But wait, the UUT is in a screen room; how can an ambient signal get inside the screen room and contaminate the test results? In a properly designed and maintained screen room, it is not difficult to achieve significant isolation between the inside and outside of the room, easily on the order of 100 dB. Rarely is the screen room the guilty party. Often your nicely shielded test cables are the culprit – the ones with the shields that properly terminate about the circumference of their associated connectors. Those carefully designed shielded cables that are fished through a pass-through or “stuffing tube” in the wall of the screen room. You remember that pass-through with a length-to-diameter ratio greater than five that ensures that only insignificant amounts of RF can pass. This is the “waveguide beyond cutoff” effect. Unfortunately, you have defeated this powerful shielding technique by converting the waveguide into a coaxial cable that connects to an antenna both inside and outside the screen room.

For this technique to work, the waveguide/pass-through, often made of bronze, must be filled with a good dielectric, usually air. Your test cable violates this condition when it passes through the pass-through into the screen room without special consideration. The shield on your test cable becomes the center conductor of a coaxial cable whose outer conductor is the screen room pass-through. Viola, you have converted a waveguide into a coaxial cable and 2 antennas! RF energy from numerous ambient sources is easily coupled onto the that shield (antenna 1) outside the screen room, passes through this short, makeshift coaxial cable, and radiates from the shield (antenna 1) on the inside of the screen room – right into your measurement antenna.

Fortunately, it is easy to fix this often-overlooked problem. Just bond your test cable shield to the screen room pass-through. At the bonding point, stuff the pass-through (that’s why it’s called a “stuffing tube”) with conductive, usually copper, wool to ensure the tube is RF tight. If this test setup is used often, a more repeatable and easier-to-install arrangement would be to mount a bulkhead connector in the screen room penetration plate. Your test cables would then need appropriate connectors inside and outside the screen room – each with properly terminated shields.

I’ve demonstrated this effect many times to “non believers.” A simple way to do this is to set up the emissions measurement system to sweep over the FM broadcast band (88 to 108 MHz) or greater. For the first test, ensure that no conductive cables pass through any of the screen room’s pass-throughs. Take a sweep and note the “amount of quiet.” Next, fish a length of hookup wire (or about any conductor) through a pass-through tube leaving a few feet on the inside and outside of the screen room as your antennas. (The effect will be more dramatic if you support the free ends of the wires above the floor.) Take another sweep and note the amount of Led Zepplin and elevator music! This usually elicits an “aha” from observers.

Stu Benner
E3 Design Engineer
EMConsulting, LLC

Several years ago the Commission to Assess the Threat to the United States from Electromagnetic Pulse (EMP) Attack, spearheaded by my congressman Roscoe Bartlett (Maryland, 6th District, Republican), issued a report about the devastating effects of a High Altitude Electromagnetic Pulse (HEMP) event on the domestic U.S. population. A rouge state or terrorist organization could detonate a nuclear device over the U.S. and potentially blast us into the Stone Age without killing people or destroying buildings. The EMP Commission’s Critical National Infrastructure Report (or the Executive Report for those interested in an introduction or ‘lighter reading’) is an excellent treatise on this risk to our way of life.

A grass-roots organization called EMPACT America has sprung up to bring together “local citizenry, civic-minded companies, and grassroots activists to help prepare communities in New York and across America to prepare for an EMP attack. Working with local city governments and first responders, EMPACT America provides for coordination, education, and consulting services, helping to create a growing grassroots movement focused on EMP preparedness and recovery.” This is one of many similar groups likely to grow out of our new world disorder.

Stu Benner
E3 Design Engineer
EMConsulting, LLC

Whether you call it interference, noise, Radio Frequency Interference (RFI), Electromagnetic Compatibility (EMC), Electromagnetic Interference (EMI), Electromagnetic Environmental Effects (EEE, E3, or E³), solving interference problems is not “black magic.” By considering basic laws of physics, EMConsulting can provide effective engineering solutions to prevent or eliminate electrical interference problems in your products.