Introduction
There are a couple of reasons for this paper. One is that folks who have an enclosure would like to verify that it is indeed shielding and operating correctly, and another is simple curiosity on how one would measure a test chamber's shielding isolation.
Simply expressed, we use a small transmitter and measure its signal both inside and outside the enclosure. We know what signal level the transmitter is radiating while it is "in the clear" and again measure the signal level when it is "in the box." The difference between the two measurements is the isolation of the enclosure. A technician may use laboratory test equipment like an RF signal generator and spectrum analyzer, but this ties up expensive test gear and requires a good understanding of operating and configuring the equipment. Whether one uses the JRE TVK-2 test kit or the labs existing test equipment, the general process remains the same.
The JRE TVK-2 Test Verification Kit
Everything you need for this measurement is included in the JRE TVK-2 Test Verification Kit. The kit contains:
JRE HPSS-1 High Power Signal Source — A synthesized RF signal source operating at 2.45 GHz, producing 250–500 mW of RF power. The 2.45 GHz frequency is chosen as a good match for antenna size, RF power generation, and ease of measurement. Since we are attempting to measure isolation greater than 100 dB, we need a high power source — and the HPSS-1 delivers. It is battery powered and completely self-contained — no external power cable is needed, which means you do not have to breach the isolation barrier with a power cord. A dipole antenna and charger are included.
JRE STA-1 Shielding Test Analyzer — A portable spectrum analyzer pre-tuned to the HPSS-1's frequency. Just turn it on — there is no frequency to set, no reference level to adjust, and no bandwidth to configure. The STA-1 is purpose-built to work with the HPSS-1 and displays the signal level directly on screen, with a dynamic range from 0 dBm to –115 dBm. This large dynamic range makes it easy to both verify overall shielding isolation and track down specific points of RF leakage. A Yagi antenna, a set of near-field probes, and a charger are included.
Carry Case — Everything is packaged in a protective carrying case for field use.
The HPSS-1 and STA-1 are also available separately if you already have one component. The TVK-2 kit is also useful for customers with walk-in shielded rooms who want to identify leak paths at door seals, penetrations, and other potential weak points.
How It Works
The concept behind the measurement is straightforward. The HPSS-1 radiates a known signal, and the STA-1 displays that signal as a pip on screen. The STA-1's screen has a 'marker' that indicates the highest signal level of the pip. In addition to directly reading the signal level, each division on the STA-1's display represents 10 dB, so reading the isolation is simply a matter of reading the marker or counting how many divisions the pip drops when the HPSS-1 is placed inside the enclosure. With the STA-1's sensitivity reaching down to –115 dBm, you have plenty of dynamic range to measure well beyond 100 dB of isolation.
To get a feel for the system, switch on both units and hold the STA-1's Yagi antenna at arm's length from the HPSS-1. You will see the pip clearly on the STA-1's display. As you move the Yagi further away, the signal drops off — just like driving further away from a local radio station or moving farther away from a street light. Power drops off as we move away. At 2.45 GHz, you will see the path loss being about 20–30 dB just by moving the antenna a meter away. Most shielding isolation measurements are at a distance of one meter, but for measuring very high isolation, getting very close to the test chamber will allow you to see even the slightest leakage.
A note on close-range readings: The STA-1 is a very sensitive instrument, and if you hold the Yagi right next to the HPSS-1's dipole antenna, the strong signal will saturate the STA-1's front end. The display won't show a nice clean pip — it will look overloaded and messy. This is completely normal and nothing to be concerned about. As you move the Yagi away and the signal level drops, the front-end saturation resolves and the STA-1 will indicate nicely. For the purposes of our isolation measurement, we are interested in the weak signal coming through the enclosure, so saturation at close range is not an issue.
For those who wish to see a clean reference reading at close range — perhaps to verify the HPSS-1's output level or to calibrate the "high water mark" more precisely — simply add a 20 or 30 dB inline attenuator to the STA-1's input. This will prevent front-end overload and allow you to bring the Yagi close to the HPSS-1 and observe a clean pip. Factor in the attenuator's value when interpreting the reading, and you will see the HPSS-1's signal level as expected. Remove the attenuator before making your isolation measurement so you have full sensitivity available.
