TEST BENCH FOR METAL MESH RF EMISSIVITY MEASUREMENTS BY RADIOMETRIC MEANS

S. S. Kristensen¹, N. Skou¹, S. S. Søbjærg¹, M. M. Bilgic², S. B. Sørensen², C. Cappellin², R. Lopes³, E. Romano³, M. Decius⁴, P. Moseley⁵

¹National Space Institute, B 348, Technical University of Denmark, DK-2800 Kgs. Lyngby, Denmark, e-mail: ssk@space.dtu.dk ²TICRA, Landemærket 29, 5., DK-1119 Copenhagen K, Denmark, ³Inegi, Rua Dr. Roberto Frias, 400, 4200-465 Porto, Portugal, ⁴TEC-KNIT CCTT GmbH, Am Böwing 10, 46414 Rhede, Germany, e-mail: m.decius@tec-knit.de, ⁵ESA ESTEC, Keplerlaan 1, Postbus 299, 2200 AG Noordwijk, The Netherlands

The requirements for the spatial resolution to be provided by upcoming and possible future spaceborne microwave radiometers require very large antennas. The size of the antenna needed for the Copernicus Imaging Microwave Radiometer (CIMR) satellite is approximately 8 meter, and approximately 12 meter for the possible CryoRad satellite. Current launchers are unable to launch such reflectors. A possible solution for deploying such large antennas is to have a large reflector antenna using a knitted mesh woven from thin metal threads that can be folded during launch and then unfolded in space. This mesh based reflector antenna design has been selected for CIMR, covering L-, C-, X-, K- and Ka-band, and is an option for CryoRad that shall cover 400 MHz to 2 GHz.

The thin metal threads constituting the reflecting antenna will have both non-zero emissivity and non-zero transmissivity. These parameters affect the radiative transfer equation and need to be known in order to calibrate the Earth surface brightness temperature to be measured. As the emissivity (and transmissivity) is very small it is inherently difficult to measure but using a radiometer and a cold target like the sky provides a solution.

Assume the brightness temperature of the sky is 10 K. Placing a perfectly reflecting metal plate in a perfectly reflecting metal bucket so that only the sky is visible, a radiometer can measure the sky brightness temperature of say 10 K reflected in the metal plate and the bucket. Replacing this perfectly reflecting metal plate by a similar sized mesh with a reflectivity of 0.01 and at ambient temperature of say 300 K, the radiometer will measure 0.01•300 K plus 0.99•10 K, a total of 12.9 K. The difference of 2.9 K is easily measured by the radiometer. The drawback of this method is that a blue cold sky is needed, hence, good weather is an issue. And the uncertainties of measurement situation like Radio Frequency Interference (RFI) and ambient temperature make repeatability difficult.

We will present the design of an indoor test bench for measuring emissivity and transmissivity of test samples of meshes for C-band and Ka-band. A shielded test bench box of approximately 2x3x2 meters is envisaged and it is expected to be installed at the ESAs European Space Research and Technology Centre (ESTEC) in Noordwijk, the Netherlands.

The advantage of an indoor test bench is that it enables control of the measurement environment. The box will shield from any incoming RFI, and absorbers on the inside of the box will eliminate unwanted reflections. The sky used in outdoor measurements will be replaced with a microwave absorber soaked with liquid nitrogen in the bottom of the box. The radiometer including the antenna horn will be placed in a shielded container inside the text bench box. A triangle shaped sample holder will allow for fast switch between three different targets: mesh sample, absorber and perfectly conducting metal plate, enabling both calibration of the measurement situation and fast repletion of measurements.

Index Terms – Radiometer, emissivity