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Abstract

A method and apparatus for detecting grain direction in wood particularly in lumber by means of high frequency radio energy like microwave radiation. The grain direction in lumber can be detected so that knots will cause no significant error to the measuring result. The knots are detected by the method according to the invention in so far as they contain deviations of grain direction. The microwave radiation is either passed through or reflected from the piece of lumber. Radiation energy is transmitted with the aid of a transmitting antenna with a certain direction of polarization and a signal of grain direction is obtained by detecting microwave field by a receiving antenna whose direction of polarization plane forms an angle of 90.degree. with the direction of polarization plane of transmitting antenna. A preferred embodiment of the invention provides for the polarization of transmitting field to be changed so that it alternatively is parallel or perpendicular to the direction of polarization plane of receiving antenna. A first signal (V.sub.s) is measured when both polarization planes are perpendicular to each other and a second reference signal (V.sub.ref) is measured when the polarization planes are parallel. The angle of the grain direction is obtained from the result of dividing the first signal with the sum of the first and second signals.

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First Claim