Measuring Shielding Effectiveness with Two Near Field Probes

Introduction

Sometimes you may find yourself needing to make a quick check on the shielding effectiveness (SE) of a material, such as plated plastic or shield gasket material. It’s possible to set up a quick measurement setup using near field probes by using a couple H-field (for magnetic field SE) or E-field (for E-field SE). You’ll also need a spectrum analyzer with tracking generator or network analyzer that covers the desired frequency range.

The use of two near field probes is not unique. In fact, I used this technique in the early 1990s to measure the SE of various plated plastics we were using at the time for oscilloscope enclosures during my time with Hewlett Packard. I even tried patenting the technique, but my lawyer discovered prior art. Both my colleagues, Doug Smith (http://www.emcesd.com) and Arturo Mediano (http://www.cartoontronics.com) have promoted this technique on their web sites and public seminars.

Measuring the SE in the near field is probably more pertinent for real products, because real enclosures are usually in the near field close to circuit boards. In fact, the results you get with this method won’t agree with the far field SE equations (SE = A + R + M) one generally finds in the literature. George Kunkel wrote an article recently deriving the equations for near field SE using circuit theory as the basis. This is referenced below in [1].

For the purposes of this article, I’ll be using a Siglent SSA3032X spectrum analyzer [2] with tracking generator and looking at frequencies in the range 1 to 1000 MHz. A pair of Beehive Electronics 100C H-field probes [3] were used. See Figure 1 for the general test setup.

The probes were clamped between erasers in a small vise to hold them an arbitrary distance apart. The erasers helped isolate the probe shafts from the metal vise (Figure 2). The probe distance doesn’t matter too much, except that they must be able to measure the sample without touching it and they must be close enough together to make a readable signal.

Connect one probe to the tracking generator output. Connect the other to the analyzer input. Try to separate the two coax cables to avoid coupling. Set up the spectrum analyzer as follows:

1. Start frequency = 1 MHz
2. Stop frequency = 1 GHz
3. Resolution bandwidth = 120 kHz (or 100 kHz) – not critical
4. Vertical scale = dBm
5. Reference Level = -20 dB
6. Preamp = Off
7. Attenuation = 0 dB
8. Tracking Generator (TG) = On (upper right on keyboard)
9. Tracking Generator Level = -20 dBm
10. In the TG menu, press Normalize
11. Turn TG = On

The SE response trace should appear in the top of the display and the top reference scale is now 0 dB. Placing any metallic sample between the probes will read out the SE directly versus frequency.

Note that the Beehive Electronics 100C probes I’m using have a sharp resonance about 560 MHz, which causes a spike in the response. I tried large paper clip loop probes and they exhibited a similar resonance. The use of the Beehive 100B (medium-sized) probes should move this resonance out of the displayed window. I didn’t have a set of these, so had to use the larger probes as shown.

I’d just ignore the resonance and continue the SE plot straight through. See Figure 3.

Here are some sample measurements. See Figures 4 through 9.

Summary

Near field shielding effectiveness is easy to measure if you have a couple near field probes and either a spectrum analyzer or network analyzer. Plated plastics and most EMI gaskets or fan shields are inferior to solid metal.

References

[1] Kunkel, A CIRCUIT THEORY APPROACH TO CALCULATING THE ATTENUATION OF SHIELDING BARRIERS, Interference Technology, 2016 EMC Shielding Guide, Included in this issue of the 2017 EMI Shielding Guide.

[2] Siglent SSA3000X-series spectrum analyzer, http://siglentamerica.com/pdxx.aspx?id=5113&T=2&tid=227

[3] Beehive Electronics 100C H-field probe, https://beehive-electronics.com/probes.html