Abstract: The invention relates to a radio frequency tag. The tag comprises a receiver for receiving radio frequency waves having an amplitude spectrum provided with a first frequency component and a second frequency component. The tag also comprises a converter associated with the receiver for generating a signal having a frequency component at the frequency difference between the first frequency and the second frequency. In addition, the tag comprises a signal circuit for processing the generated signal and a coupler for forwarding the generated signal from the receiver towards the signal circuit. The signal circuit comprises a transmitter for transmitting the generated signal as a transmitted radio wave.

Abstract: The invention relates to an electrical measuring device for performing an electrical impedance measurement in a contactless manner. The measuring device comprises a measuring unit which is provided with the impedance to be measured and a passive resonance circuit connected thereto for generating a measuring signal to be wirelessly received by a separate active transmitting and receiving unit for determination of the electrical impedance, upon wireless reception of an interrogation signal transmitted by the active transmitting and receiving unit. Further, the measuring unit is provided with an additional reference circuit which is preferably connected to the resonance circuit for, depending upon the interrogation signal, generating a reference signal to be received by the active transmitting and receiving unit.

Abstract: A method for determining the degree of hardening of a hardenable material determines the complex electrical permittivity. The .di-elect cons."(t=0)/.di-elect cons."(t) ratio, where .di-elect cons."(t) is the imaginary part of the complex electrical permittivity .di-elect cons.'(t)-j.di-elect cons."(t), is determined as a measure of the strength of the material at an instant in time t. The .di-elect cons.'max/.di-elect cons.'(t) ratio of the real part of the complex permittivity can also be determined as a measure of the strength. These ratios are similar in shape to the curve of the strengths of the hardenable material with time.