Evaluate Shielding Effectiveness With Your VNA

The effectiveness of different materials for shielding can be tested using near field probes and your vector network analyzer (VNA).

A VNA is a typical instrument for RF/MW engineers working with antennas, filters, amplifiers, and many circuits and components in wireless and communication business.

But, this instrument is really useful for design engineers in a lot of applications during testing and troubleshooting EMI/EMC problems.

If you are not familiar with this instrument, try to know how it works. You will discover a really powerful for your lab.

I like portable instruments as Rohde and Schwarz FSH and Keysight Fieldfox families:  instruments including network analyzer, spectrum analyzer with tracking generator, and cable tester. Possibilities are only limited by your imagination.

This article will show you a well-known technique to evaluate the shielding effectiveness of a material in a frequency range. The technique is especially useful to compare different materials, different thickness for same material, or how a plastic material can affect in the frequency range of your interest (including plastic materials with metallic particles).

For this measurement you need: 1) your VNA; 2) two passive magnetic near field probes (I prefer identical probes); and 3) two coax cables.

Follow this procedure:

1. Set your network analyzer to measure S21 in the frequency range of interest. Remember S21 is a measure of power transfer from port 1 to port 2. As an example, in Figure 1 the analyzer is configured in that way in the frequency range of 30MHz to 600MHz.
2. Connect near field probes to ports 1 (out) and 2 (in) of your analyzer using the coax cables.
3. Place the probes close one to the other with good orientation for maximum coupling.
4. Calibrate and normalize so a flat response appears  in your instrument. With the calibration and normalization procedure any resonance or frequency dependence of the setup is removed.
5. Save in a reference trace the measurement result.
6. Without movement of the previous setup (this is critical!!!), introduce the material under test between the probes as shown in Figure 2.

Note the difference between the reference trace and the actual measurement. The measured value of S21 is reduced because less power goes from port 1 to port 2. That reduction is related with the shielding effectiveness of that material in the frequency range of interest.

In our example response is more than 30dB with a near flat response (sometimes you can find resonances at special frequencies).

In Figure 3 a picture of a different experiment is included. No normalization was applied but you can see the difference between coupling without shield (top yellow) and with shield (bottom light blue) on screen.

Obviously, this experiment cannot replace a full and complete test but intuitive and practical information can be obtained in a very fast and useful process.

My final advice: play with VNAs in your EMI/EMC troubleshooting problems.