Juha Mallat

The Odin satellite - I. Radiometer design and test

The Sub-millimetre and Millimetre Radiometer (SMR) is the main instrument on the Swedish, Canadian, Finnish and French spacecraft Odin. It consists of a 1.1 metre diameter telescope with four tuneable heterodyne receivers covering the ranges 486-504 GHz and 541-581 GHz, and one fixed at 118.75 GHz together with backends that provide spectral resolution from 150 kHz to 1 MHz. This Letter describes the Odin radiometer, its operation and performance with the data processing and calibration described in Paper II.

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.

Dielectric rod waveguide antenna for W band with good input match

Antennas based on dielectric rod waveguide made of GaAs and fused sapphire cut along its optical axis inserted in the end of a standard WR-10 metal waveguide are investigated with computer simulations and experiments in the W band. The radiation pattern is found to be similar to that of the open metal waveguide end, but less frequency-dependent beamwidth and input match: measured return loss is more than 25 dB.

Low-loss sapphire waveguides for 75-110 GHz frequency range

Low-loss dielectric waveguides are promising for use instead of metal ones, but problems in transitions have to be overcome. A simple and effective structure made of a monocrystalline sapphire waveguide has been designed. Experimental results at 75-110 GHz indicate good matching with metal waveguides (VSWR /spl les/1.13) and low insertion loss (0.05-0.35 dB for 47 mm dielectric section).

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.

Monostatic Reflectivity Measurement of Radar Absorbing Materials at 310 GHz

This paper presents monostatic reflectivity measurements of radar absorbing materials at 310 GHz in a phase-hologram-based compact range. The radar cross-section method was used and the backscattered reflection was measured with horizontal and vertical polarizations in plane-wave conditions. Transmission was also studied. The reflectivity was measured over an incidence angle of 0deg-45deg. The reflectivity of Thomas Keating Terahertz RAM at normal incidence was found to be -56 dB-the smallest of the studied materials. The reflectivity of carpet material measured was also below -40 dB and it was found to be suitable for use as an absorber. The results are in line with those available from previous studies of reflectivity and complement them with new materials, frequency, and angle information

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.

Phase-hologram-based compact RCS test range at 310 GHz for scale models

A compact radar cross section (RCS) test range based on a phase hologram has been developed for scale-model measurements. The phase hologram converts the feed-horn radiation to a plane wave needed for RCS determination. The measurements are performed at 310 GHz using continuous-wave operation. A monostatic configuration is realized using a dielectric slab as a directional coupler. The main advantage of a scale-model RCS range is that the dimensions of radar targets are scaled down in proportion to the wavelength. Therefore, RCS data of originally large objects can be measured indoors in a controlled environment. Test objects such as metal cylinders and simplified targets have been measured. The feasibility of the phase-hologram RCS range has been verified. The operation and measurement results of the monostatic measurement range are reported here. A comparison with simulated results is also included

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.