Abstract: An yttrium-iron-garnet (YIG) tuned reflection filter is placed in the magnetic field adjacent a High Voltage Direct Current Transmission Line. A YIG filter receives a microwave signal from a remote transmitter and the same signal is modified by the YIG filter and returned to a remote receiver wherein the current in the transmission line is evaluated.

Abstract: A ferromagnetic element tuned oscillator is located close to a current carrying conductor and is provided with a power supply energizing means. Means are provided for transmitting the information carried by the electro-magnetic wave from an extra high voltage potential tuned oscillator to a receiving means at ground level together with frequency measuring means. A frequency discriminator converts the received signal frequency variations into an output signal which is an analogue of the current flowing in the extra high voltage conductor. The output signal from the tuned oscillator can be transmitted either through free space or a dielectric wave guide, which provides the necessary isolation between the extra high voltage conductor and the receiver.