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MagneticFieldProbe.php    9712 Bytes    23-04-2024 11:15:29

A DIY Magnetic Field Probe

What's all this EMI stuff anyhow ?

EMI (Electro Magnetic Interference) deals with unintended transmitted or received signals which trouble your circuit or experiment. Even so circuits developped and used at an university need not to pass all rigid tests for consumer electronics, you mayst shoot yourself in the foot if they do not. Here we present a Magnetic Field Probe and explain how to use it.

A DIY Magnetic Field Probe

A DIY PCB H-Field-Probe, useful to find sources of radiation

✈ Motivation

Circuits which work fine in your (shielded) lab may not do so, when sent outside to the "real world". Maybe they are not robust enough. Maybe high-impedance traces act as a receiving antenna for some radio-frequency garbage out there. Maybe your clock lines act as transmitting antennas and therefore you shoot yourself in the foot.

A lot of maybe up there. With the H-Field Probe we get a clear GO or NO-GO.

✈ Circuit Description

H-Field Probes may be constructed with coaxial lines. They may be build with coaxial cables or Coplanar Waveguides. We used the PCB approach here, as we can order many units up to our specifications.

A H-Field-Probe is in fact an electrically small loop antenna. As it is electrically small, the current is constant around the loop. The shape of the loop does not matter. It only depends on the current and the area spanned by the loop.

We used a 50 Ω coplanar waveguide here and did not change the geometry for the loop, in order not to introduce jumps in impedance.

A DIY Magnetic Field Probe

A DIY PCB H-Field-Probe, useful up to more than 2 GHz.

✈ Performance - Coupling Loss vs. Frequency

Magnetic Field Probe
Measurement Setup : A terminated 50 Ω line
Magnetic Field Probe
Coupling Loss up to 3 GHz

✈ Some Physics :-)

Farradays law says, that a magnetic field through the probe loop generates a voltage. And the induced voltage is proportional to the rate of change of the magnetic flux through a circuit loop:

V = 2 * π * f * B * A

The diameter (area) of the loop determines:

• The Sensitivity of the Probe
• The Frequency Response
• The Geometry Form Factor (Size)

The larger the loop, the more H field lines will cross the loop, hence better inductive coupling results in higher sensitivity. However, a larger loop has a higher inductance and therefore a lower resonance frequency. So a smaller loop gives a higher frequency response but is less sensitive.

Unfortunately the DC-to-Daylight Probe is not invented yet :-(
That's why you want to make a set with different sizes to comply with different frequency ranges.

✈ Usage

A Magnetic Field Probe is usually used with a spectrum analyser or an oscilloscope. In case you are hunting for very low (levelled) signals, a low noise preamplifier may be useful.

• Use a Magnetic Field Probe to hunt for currents.
• Use an Electric Field Probe to hunt for voltages.

The probe is swept over the pcb. If the signal level increases, you get closer to the "hot spot".

Magnetic Field Probe
The probe is swept over the pcb ...
Magnetic Field Probe
whilst watching the screen with the other eye :-)

The limiting values - in case you want the CE-logo on your device - are available here :


✈ Downloads

A DIY Magnetic Field Probe

A DIY PCB H-Field-Probe, useful up to more than 2 GHz. Top Unit with a Preamplifier

✈ Understanding Near Field Probes ... by Rohde & Schwarz

✈ Share your thoughts

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