Waveguides are an important part of preventing signals from escaping a room or facility. They are used for specialty construction projects, like an MRI room in a hospital or a Sensitive Compartmented Information Facility (SCIF) used by members of the U.S. Intelligence Community. If you know you have a need for waveguides, it’s important to understand how different types of waveguides are used and how to choose one that’s right for you.
How do Waveguides Work?
Waveguides essentially act as a boundary that signals of a certain range of frequencies cannot pass through. Depending on the type of waveguide, signals are blocked or grounded. In many cases, this helps to keep a signal from being disrupted as it passes from one place to another. In some cases, like in secure facilities, waveguides are used to keep signals from leaving the facility that could be picked up by adversaries and used to gain information. Waveguides are made of metals with a low bulk resistance like brass, since these types of metal can better block electromagnetic signals.
Honeycomb Waveguides
Honeycomb waveguides are named for the hexagonal pattern that makes them up, resembling the inside of a beehive. These waveguides are used in vents because the small holes allow air to pass through but not the emanations. Because of the size of the holes, cables can’t be run through a honeycomb waveguide.
You can also see honeycomb waveguides in your home. They are used on the doors of microwaves to prevent radiation from escaping during use.
Honeycomb waveguides come with holes in a variety of sizes. Smaller holes will be able to block higher frequency of signals, but that also means less air is able to pass through. The need for radio frequency (RF) shielding needs to be balanced against the airflow of the space so that it can remain comfortable to be in. You may need to go with a larger-sized hole in the waveguide to meet airflow standards. There are tests that you can run to check that enough air is flowing through the waveguide and into the area.
Threaded Pipe Waveguides
Pipe penetration waveguides are used for wall penetrations where cables need to leave the space. These could be fiberoptic cables, control wires for imaging equipment or sprinkler pipes. These look like a metal pipe sticking out of the wall with cables running through it. It will then be stuffed with copper wool or mesh to further block the signals, and a sound block can also be added if there is a requirement for acoustic protection.
These waveguides come in a variety of shapes and sizes to fit the penetration.
Shapes
Pipe penetration waveguides can be purchased in circular, rectangular, oval and square shapes. Circular or cylindrical waveguides are the most common, since these waveguides are typically being placed in drilled holes, which are round. Circular waveguides are also threaded, which allows for better shielding. The threads of the waveguide are able to be tightened like a nut on the secure side and clamped on the unsecure side without making any further holes in the wall. An oval waveguide, for example, requires flanges that will need to be screwed into the wall. Any time more holes are introduced, there’s a higher likelihood of more signals escaping.
Sizes
Sizes of pipe penetration waveguides are based on a set ratio. For secure spaces, this is a 7:1 ratio of length to inside diameter according to the Best Practices Guideline for Architectural Radio Frequency Shielding. This means that a one-inch diameter waveguide needs to be seven inches long. This is an important ratio to keep in mind. The diameter you need will be dependent on how many things you’re trying to pass through the waveguide. If you have a lot of cables that need to pass through, like for fiberoptics, this means you’ll also have a long waveguide that will stick out from the wall significantly. Walls for a facility like a SCIF are typically five to six inches thick, while the largest waveguide is four inches in diameter and 28 inches long, so you need to account for that in construction. Other industries may use a 4:1 ratio for their waveguides.
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