Tomi Koskinen

Millimeter-wave beam shaping using holograms

We synthesize amplitude- and phase-type computer-generated holograms (diffractive gratings) for shaping millimeter-wave fields. We design holograms using quasi-optical back-propagation and rigorous optimization methods adopted from diffractive optics. We present experimental results from a plane-wave-generating hologram and a custom-designed field shaper at 310 GHz. Holograms can be applied, e.g., in a compact antenna test range and we propose their use for alignment purposes.

Hologram-based compact range for submillimeter-wave antenna testing

A hologram-based compact antenna test range (CATR) is being developed to overcome challenges met in antenna testing at submillimeter wavelengths. For the first time, this type of CATR has been built for testing of a large reflector antenna at submillimeter wavelengths. The CATR is based on a 3-m computer-generated hologram as the focusing element. This paper discusses the design and the construction of the CATR, and the verification of the CATR operation with quiet-zone tests done for the CATR prior to the antenna testing. Assembly of the CATR, testing of the 1.5-m reflector antenna at 322 GHz, and the disassembly were all done within two months in 2003. The quiet-zone field measurement results are analyzed in this paper. The CATR was concluded to be qualified for antenna testing. The antenna testing is described in a separate paper.

Testing of a 1.5-m reflector antenna at 322 GHz in a CATR based on a hologram

Hologram-based compact antenna test range (CATR) is a potential method for testing large antennas at submillimeter wavelengths. This paper describes testing of a 1.5-m single offset parabolic reflector antenna with a 3-m-diameter hologram-based CATR. This is the first time such a measurement is carried out at submillimeter wavelengths. The antenna tests were done in a CATR that was specifically designed and constructed for these tests. The measured radiation pattern at the frequency of 322 GHz is presented. The measured pattern corresponds reasonably well to the simulated pattern of the antenna. The effect of the quiet-zone field nonidealities on the measurement results and the reasons for the discrepancies in the measured antenna beam are discussed.

Holograms for shaping radio-wave fields

Holograms—diffractive elements—are designed and fabricated for shaping millimetre-wave radio fields. Methods for the synthesis of hologram elements are discussed and several beam shapes are tested: plane waves, radio-wave vortices and Bessel beams. Here we present an overview of the methods applied and results obtained with quasi-optical hologram techniques using both amplitude and phase holograms.

Antenna Tests With a Hologram-Based CATR at 650 GHz

A hologram-based compact antenna test range (CATR) is designed, constructed, and used to test a 1.5-m antenna at 650 GHz. The CATR is based on a 3.16-m-diameter hologram as the collimating element. So far, this is the highest frequency at which any CATR has been used for antenna tests. The quiet zone is measured and optimized before the antenna tests. The measured antenna pattern results at 650 GHz are analyzed and compared to the simulated patterns. Feed scanning antenna pattern comparison technique is used to correct the antenna pattern. These tests show the hologram CATR to be promising for antenna measurements up to 650 GHz.

Experimental study on a hologram-based compact antenna test range at 650 GHz

This paper studies the feasibility of a hologram-based compact antenna test range (CATR) for submillimeter-wave frequencies. In the CATR, a hologram is used as a collimating element to form a plane wave for antenna testing. The hologram is a computer-generated interference pattern etched on a thin metal-plated dielectric film. Two demonstration holograms of approximately 1 m in diameter were designed for 650 GHz, and they were manufactured on two different Mylar films. The holograms were illuminated with a horn, and the plane-wave field was probed at 644 GHz. The measured amplitude and phase ripples were 2 dB and 15/spl deg/ peak-to-peak for one of the holograms. A higher quiet-zone field quality can be achieved by increasing the manufacturing accuracy by further manufacturing tests. After this, the hologram-based CATR should have a potential for high-quality antenna tests at frequencies up to 650 GHz.

Dual reflector feed system for hologram-based compact antenna test range

Manufacturing of large computer-generated submillimeter wave holograms with high pattern accuracy has been the main challenge in the development of hologram based compact antenna test ranges (CATRs). Illumination of the hologram with a shaped beam produced by a dual reflector feed system (DRFS) simplifies the hologram manufacturing by eliminating the narrow slots in the hologram pattern. In this paper, the design of a shaped dual reflector feed for a hologram CATR is described. The simulated and measured illumination field amplitude and phase at 310 GHz are presented and compared to the desired hologram illumination. The measured amplitude is within /spl plusmn/0.5 dB from the design objective in the most significant central region of the illuminating beam. Measurement results of the quiet-zone field of a demonstration CATR illuminated by the DRFS are presented and compared to the measured quiet-zone amplitude and phase of a hologram fed directly with a corrugated horn. The quiet-zone diameters of the both holograms are over 0.25 meters and the measured root mean squared (rms) amplitude and phase ripples are below /spl plusmn/0.4 dB and /spl plusmn/5/spl deg/, respectively. Further improvements to the hologram CATR, such as greater tolerance to manufacturing errors, are also discussed.

Amplitude-only vs. complex field measurements for mobile terminal antennas with a small number of measurement locations

Amplitude-only and complex field measurements for characterizing 3D radiated fields of mobile phones with a small number of measurement locations by applying field expansion methods are examined. The uncertainties of the determination of the total radiated power and the radiation pattern are compared. Examination is carried out by computer simulations for several models of mobile phones beside a head phantom. The results provide guidelines for choosing a reasonable number of measurement locations for amplitude-only and complex field characterization. The results also show that complex field measurements provide lower uncertainty of the determination of the radiation pattern of a mobile terminal antenna beside a head phantom compared to amplitude-only measurement, when the number of measurement locations is relatively small.

Studies on an amplitude hologram as a submillimeter-wave collimator at circular polarisation

A hologram can be used as a collimator in the submillimeter-wave compact antenna test range (CATR). Previously, when a horn has been used as the feed, operation of the hologram has been limited to the vertical electric field polarisation. Use of a dual reflector feed system as the feed allows us to design holograms that operate also at the horizontal polarisation. Here, we show by simulation at 310 GHz that the operation of such holograms can be almost identical at both linear polarisations, thus, letting us to assume that these holograms can be used also at the circular polarisation.

Development of a hologram-based CATR for testing a very high gain antenna at 650 GHz

A compact antenna test range (CATR) based on a radio frequency hologram is a potential method for testing high-gain antennas at submillimetre wavelengths. Within a European Space Agency (ESA) project, a 1.5 m reflector antenna, ADMIRALS Representative Test Object (RTO), will be tested at 650 GHz during autumn 2006. For these antenna tests, MilliLab / Radio Laboratory at the Helsinki University of Technology (TKK) is designing and constructing a CATR based on a binary amplitude hologram having a diameter of 3.16 m. The designed quiet-zone width is about 2 m. A plane-polar scanner is designed for the quiet-zone field verification. The dynamic range is estimated to be 32 dB in the quiet-zone field tests and 78 dB in the tests of the high-gain test antenna.