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Designing An Open Area Test Site (OATS)

Updated on July 26, 2015
For the best EMC test results, locate the end of humanity and plant your antenna there.
For the best EMC test results, locate the end of humanity and plant your antenna there.

Designing an Open Area Test Site, commonly referred to as an OATS, can be a challenging, complex, and mystifying experience. At the company I work for, we recently expanded our horizons and built our own OATS facility which included a year of team meetings, discussions with contractors, and finally the actual construction of the building. It was a long process with caveats around every corner, from frame-build specifics to power requirements. Throughout the development of the building, I learned quite a few things myself. Does this give me authority on the subject? Hardly, but first-hand accounts are always a welcomed breath of fresh air . . . especially if you’re suffocating under the prospect of having to build an OATS, yourself. Here are list of things to think about, remember, or ponder while developing your facility:

1) Is this really what you need? It’s easy to recognize that third party labs have increasingly developed open area test sites and a common thought is “if a certified lab is doing it, it can’t be a bad thing.” That’s true, to an extent, but if you’re in line to test flash drives or some other small item, the chances are that you do not need an OATS. Sometime all you really need is a three-meter semi-anechoic chamber. They are not ideal but good enough to get you in the right frame of mind. I’ve even seen a company or two that has a piece of sheet metal bolted to the ground outside (acting as a reference ground plane) and when they need to test a product, they roll out an antenna and go for it. The requirement of a ten-meter OATS is mostly dependent upon the size of your product and the level of precision for which you’re aiming. The point being, there are alternatives (heck, even the OATS can be considered an alternative test site.)

2) Considering ambient noise, is it feasible? It’s a known fact that pretty much anywhere in the United States, FM radio, AM radio, Television, and cellular signals will be bouncing around in the air. At an Open Area Test Site, these signals are to be expected. However, if your facility is in the middle of a major city, you’ll also see everything from air traffic control signals to school bus drivers complaining about kids at three o’clock. There can be so many spikes and so much ambient noise that having an OATS is impractical. You’ll want to take a preliminary scan of the area just to see what is out there.

If your typical weather looks like this, you might need a roof.
If your typical weather looks like this, you might need a roof.

3) What are the natural requirements? What I mean by this is: do you live in Indiana or do you live in southern New Mexico? For anyone living in the Midwest, weather is obviously an issue. Unless you’re going to be hauling your equipment in and out of your facility with ease, you would probably want to develop a frame structure around the OATS. However, if you’re going to be testing in a typically dry climate, the bolted down sheet metal previously mentioned would probably work out just fine. Often, companies like having a structure for the sake of security and aesthetics so a frame may be ideal, regardless of the weather conditions.

A Structural Insulated Panel (SIP) or, from a distance, a huge ice cream sandwich.
A Structural Insulated Panel (SIP) or, from a distance, a huge ice cream sandwich.

4) What materials should be used in construction? If you’ve come to the conclusion that you need a structure around your OATS / ground plane, the next step will be determining what to build and how to build it. If you’re feeling particularly frugal you can build a pseudo-canopy with tarps and PVC but I cannot attest to the performance of such a design. What most will do is build the equivalent of a pole barn.

The thing to remember with an OATS is that we are trying to allow the noise emissions from the EUT to spread out over an open area, unhindered. This means no reflective surfaces which can act as antennas (copper, aluminum, steel, etc.) Wood acts as a non-reflective material rather convincingly up to 1 GHz (though some case studies exist where wood is reflective at much lower frequencies) so the frame build can be rather straight-forward. Look into fiberglass as an alternative wherever feasible. Try to keep rebar as close to the surface as possible (ideally completely underground.) Use the minimal number of nails that can safely keep the building together (and don’t tell your insurance company about it.) Look into nontraditional materials such as structural insulated panels. The list goes on and on but the same principle applies: the less metal the better.

5) Where will equipment be used / stored? At a cursory glance this seems like an insignificant concern but once we think about distances and standard requirements, it becomes more serious. For instance, per the test standards, measurements must be made from 30MHz to 1GHz, which would mean at least two antennas will be needed (and I’ve heard rumblings that bi-log antennas are being frowned upon.) Measurements must be made from heights of one meter to four meters so that means a mast with potential motor controls will be necessary. Hauling this around is no fun.

Likewise, receivers, analyzers, pre-amplifiers, and maybe a PC or printer will be needed . . . which, on paper, sounds easy to handle . . . until you’re the one carting $50,000 worth of equipment across a field in between wind gusts. Storage is ideal and even better is a control room where measurements and data collection can take place. The key to remember is that because all of these devices need to plug into outlets, there will be long lengths of copper and other reflective materials. The control and/or storage rooms would need to be a certain distance outside of the measurement area (an ellipse with a major radius of 20 meters and a minor radius of approximately 17.32 meters for a ten-meter OATS) also known as the ‘obstruction-free’ area. It may also be practical to place the control room below-grade so that the equipment in the control room (and the associated switchgear) does not disturb the test results.

6) How will the tests be performed? Since every angle of the product should be tested to verify that it is not spewing out electromagnetic trash, you’ll have to find a way to monitor all four corners and sides. For smaller products, it might be easy to do this by hand but this is still likely to be time-consuming and tedious. Most test site developers settle on the use of a turntable that is either motor-controlled or pneumatic in nature. This way, the equipment under test can be rotated and monitored all at once without having to break up the testing for the sake of moving the product around. Time and money are saved which means somewhere a Project Engineer is smiling.

7) What do local codes/regulations entail? Will you need handicap access? Are bathrooms required? Is there adequate space for desired occupancy levels? These are just a few of the numerous ‘particulars’ that have to be thought of when building the OATS facility and, in all likelihood, even if you think everything through, your local fire marshal will bring up that one thing you never would have imagined. At the company which happily employs my services, we were told that our driveway to the building would have to be expanded because of a local code allowing fire truck access (even though we had designed it wide enough for a forklift or automobile.) It was the last thing on our minds but a relatively costly addition to our already large financial request. Remember, for however much we desire EMC perfection, we must also keep the legal beagles happy.

In a nutshell, those are the checklist items to think about when planning to construct an Open Area Test Site. Does this encompass everything? Absolutely not, but these are the main points to remember when the boss asks you why you are sending products to a nationally-recognized test lab for thousands of dollars and if there is anything in-house that is feasible. I should also note that if someone in management begins to get a funny idea about certifying results and testing third-party equipment within the facility it should be mentioned that a whole new can of worms is being opened in regards to lab certification and standardization. NIST (National Institute of Standards and Technology,) NVLAP (National Voluntary Laboratory Accreditation Program), ISO (International Organization for Standardization), and good old ANSI (American National Standards Institute) standards are all used in certifying a laboratory and encompass all the requirements necessary. The FCC outlines details for registration of a test facility and associated conditions.

Oh, and be sure to have as many individuals involved as possible as to not overlook something. So have fun, and don’t forget to remember that you’ll probably forget something.

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