Martti Tiuri

The complex dielectric constant of snow at microwave frequencies

The complex dielectric constant of snow has been measured at microwave frequencies. New and old snow at different stages of metamorphosis have been studied. The results indicate that the complex dielectric constant is practically independent of the structure of snow. For dry snow, the dielectric constant is determined by the density. For wet snow, the imaginary part and the increase of the real part due to liquid water have the same volumetric wetness dependence. The frequency dependence of the complex dielectric constant of wet snow is the same as that of water. A nomograph for determining the density and wetness of wet snow from its dielectric constant is given. A snow sensor for field measurement of the dielectric constant has been developed. It can be used for determining the density and the wetness of snow by a single measurement.

Snow Fork for Field Determination of the Density and Wetness Profiles of a Snow Pack

A radiowave sensor (a snow fork) for determining the density and wetness profiles of a snow pack with a single measurement has been developed. The snow fork is based on the measurement of the dielectric properties (real and imaginary part) of snow around 1 GHz. Due to the open structure of the resonator the measurement is nondestructive. Automatic measuring equipment guarantees instantaneous measurement results that can be recorded in the field.

Theoretical and Experimental Studies of Microwave Emission Signatures of Snow

The brightness temperature of snow in Finland has been studied theoretically and experimentally at 5,12, and 37 GHz for satellite remote sensing applications. A snow model consisting of ice spheres covered by a water shell has been used in theoretical calculations taking into account scattering and absorption. The brightness temperature of a natural snow field on the bare ground and on the ground covered with aluminum sheets has been measured from a tower. The experimental brightness temperatures are compared with calculated ones and show a reasonably good agreement. Experimental results also show that relatively small changes in the snow conditions cause large changes in the brightness temperature. Possible methods for using satellite observations in the remote sensing of snow are suggested.

Microwave Instrument for Accurate Moisture Measurement of Timber

In this report a new instrument developed for moisture measurement of timber is described. In order to determine the dependence of the dielectric properties of moisture many measurements of the properties of timber were performed at 10 GHz. The results showed that the attenuation through timber is dependent on moisture content, on density, and on temperature. In the developed instrument the attenuation of timber is measured at 10 GHz, the density is determined by ¿-ray attenuation and the temperature by an infrared detector. A microprocessor calculates the moisture content. Field tests have shown that the correlation coefficient between the moisture content determined by the instrument and measured directly is 0.993 and the standard deviation of the moisture per cent is 0.68.

Microwave Methods for Strength Grading of Timber and for Automatic Edging of Boards

Microwave methods have been succesfully applied in the saw-mill industry to the strength grading of timber and automatic edging of boards. Microwave detection of defects in sawn goods is based on inhomogenity or anisotropy associated with imperfections of wood material. A microwave bridge with freespace transmission through the timber in both branches of the bridge is used for the knot measurement of the strength grading equipment. Slope of grain is determined as a change of polarization of the transmitted microwave field. In the automatic edging system the microwave detection of knots is used together with optical methods. The knot detection is achieved using TM01-mode detectors to catch the dielectric wavequide mode HE11 associated with the knot.

Microwave Emission Characteristics of Snow Covered Earth Surfaces Measured by the Nimbus-7 Satellite

The satellite Nimbus-7 is mapping the brightness temperature of the Earth surface at several microwave frequencies. Its observations can be applied to the remote sensing of snow cover. In this report the effects of different types of surfaces, such as forests, boglands, farm lands, and lakes are studied. It seems to be possible to determine the water equivalent of a dry snow cover, if the surface effects are taken into account by comparison with observations taken before the snowfall.

Microwave Method for Measurement of Fiber Orientation in Paper

A microwave method for the measurement of fiber orientation in paper, is described. The method uses a dual mode microwave cavity where the modes have equal resonant frequencies and mutually perpendicular electric fields. The paper is inserted through the cavity so that the electric fields of both modes are in the plane of the paper. If the orientation of the fibers in the paper is not random the resonant frequencies will differ due to the dielectric anisotropy of the paper. This difference is measured and can be calibrated to give the amount of fiber orientation. Moisture and thickness variations in paper can cause calibration errors. However, if the variations are within certain limits, these errors can be eliminated by a suitable selection of the resonator bandwidths.

Radiometer studies of low-salinity sea ice

Theoretical and experimental studies of ultra-high-frequency (UHF) and microwave radiometry to sense remotely the thickness and other characteristics of low-salinity (less than one per mille) sea ice are described. The experimental studies used a 600-MHz and a multichannel, multibeam, 4.7-GHz radiometer carried by a helicopter to measure the brightness temperature of different types of sea ice in the Gulf of Bothnia in 1975 and 1976.The developed theory and experiments show that the brightness temperature of low-salinity sea ice oscillates as a function of ice thickness at UHF frequencies. Due to higher attenuation, the oscillations quickly die away at 4.7 GHz and the brightness temperature reaches an almost constant value when the thickness exceeds a few decimetres. By using a three-channel UHF radiometer with suitably selected center frequencies, the oscillations can be reduced and the effective brightness temperature will grow more linearly with the ice thickness.Experiments show that ice ridges appear as thick ice at 600 MHz and as thinner ice at 4.7 GHz, thus allowing them to be detected by radio techniques.

Microwave Sensor for Snowpack Wetness and Density Profile Measurement

A microwave sensor for fast field measurements of snowpack wetness and density profiles is described. The sensor is an open resonator that can be pushed into snow. The resonant frequency (around 1 GHz) and the Q factor are automatically measured as a function of depth and converted to wetness and density values.

A Circularly Polarized Traveling-Wave Chain Antenna

A novel traveling-wave antenna array with bent conductors as elements can give circularly polarized radiation. An expression for the far-field has been derived, from which the lengths and bending angle of the conductors can be calculated for circular polarization. A useful application would be printed circuit microwave antenna for different purposes. Antennas have been made and work well over reasonable large bandwidths. They can also function on their broadside frequency.