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High-resolution radars with large frequency bandwidth are widely used for the detection and location of subsurface objects (Daniels et at. 1988). The most common wideband signal is the very short pulse (impulse) utilized in short pulse radars (impulseradars). In the ideal case the impulse is a one-sided sinusoidal or gaussian signal (Esselle & Stuchly 1991). In practice the signal contains a few oscillation periods at a particular operation frequency. Frequencies of the order of 100 MHz are used in the measurement of large objects (e.g. ground), where high penetration depth is needed and range resolution of 0.1-1 m is adequate. If range resolution of the order of centimetres is desired, the frequency of the radar must be 1000 MHz or more.

A new instrument for accurate moisture and density measurement of timber is described. To determine the optimum frequency the dielectric properties of Finnish spruce and pine have been meas...

Different dielectric sensors for measuring the liquid water content of snow are compared and described in detail. The instruments make use of the significant difference in the dielectric properties of ice and liquid water at radio frequencies; they are operated with frequencies ranging from 1 MHz up to 1.3 GHz. Plate condensers in connection with ac bridges are used as sensors in the frequency range up to 100 MHz whereas open resonators are used in the GHz regime. Test measurements with the different sensors on homogeneous samples like dry sand and mixed and prepared snow showed the same results for the dielectric constant: the discrepancies are less than 1%. In the natural, inhomogeneous snow cover, the special properties of the different sensors appear. Snow wetness is calculated from the measured dielectric constant and the snow density using the model of Polder and van Santen. The comparative field measurements were made with Alpine snow in the Stubai Alps in Austria.

A microwave instrument for non-destructive testing of the homogeneity of supercalendar rollers used in paper mills is described. The sensor consists of a flat coaxial transmission line a half wavelength long. A sensitivity of more than 0,1% is achieved in measuring small changes in the dielectric constant. Measurement of absolute values of the dielectric constant is also possible.

The BL Lac-type object OJ287, with a redshift z = 0.306 and a luminosity exceeding 1047 erg s−1 at maximum light, is among the brightest and most violently variable objects of its class. Rapid variations, with timescales of a few hours or less, have been reported over a wide range of wavelengths1,2 and also in polarization3, imposing strong constraints on the size and dynamics of the source4,5. The detection of periodic variations in OJ287, or in any other compact extragalactic source, would be even more important, as these could be related to rotation or orbital motion in the source. So far the only searches sensitive to periodic variations shorter than 1 h are the photometric observations of Visvanathan and Elliot6, Frohlich et al.7 and Kiplinger8, in which a 40-min period in OJ287 was respectively found, confirmed and not detected. We report here the detection of a 15.7-min periodic flux variation in OJ287 in April 1981 at 37 GHz. The source has been subsequently monitored for 265 h at 22 and 37 GHz, as well as in optical photometry, and the same period has been seen in simultaneous observations. The peak–peak amplitude of the variation is 1–10% of the total intensity. Other periods may also exist.