresponding theoretical minima.

ON THE INFLUENCE OF INCOMPLETE RADIATION PATTERN DATA ON THE ACCURACY OF A SPHERICAL WAVE EXPANSION

The accuracy of a spherical wave expansion is examined when the expansion is calculated from incomplete data of the radiation pattern, i.e., when field data on a part of the far-field sphere is missing. The effect of antenna size and truncation index on the interpolation capacity of a SWE is examined by using an analytical expression for the radiation pattern of wire antennas of different lengths. The error of the SWE is seen to increase drastically when the smallest diameter of the dead zone surpasses the length of a period of the highest included wave function. The influence of the size and shape of the dead zone is studied by the aid of a measured pattern, of which a part of the field data is ignored. Two different ways are proposed for estimating the accuracy of the obtained SWE in a practical instance, when the field in the dead zone is unknown.

Rapid beamsteering reflectarrays for mm-wave and submm-wave imaging radars

Recent developments in millimetre to submillimetre-wave imaging radars with excellent ranging resolution provide an attractive route towards stand-off imaging of concealed explosives at ranges up to several tens of meters. Present systems typically rely on only one transceiver, coupled with an optomechanical scanning system for image formation. This limits the image acquisition speed to several seconds/frame. Frame rate can in principle be increased with increasing the channel count but this adds substantially to the system complexity and cost, while only providing a modest speed increase. In this paper we present preliminary designs for rapid electronic beam steering system that could provide a way towards real-time millimetre-wave to submillimetre-wave imaging radars.

Reflectarray for 120-GHz beam steering application: design, simulations, and measurements

Development of a 120-GHz FMCW radar with a reflectarray as focusing element is described. The reflectarray is realized on a 150-mm silicon wafer and it has 3700 phase-modulating elements on it. The phase shifters have four discrete values to cover full phase modulation with 90° steps. The reflectarray element is realized with a conductor-backed coplanar waveguide patch antenna with a phase shifter coupled to it. The required phase modulation for each reflectarray element is determined with an in-house physical optics simulation combined with genetic-algorithm-based optimization. The reflectarrays are developed in two stages. First, preliminary reflectarrays with static phase shifters have been manufactured and tested at 120-GHz antenna measurement range. The static reflectarrays are found to perform as designed in their capability to steer the beam to a desired direction and to a distance of 3 m. The reflectarrays have -3-dB beam width from 1.1° to 1.3° depending on the beam tilt. After the preliminary verification with the static phase shifters, the reflectarrays will be assembled together with actively controlled MEMS-based phase shifters. The MEMS switches are controlled with dedicated high-voltage CMOS electronics, forming a system-in-a-package (SiP). First, the MEMS phase shifters are modeled, are being fabricated, and will be measured separately to verify their phase-shifting capability.

Reflectarray Design for 120-GHz Radar Application: Measurement Results

In this paper, we present design and experimental results on reflectarrays at 120 GHz. The offset-fed reflectarrays consist of conductor-backed coplanar patch antennas with phase-shifting stubs. Three 138-mm reflectarrays are lithographically fabricated and evaluated in a near-field measurement range. Their measured beam patterns are compared to the theoretical ones. The theoretical -3-dB beam width is 60-64 mm at 3-m distance from the reflectarray. Measured beam widths of the different reflectarrays deviate less than 10% from the theoretical values. The beam pointing is found to be close to theoretical, whereas the sidelobe level is up to 5 dB higher. The efficiency, alignment accuracy, and surface shape of the reflectarray are studied with near-field imaging of the reflectarray aperture field. The measured average efficiency is 0.11 whereas the predicted average efficiency is 0.54. The low efficiency is most likely due to over-etching of the structures of the reflectarray element, and could be improved in future fabrication processing rounds. Beam pattern measurement close to the main beam is well suited for evaluating the beam width and pointing accuracy, but it gives little information on the element performance. We propose near-field imaging of the reflectarray to evaluate both element efficiency and phase shift.

