Introduction
One of the most common questions we get — usually right after a customer takes delivery of their first chamber — is some variation of "how do I actually hold my device in place inside?" It seems like a small detail next to the headline numbers of shielding isolation and filter performance, but how you mount your DUT (Device Under Test) has a real effect on test repeatability, on antenna behavior, and on how easy your day-to-day testing is going to be. This paper walks through how JRE chambers are set up for DUT mounting, what works well, and a few things to avoid.
The Removable ABS Floor
Every JRE test chamber ships with a removable 0.062" thick ABS plastic floor piece sitting on top of the RF absorber foam. Its primary job is to protect the foam from being torn up by tools, fixtures, and devices being placed in and out of the chamber, but it doubles as a perfect mounting surface for DUT fixtures.
The floor lifts straight out — no fasteners. Most customers pull it out, take it back to their bench, drill or tap it to suit their fixture, and drop it back into the chamber. You can mark it up, mount threaded inserts in it, or screw a custom acrylic or plastic fixture directly to it. If you ever need a clean one, replacements run roughly $25 to $50 depending on the chamber size, so you can keep multiple floor pieces drilled for different DUT configurations and swap them in and out. That alone makes the chamber more flexible than people expect.
The floor is RF-transparent — plastic does not interfere with RF at the frequencies our chambers operate at, so anything you mount to it sees the absorber foam underneath as if the plastic were not there. You are not giving up any absorber performance by using the floor as a mounting platform.
Why Plastic, Not Metal
We get the occasional request for a metal mounting plate or threaded inserts in the chamber floor itself. We push back on this for a couple of reasons.
First, any sizable piece of metal inside the chamber is a reflector. The whole point of the LS-30 absorber foam lining the inside of the chamber is to soak up reflections so your antenna and DUT are not fighting standing waves and multipath. Bolt down a metal sheet on the floor and you have just created a reflective surface right under the DUT, which is exactly where you do not want one. Plastic, ABS, acrylic, polycarbonate, even wood — all RF-transparent at our operating frequencies, and all good fixture materials.
Second, threaded inserts permanently installed in the chamber floor lock you into one fixture geometry. The whole philosophy of JRE chambers is that they should not become obsolete when your DUT changes. Removable plastic gives you that flexibility for free.
If you absolutely need to mount something heavy — a large fixture, a turntable, a robotic positioner — talk to us. We can sometimes accommodate special mounting provisions during build, but in nearly every case we will steer you toward a non-metallic fixture sitting on the ABS floor first.
Antenna Placement
Antennas mounted inside the chamber are usually attached to the I/O plate via SMA bulkhead feedthrough. The JRE BBA-1 broadband antenna and the ANT-245 2.4/5 GHz dipole are the two we see most often, and both have male SMA connectors that screw directly onto the bulkhead.
A common worry is that an antenna mounted right at the I/O plate will be too close to the metal plate to perform well. In free space, that would be a real concern. Inside a small test chamber it is not — you are already in the near field, surrounded by walls, with the absorber foam handling reflections. As long as the antenna is firmly threaded onto the bulkhead, its proximity to the I/O plate does not matter for testing purposes. Whatever effect the plate has on the antenna's match becomes part of your fixed test geometry, and the gold-standard benchmarking process (described below) characterizes the entire RF path from antenna to DUT as a single repeatable system.
If you would rather give yourself flexibility on antenna placement, you can run a short coax pigtail from the bulkhead to position the antenna somewhere else in the chamber. Either approach works fine — direct mount is simpler, pigtail gives you more control over geometry.
A few practical notes on the antenna-to-DUT relationship:
- Keep the geometry repeatable. Whatever distance and orientation you settle on between antenna and DUT, write it down and replicate it every time. Inside a small chamber you are firmly in the near field of the antenna — the far field does not begin until 10 to 20 wavelengths away, which at 2.4 GHz means about 1.25 to 2.5 meters, far larger than any benchtop chamber. Near-field measurements are perfectly valid and repeatable, but they are exquisitely sensitive to geometry. Move the DUT half an inch and your reading changes.
- Use a gold-standard device to benchmark. Since absolute path loss inside a small chamber is hard to predict from theory, the way to get useful data is to characterize a known-good DUT first and then measure subsequent units against it. This is covered in detail in our paper How to Radiate Signals Into and Out of an RF Shielded Test Enclosure Using Antennas.
