of variations of the above can be envisaged that nevertheless falls in under the main principles of the invention in the preferred embodiment an isolator is used with defined thermal coefficient of expansion in order to change an impedance mismatch one can imaging that this isolator is replaced with a bimetallic arm 25 as known from thermostats wherein the extreme point of the arm 26 various a distance to the signal conductor based on the temperature of the arm as shown in fig2 b the arm can also be provided using non metallic parts such as plastic of different thermal coefficients of expansion alternatively the extreme point 26 of the arm 25 can move along the signal conductor and thereby modulate phase instead of amplitude for the reflected signal as shown in fig2 c with the increased complexity that this entails one will on the other hand achieve a large change in the reflected signal the bimetallic structure can be provided as a micro electro mechanical system mems fabricated using typical semiconductor conductor processes in for instance silicon an embodiment as mems gives a rise to further possibilities for integrating a filter enabling the pearl to give a response on a predetermined frequency band such a system can be attached to the heating cable using direct electrical connection frequently known as vampire connector this gives a rise to a large degree of uniformity in production simple attachment during production and due to the small distances also.