Making the Measurement
With the HPSS-1 switched on, place it inside the enclosure with the dipole antenna in the vertical position. Slowly close the door and watch the STA-1's display — you will see the signal getting weaker and weaker until it is almost impossible to see, the pip becoming buried in the grassy noise floor.
On the STA-1, you should be able to see down 80–90 dB on the screen (8 to 9 divisions, each division being 10 dB). So right off the bat, we can see that the enclosure has a minimum of 80 to 90 dB of isolation — and this is with the Yagi antenna mere centimeters from the enclosure.
Here is the key: industry practice is to measure isolation at 1 meter distance. At 2.45 GHz, the path loss at 1 meter is approximately 20 dB. Since we are measuring with the Yagi held close to the enclosure (centimeters away), we simply add that 20 dB path loss factor to our reading to reference back to 1 meter. So if we read 80–90 dB on the STA-1 at close range, the actual isolation at 1 meter is 100–110 dB.
This is a fast, easy, reliable test of the shielding. It is far easier and every bit as reliable as using extremely sensitive lab equipment in an anechoic chamber — we simply add in a conservative 20 dB for the extra path loss at 1 meter distance and arrive at the same answer.
With the door closed and the HPSS-1 inside, slowly move the Yagi around the enclosure — along the door edges, around the I/O plate, over seams, and near connector entry points. You are "sniffing" for any localized signal leaks. Typically, you will see the signal pip very near the bottom of the display, indicating a weak signal and effective RF shielding. If you find leakage beyond expected limits, check that the door hinges are firmly seated and tightened, all screws on the I/O plate are tight, and connectors and cabling are properly installed. Most leakage problems are mechanical in nature.
It's not a bad idea to perform this test periodically to ensure your enclosure is operating correctly, especially after reconfiguring the I/O plate or after the enclosure has been shipped or severely jostled about in the lab.
Using the Near-Field Antennas to Pinpoint Leaks
The Yagi antenna included with the STA-1 is a far-field antenna — it is excellent for the overall isolation measurement described above, but its capture area is relatively broad. When you need to pinpoint the exact location of a small leak, the TVK-2 kit includes a set of near-field antennas designed for precisely this purpose.
The kit includes three H-field (magnetic) antennas and one E-field (electric) antenna. The E-field antenna is the simplest looking — just a straight length of PC board material. Don't be fooled by its simple appearance; buried inside its layers is a probe responsive to the electric field component of an RF signal. The H-field antennas have a loop or larger end and are responsive to the magnetic field component — the RF's magnetic field induces a voltage into the antenna's loop, just like an electrical generator where a moving magnetic field generates electricity. The different loop sizes allow progressively more precise locating of leakage, with the smaller loops giving finer resolution.
Why both E-field and H-field antennas? This is where it gets interesting. Consider a break in the conductive door gasket — perhaps from excess wear, something caught between the surfaces, or a bent door causing an alignment problem. This creates a "void" in the continuously conductive shell that is required for full isolation. The E-field and H-field behave differently at this void, and understanding why helps you interpret what the STA-1 is showing you.
At the center of the shielding void, the two conductive surfaces are not in contact, so a voltage can develop between them — the E-field is at its maximum here. But since there is no contact, no current flows, so the H-field is at its minimum.
At the edges of the shielding void, the conductive surfaces are in contact — a short circuit. No voltage can develop between short-circuited surfaces, so the E-field is at its minimum. But the short circuit means current is flowing, which generates a magnetic field — so the H-field is at its maximum at the edges.
In practice, you would use the E-field antenna to find the center of a void (where the gap is widest) and the H-field antennas to find the boundaries (where the gasket contact resumes). By switching between the two, you can precisely map the extent of even a very small leak. The smaller H-field loop antennas are particularly useful for isolating leaks at I/O plate screws — a loose screw that is not making proper contact will show a clear signal that drops away when the screw is tightened.
Final Thoughts
Measuring shielding isolation is not difficult and quite enlightening. The JRE TVK-2 Test Verification Kit provides everything needed in a single, portable package — just turn on both units and start measuring. Between the Yagi for overall isolation verification and the near-field probes for pinpointing specific leaks, the kit covers both the quick health check and the detailed diagnostic.
To watch the test procedure in action, watch the video below. For more information on the TVK-2, HPSS-1, or STA-1, visit our test equipment page.
