• Skip to primary navigation
  • Skip to main content
  • Skip to primary sidebar
  • Skip to footer
  • Advertise
  • Subscribe

Test & Measurement Tips

Oscilloscopes, electronics engineering industry news, how-to EE articles and electronics resources

  • Oscilloscopes
    • Analog Oscilloscope
    • Digital Oscilloscope
    • Handheld Oscilloscope
    • Mixed-signal Oscilloscope
    • PC-based Oscilloscopes – PCO
  • Design
  • Calibration
  • Meters & Testers
  • Test Equipment
  • Learn
    • eBooks/Tech Tips
    • FAQs
    • EE Training Days
    • Learning Center
    • Tech Toolboxes
    • Webinars & Digital Events
  • Video
    • EE Videos
    • Teardown Videos
  • Resources
    • Design Guide Library
    • Digital Issues
    • Engineering Diversity & Inclusion
    • Leap Awards
    • White Papers
  • Subscribe
You are here: Home / Featured / How to locate EMI emissions with near-field probes: Part 1

How to locate EMI emissions with near-field probes: Part 1

September 14, 2023 By Kenneth Wyatt Leave a Comment

Near-field probes let you find the sources of near-field and far-field EMI that ruin your design, and your day. You don’t need to know Maxwell’s Equations to troubleshoot your designs.

Most product designers are familiar with near field probes such as the H-field (loop) probe and E-field (stub) probe. You may also know how to use these probes with a spectrum analyzer to identify emissions sources on PC boards, cables, or enclosure seams. Many don’t, however, understand how to troubleshoot and mitigate EMI.

To understand the near field, consider a “point source” (electrically small) radiating E-field dipole antenna (Hertzian dipole). In the near field, the radiating electromagnetic waves will be spherical and will gradually become planar (plane waves) in the far field. As you move away from this radiating point source, the E-field (measured in volts per meter) drops off with distance. The same principle applies to an H-field (loop) point source.

In the near field, the equations for the electromagnetic (electric and magnetic) fields are complex and include factors of 1/r, 1/r², and 1/r³. Where r is the distance away from the point source radiating antenna. In other words, the field strength tends to drop off quickly in the near field. In the far field, the measured E-field drops off by a factor of 1/r primarily, as the other r terms become insignificant.

Because this conversion from spherical to planar waves is gradual, it’s difficult to definitively say where the exact transition point occurs. Most EMC engineers assume the point is at distance = λ/2π or about 1/6th of a wavelength (or, in normalized terms, r/l = 0.16). For example, at 30 MHz, this distance is about 1.6 m from the source.

If you were to plot the wave impedance (Zw = E/H, E-field and H-field plots) versus frequency for a point source (dipole or loop) antenna, we’d find that in the near field, the E-field was high (over 5 kΩ) and the H-field is minimal (about 20 Ω). If the emission source has a high current and low voltage (E/H < 377), the near field is predominantly magnetic (H-field). If the source has a low current and high voltage (E/H > 377), the near field is predominantly electric (E-field).

Wave impedance near far field
Figure 1. TheFigure 1. The wave impedance for electric and magnetic field sources as a function of r/λ. Image: Arturo Mediano.

As we move out to the far field, the E-field and H-field wave impedances converge to approximately Zw = 377 Ω in free space (Figure 1).

We can use the concepts in the theory above to apply to measured currents and voltages on PC boards, wires, and cables. What we measure are not really “point sources” although the electromagnetic physics is the same, even for larger structures such as circuit traces, power conversion components, or cables.

Most near-field probe kits come with both E-field and H-field probes. Deciding on H-field or E-field probes depends on whether you’ll be probing high switching currents — high di/dt for circuit traces, cables, etc., or high switching voltages, dV/dt for switching power supplies, etc., respectively. Both are useful for locating leaky seams or gaps in shielded enclosures.

Summary
While near-field probes are useful for identifying the various energy sources on boards, cables, or systems, they are only one component of a comprehensive troubleshooting philosophy, which will include RF current probes for measuring cable currents and close-spaced antennas to troubleshoot actual system emissions. The best part? All this may be done right on your own bench, rather than at the EMC compliance test lab.

We’ll discuss how to select and use these probes in Part 2 of this series — Part 3 will be available September 21. These later articles will describe additional tools useful for troubleshooting radiated emissions issues.

References
Wyatt, Kenneth, Create Your Own EMC Troubleshooting Kit (Volume 1, 2nd Edition), Amazon, 2022
Wyatt, Kenneth, Workbench Troubleshooting EMC Emissions (Volume 2), Amazon, 2021

You may also like:


  • IEEE EMC Symposium 2023: Exhibits

  • IEEE EMC Symposium 2023: Presentations

  • EMC chamber fits into small spaces

  • EMI testing for IoT transceivers
  • PCB schematic generation
    The challenge of testing obsolete PCBs
  • Create Your Own EMC Troubleshooting Kit
    Book Review: Create Your Own EMC Troubleshooting Kit
  • EMC book
    New book covers EMC troubleshooting with test instruments

Filed Under: Bench Test, EMI/EMC/RFI, FAQ, Featured, spectrum analyzer Tagged With: EMIprobes, FAQ, spectrumanalyzer

Reader Interactions

Leave a Reply Cancel reply

You must be logged in to post a comment.

Primary Sidebar

Featured Contributions

Why engineers need IC ESD and TLP data

Verify, test, and troubleshoot 5G Wi-Fi FWA gateways

How to build and manage a top-notch test team

How to use remote sensing for DC programmable power supplies

The factors of accurate measurements

More Featured Contributions

EE TECH TOOLBOX

“ee
Tech Toolbox: 5G Technology
This Tech Toolbox covers the basics of 5G technology plus a story about how engineers designed and built a prototype DSL router mostly from old cellphone parts. Download this first 5G/wired/wireless communications Tech Toolbox to learn more!

EE TRAINING CENTER

EE Learning Center

EE ENGINEERING TRAINING DAYS

engineering
“test
EXPAND YOUR KNOWLEDGE AND STAY CONNECTED
Get the latest info on technologies, tools and strategies for EE professionals.
“bills

RSS Current EDABoard.com discussions

  • Why do fill dummy(logic)on the chip(layout)
  • Why need use TOPmetal Stacking?
  • Monte-Carlo simulation error on ADE-XL
  • Snooping Around is All
  • Identification of a 6 pin smd chip (sto-23-6) marked E2

RSS Current Electro-Tech-Online.com Discussions

  • Does US electric code allow branching ?
  • Fun with AI and swordfish basic
  • using a RTC in SF basic
  • Faulty heat air gun (dc motor) - problem to locate fault due to Intermittent fault
  • Sump pit water alarm - Kicad 9
Search Millions of Parts from Thousands of Suppliers.

Search Now!
design fast globle

Footer

EE World Online Network

  • 5G Technology World
  • EE World Online
  • Engineers Garage
  • Analog IC Tips
  • Battery Power Tips
  • Connector Tips
  • DesignFast
  • EDA Board Forums
  • Electro Tech Online Forums
  • EV Engineering
  • Microcontroller Tips
  • Power Electronic Tips
  • Sensor Tips

Test & Measurement Tips

  • Subscribe to our newsletter
  • Advertise with us
  • Contact us
  • About us

Copyright © 2025 · WTWH Media LLC and its licensors. All rights reserved.
The material on this site may not be reproduced, distributed, transmitted, cached or otherwise used, except with the prior written permission of WTWH Media.

Privacy Policy