Novel electromagnetic field probes with ultrasonic transmission lines for field measurements with minimum interaction

Abstract

Electromagnetic field probes are described which use ultrasonic transmission lines for signal transmission from a small electric or magnetic measuring dipole to the data processing unit. These transmission lines are made of nonmetallic material with low permittivity and permeability so that its interaction with the field to be measured is minimum. In particular, there is no evidence of energy leakage via surface or sheath waves, which normally cause problems when usual metallic connecting cables are used. This is especially important when measuring near fields of antennas or fields in resonators with high Q factors. Wide-band operation was achieved by amplitude modulating the field to be measured at a low frequency. The purpose of this modulation is to create a low-frequency (kHz) signal which can be transmitted via an ultrasonic line, designed to resonate at this low frequency. The radio frequency (rf) itself can extend over a broad range since the ultrasonic line does not transmit this frequency directly. Since the ultrasonic line is operated at a low frequency, its design and manufacture including ultrasonic transducers are essentially simple, even for field probes working in the gigahertz range. The design of the transmission line and of the measuring dipoles with demodulation circuitry are described. The probe performance is discussed for a magnetic field probe used for field measurements in resonant antennas for magnetic resonance imaging in the frequency range up to about 200 MHz. A typical field measurement result is presented for this application.