You’ve been tasked by management to build a new, from scratch, EMC test facility. Among other items, you need to decide between an Open Area Test Site (OATS) or an RF Semi-Anechoic Chamber (SAC). How do you decide? To someone who has been there, done that, the choice is clear, but what are some factors that might sway the decision one way or the other?
Design of an OATS
An OATS is defined in the standards basically as a large area, free of objects above the ground that might reflect RF energy. The standards go on to generally require a metallic ground plane and the minimum clear area. This clear area is stated as an elliptical area whose major axis is twice the measurement distance and whose minor axis is the square root of 3 times the measurement distance. A pure OATS would not have a weather protective cover over the EUT or the whole site.
Design of a SAC
In simple terms, a SAC is an RF shielded box lined on the walls and ceiling with RF absorbing material to simulate an ambient free OATS. In the past the design of a good SAC was as much art as it was science and many different designs were tried as engineers aimed for specifying a chamber that would meet the same requirements as an OATS while not breaking their employer’s bank. Today chamber manufacturers have much better absorbing material to work with and once they are consulted on the size requirements the design work for the customer becomes one of laying out the facility in a manner to allow convenient testing of the products envisioned.
What are some of the considerations and “gotchas” in an OATS?
While the concept of an OATS is fairly simple, the details make them a bit more complicated.
First, and foremost, the OATS must be built in a location that satisfies seemingly conflicting requirements. The OATS should ideally be built in a location that is convenient for the company. Travel distances and times between the development facility and the OATS should be kept to a minimum to avoid non-productive time for personnel and hardware moving between the sites.
The OATS must be built in a place that have minimal, if any, ambient RF signals. This means that it should be built in a location where radio (especially FM broadcast) and television reception is terrible. Otherwise, the operator at the OATS will have to identify every signal received and decide if it is from the EUT or is an ambient signal. And if this is the only emissions test facility available there is the distinct possibility (seen by the author) of missing an emission entirely because it is buried by a local radio or TV station.
The minimum dimensions for the clear area ellipse have been shown by a number of engineers over the years to be inadequate. Engineers at HP in Cupertino, CA determined that these dimensions should be doubled and further investigation by an engineer at Intel Corporation in DuPont, WA showed that even with doubling the dimensions it was necessary to take further mitigating actions to meet NSA requirements. For a 10 meter OATS (10 meters being the measurement distance) this requires a large area of ground, ideally remotly located. The author built what might be the ultimate OATS nearly 30 years ago when he built a 30 meter OATS with a double sized clear area ellipse and then wound up only using it at 10 meters. Clearly overkill by accident, but it eliminated any problems with reflecting objects above the ground. The site did, however, have other challenges.
What are some of the considerations and “gotchas” in a SAC?
The first consideration is financial. Will the company commit to paying the cost of a SAC? A 10 meter SAC is a major facility and is not cheap. Prices have been as low as about $1,000,000 and have gone significantly higher, depending on size and features desired.
Once you get past the cost issue, what are some other considerations?
Where will the SAC be located? Does the company already have a facility where the chamber will fit? Is the ceiling high enough? Or is the company building a new facility where the chamber will be located? Is there enough space for the chamber, control room (or desk) and preparation area for test samples? How about storage space for test equipment and test samples? How will the facility be laid out, both for operational and convenience considerations? If the SAC is going in an existing building are there adequate facilities for it? Power, air conditioning, compressed air? If you are marketing your product internationally, how will you provide power at frequencies and voltages not normally seen in the location where the lab is being built?
OATS vs SAC
Let’s take a look at some advantages and disadvantages of an OATS vs a SAC.
An OATS is a fairly simple facility, in principle, to design and build. There are a number of details that one learns by trial and error, but the basics are simple. It has the advantage over a SAC in that it is less expensive to build. I didn’t say “cheap”, I said less expensive. OATS facilities can be built for a few hundred thousand dollars plus the cost of the land. They can be significantly more expensive, depending on the size, how the ground plane is constructed and how the EUT or even the site is to be weather protected. Land is a major cost factor in any metropolitan area. “Less expensive” always appeals to bean counters regardless of other factors.