Analytical Solution for Characteristic Modal Power Distribution and Truncation Limit for Spherical Wave Expansion of Antenna Radiation Pattern

—In this paper the Spherical Wave Expansion (SWE) is used to represent an antenna radiation pattern. An analytical expression for the characteristic modal distribution of power radiated by an arbitrary antenna is derived from the assumption of equal average amplitude for each mode over the surface of the minimum sphere enclosing the antenna. The relative power error of a truncated expansion is obtained from the derived modal power distribution. The result offers a convenient way to determine an appropriate truncation limit of the modes in a SWE as a function of the desired accuracy and antenna size. This method is more accurate than the formulas given previously in the literature. Moreover, it allows one to choose the accuracy of the resulting field, contrary to the earlier formulas. The derived truncation limit is compared with expressions given previously in the literature and with analytically and numerically determined radiation pattern examples. The modal power distributions are also compared with the calculated examples.

Towards video rate imaging at submillimetre-waves — Finnish developments of passive multi-band imaging and holographic submm-wave beam steering at VTT

Imaging at submillimetre-wave (SMMW) frequencies is of considerable interest for security applications due to potentially superior performance at longer stand-off ranges in comparison to mm-wave imaging thanks to reduced diffraction. We have demonstrated passive broad-band video rate imaging system operating at a centre frequency of about 600 GHz, capable of 10 frames/second imagery of a 2 m × 1 m field-of-view at a stand-off distance of 5 meters. In addition, a multi-band system centred around 250, 450 and 720 GHz will be discussed. Multi-band imagery is interesting given its potential for rudimentary materials differentiation, a capability that would substantially benefit e.g. security imaging applications. The passive imaging activities are complemented by activities towards developing rapid electronic beam steering capability for imaging submm-wave radars. Results from two projects aiming at constructing such beam steering systems at 120 GHz and at 650 GHz are presented.

Developments towards real-time active and passive submillimetre-wave imaging for security applications

Both active and passive submillimetre-wave stand-off imaging systems are under development for security imaging applications. The drivers for operation at higher frequencies have been the desire for better image resolution, smaller optics package, reduced susceptibility to specular reflections from the human skin and capability for stand-off imagery. In this paper we summarise our efforts on devices, components and systems which could eventually pave the way for fast, real time, high-resolution imaging systems for the submillimetre-wave range.

Optimum Transparent Absorbers of Electromagnetic Waves

Electromagnetic wave absorbers consisting of thin transparent sheets of conducting medium are studied in this paper. In particular, a result known since the 1930 s that, for plane waves normally incident on a conducting sheet, the maximum achievable absorption is one half of the incident power is proved and generalized to multiple layers. The case of arbitrary incidence angle and polarization is discussed and illustrated with examples, and the resistance providing maximum absorption when the angle of incidence and polarization is arbitrary is derived. We also consider an absorber consisting of two resistive sheets and optimize its resistances to yield the best possible absorption for normal incidence, such that the total absorption at all frequencies is ges 0.5. Finally, conditions for an optimal absorber for a spherical sheet surrounding a dipole antenna are derived.

Near-field measurements of submillimeter-wave reflectarrays

We present results of experimental characterization of static 650-GHz reflectarrays. The reflectarrays are based on 123-μm circular microstrip patch antennas with tuning stubs as phase shifters. The static reflectarrays are considered as predecessors for active reflectarrays, and therefore the reflectarray elements have two discrete phase-shift values: 0° and -180°. The reflectarrays have 95000 elements, and they have separation of 400 μm. The reflectarrays are fabricated on 150-mm silicon wafers with a ground plane and a 20-μm polyimide substrate atop. The fabricated static reflectarrays are characterized in a near-field measurement range and their beam patterns at the focusing distance of 20 m are calculated with plane-to-plane transform. At this high frequency, fabrication tolerances are difficult to meet and, e.g., over-etching of the antenna and phase-shifting structure may offset the resonance frequency of the reflectarray element by more than its bandwidth.