Cable Management Inside the Chamber
When your DUT has cables running from it to the I/O plate — USB, Ethernet, power, SMA pigtails, whatever — keep them as short and tidy as practical, and try not to drape them across the antenna or the DUT itself. A loose cable lying on top of the antenna will pull its match around. Velcro cable ties or simple plastic clips on the ABS floor work well for keeping things organized.
A common mistake is leaving service loops of excess cable coiled up inside the chamber. A coiled cable is an inductor, and an inductor near an antenna is going to couple. Trim cables to length when you can, and lay any excess flat rather than coiling it.
What About Larger DUTs?
For larger devices — laptops, mesh AP base stations, full computer systems — the same principles apply, just at a larger scale. The ABS floor on the JRE 2525, JRE 2830, or JRE 3036 gives you plenty of real estate to build a substantial fixture. Customers running automated test stations have built spring-loaded probe fixtures, lift-and-press fixtures, and cradle fixtures, all mounted to the ABS floor.
If your DUT has its own stand, feet, or mounting bracket, it can usually just sit on the floor as-is. Not every test setup requires a fixture — sometimes the DUT just needs to be placed consistently.
Drilling the Chamber Walls
While the ABS floor handles most fixturing needs, occasionally a customer needs to mount something directly to a chamber wall — a bracket, a permanent fixture rail, a pivot point. This is allowed, but with one important rule: every hole you drill must be filled with a tight-fitting metal screw. Stainless steel hardware, like what we use throughout our chambers, is the right choice. A drilled hole sealed with a properly tightened metal screw is mechanically and electrically equivalent to no hole at all — the metal-to-metal contact maintains shielding integrity. Just make sure every hole gets a screw, and that no hole is left open or filled with a plastic fastener.
To get to bare metal where you want to drill, you can cut the absorber foam back with a sharp X-Acto knife. The foam is forgiving — neat cuts heal up visually with a little care, and trimmed foam in a localized spot does not meaningfully affect the chamber's overall absorption performance.
That said, drilling a finished chamber by hand is awkward. The chambers are large, the walls are not always easy to clamp or brace against, and getting a clean hole pattern by yourself on the bench takes patience. If you know up front that you need holes in the chamber walls — for a test bed, a fixture rail, a mounting bracket, or any other purpose — let us know and in most cases we can build the pattern in here at the factory during fabrication.
This is one of the advantages of doing all our sheet metal work in-house. We run a 6 kW fiber laser cutter on the shop floor, which lets us cut hole patterns of just about any geometry, cleanly and accurately, as part of the normal fabrication process. To my knowledge, no other RF test chamber manufacturer has this capability under their own roof — most are sending sheet metal work out to a job shop, which is one of the reasons custom modifications are slow and expensive elsewhere. For us, a custom hole pattern is a routine setup change on a machine we are already running for your chamber.
Typical pricing for adding a custom hole pattern runs $150 to $200, regardless of how many holes are in the pattern, since the laser cuts them all in one operation. If you need patterns on multiple walls of the same chamber, each additional pattern is around $125. Every job is a little different, so this is a guideline rather than a hard quote — but it gives you a sense of the order of magnitude.
A Few Things to Avoid
A short list of things we have seen go wrong over the years:
- Metal toolboxes, brackets, or enclosures inside the chamber. Same problem as the metal floor plate — they reflect RF and create standing waves that distort your readings.
- Foil-backed foam or conductive padding. Sometimes well-meaning customers add their own padding to cushion a DUT, not realizing the padding is conductive and is essentially a piece of sheet metal in the worst possible place. Plain plastic, plain foam, or the original ABS floor — that is the right answer.
- Fixtures with long unsupported lever arms. Closing the chamber lid on a fixture sticking up too high can crush the seal. Always check clearance with the lid closed before committing to a fixture design.
- Open holes or plastic-filled holes in the chamber walls. As noted above, drilled holes are fine — but only when filled with a tight metal screw. A plastic plug, a piece of tape, or an open hole is an RF leak.
Final Thoughts
DUT mounting is one of those areas where the simple answer is also the right answer. Pull out the ABS floor, drill it for your fixture, drop it back in. Keep metal out of the test volume. Keep cables short and tidy. Use a gold-standard device to benchmark your setup. Do those four things and your chamber will give you repeatable measurements for years.
If you have a particular DUT, test setup, or wall hole pattern you are working on and you want to talk through it, give me a call or drop an email — happy to think through it with you.
