In modern compact electronic circuits, there are several components on a confined PCB area, and so, the electromagnetic effects induced from one component may affect the surrounding components. This disturbance is known as Electromagnetic Interference (EMI). The source of the radiation must be identified in PCBs to predict the EMC of electronic circuits. Electromagnetic compatibility (EMC) is the ability of electrical equipment and systems to function acceptably in their electromagnetic environment, by limiting the unintentional generation, propagation, and reception of electromagnetic energy which may cause unwanted effects such as EMI. Near-field scanning is the general method for identifying radiation sources in PCBs.
Electromagnetic near-field scanning
Near-field scanning is used for estimating localized electromagnetic emissions. The far-field is a region in which the field acts as normal electromagnetic radiation. But in the near-field region, electric and magnetic fields can exist independently. Additionally, in the near-field region, one type of field can dominate the other. E-field and H-field probes are used for electromagnetic near-field scanning. E-field probe measures point-to-point the electric field strengths locally. On the other hand, the H-field probe measures the magnetic field strengths locally.
E-field and H-field probes: An overview
The E-field probe consists of a short dipole antenna, a detector diode, a non-perturbing transmission line, and a readout device. Three orthogonal dipoles are generally used in an E -field probe to provide a response that is nearly isotropic for all polarizations of the incident field. Optical fibers, together with a suitably modulated light source, may be used to form a wide-band non-perturbing data link from the dipole and detector to a remote readout .
The H-field probe consists of a printed circuit magnetic loop and a transmission line. The magnetic flux from the loop will induce a high-frequency waveform that travels through a transmission line to the receiver. The transmission line is mostly a microstrip line.
Implementing near field E-field and H-field probes on a printed circuit board is difficult mainly because it involves designing and implementation of non-perturbing transmission lines.
The below shown near-field probes are designed by Ketan Desai. He is a student of Electrical Engineering at the University of British Columbia.
The complete project including Gerber files can be found at: https://github.com/ketszim97/NearField_PCB_Probes#readme
Reference: H. Bassen and G. Smith, “Electric field probes–A review,” in IEEE Transactions on Antennas and Propagation, vol. 31, no. 5, pp. 710-718, September 1983, doi: 10.1109/TAP.1983.1143126.