On the other hand, an OATS should be built in a location where the RF ambient is minimal. This should preclude locating an OATS in a metropolitan area as there are numerous radio and TV stations and other radio services in operation. Over the years the author has seen OATS facilities that were even built on the roof of a building. The “land” was cheap, but you had to ask what made any difference between putting a TV antenna on top of a mast on the roof (these were the days when cable wasn’t as ubiquitous as it is today) and putting an OATS on a roof. Not the best place to put a test facility where a low ambient was needed. In any case, the low ambient requirement generally requires placing the OATS well outside a metropolitan area and raises the operating costs due to travel time to get to/from the site. And, while this requirement may be met by a given location at the time construction is commenced, the author has seen excellent OATS locations rendered unusable over time due to increasing RF ambient signals, resulting in the abandonment of the facility after a number of years.
While the design of an OATS seems simple, the author has experienced problems relating to building it “too well”. That 30 meter OATS referred to earlier had its ground plane on a concrete slab, shot in with a laser level. Very flat. A great idea, but even in California it does rain and a perfectly level concrete slab doesn’t drain very well, resulting in water getting in to underground fixtures like cable connection boxes and the like. When he built OATS facilities in the northwest (where it is known to rain a lot) the ground planes were on wood decks, where drainage wasn’t an issue. Be aware of propagation velocity differences between different materials that may be used to construct a cover over the EUT. Have a uniform design in the wall between the EUT and the antenna, or else unexpected results may be found. Ask the author over a beer about this matter sometime, it is educational.
An OATS requires periodic maintenance to prevent operational problems. Haul ropes and guy ropes for antenna masts must be inspected or replace periodically as they deteriorate due to the UV in the sun’s rays. Allowed to progress too far and this deterioration can result in antenna damage (the voice of experience here). Oh, and what happens when lightning strikes an antenna mast? It has happened.
Comparing an OATS and its issues with a SAC, the SAC has the following advantages and disadvantage (a major one).
A SAC can be built anywhere you have the space for it. Due to its design it is impervious to interference from local radio and television broadcast stations. Even an amateur radio station with a (legal) 1500 Watt amplifier and a nice beam antenna across the street from the chamber is not an issue. A SAC must be properly designed and built, but today this isn’t a major difficulty. Weather protection isn’t an issue as the SAC is built indoors.
Conveniently locating a SAC minimizes the time and cost associated with personnel traveling to/from the lab. It may even be located down the hall from the development group, making it very convenient for all testing from initial experiments to final qualification testing.
Periodic maintenance of antenna mast mechanisms is greatly reduced, as is the need for guy ropes. There aren’t any winds inside a chamber to deal with, unlike at an OATS. There still are things that should be checked and cleaned, but they are not needed as often. You do need to protect absorber material from curious people who are unaware of the damage they can cause just by squeezing the material to see what it feels like. You might consider mounting a sample outside the chamber where they can satisfy their curiosity without causing harm to the working material.
The major disadvantage of a SAC compared to an OATS is cost. An OATS can be built for significantly less initial outlay of cash than a SAC. Over time, however, a SAC is a far better choice. Yes, the author is showing his bias.
Conclusion
The author freely admits his bias in favor of a 10 meter RF semi-anechoic chamber over an OATS. It comes from decades of experience with both types of facilities. A SAC can be built in a location far more convenient for the company. It takes far less space than an OATS and typically requires less ongoing maintenance. It does not suffer from existing or future RF ambient signals. While initially more expensive to build (neglecting land acquisition costs) it can be less expensive over the life of the facility due to reduced travel and maintenance expenses. The choice is left to the reader (and his company), but you have to ask yourself why so few new OATS facilities are being built, while chamber manufacturers stay in business.
Keep in mind, as well, that standards change over time. Whichever way you go (OATS or SAC) it is always nice to be clairvoyant and design the facility so that if modifications become necessary they can be incorporated in the future without having to build a new facility from scratch. Yes, the standards committees do try to avoid such things, but sometimes a seemingly innocuous change can have a significant impact on lab design. Leave yourself some wiggle room.
