Anechoic vs reverberatory chambers for RF testing

[rhb]

I'm starting this thread by request via a PM.  I shall try to keep my English as simple as possible.  If I am confusing please quote the problem statement and ask me to restate it.  I'll also limit treatment to the basic concepts until we get to the mathematics and data processing.

There are two different approaches to test chambers;

anechoic - chambers with "reflectionless" walls

reverberatory - chambers which have reflective walls

Anechoic chambers are the most familiar.  They have been around for a long time.  The first ones used horse hair covered with powdered iron and carbon that was formed into the familiar wedges and left to sit until the mixture dried and hardened. 

I do not know anything more about the original material composition but assume paint or some other binder was used.  Aside from being fragile, they were very combustible and after several spectacular fires new materials were developed to meet fireproofing requirements.

The  pyramidal shape provides a means of gradually increasing the reflection coefficient.  If the tip size and spacing is small relative to a wavelength the reflection coefficient is the average of the tip area to the total.  As the pyramid increases in dimension it increases the average reflection coefficient based on the area.

An alternative to pyramids is to use mixtures which gradually increase the proportions of iron and carbon and are applied from maximum iron and carbon to minimum in successive layers.

The walls of an anechoic chamber are *not* reflectionless.  That is physically impossible.  What they do is substitute many small reflection coefficients so that the total reflection is spread out over time.  In numerical modeling these are implemented by surrounding the model volume with many cells of increasing reflection coefficient.

The result is that the impulse response is spread out from a spike to a shallow rectangle or some other profile.

Anechoic chambers are challenging to construct without a means of measuring the reflection coefficient between layers.  A pyramid of constant properties is the easiest way of creating a succession of small reflection coefficients.  As the total area increases the difference in the reflection coefficient is simply the ratio of the areas relative to a wavelength.

No matter what the construction method for making the wall coverings measuring the reflection coefficients is difficult.  this is why the pyramidal form was the first and still most popular construction technique for professional chambers.

Reverberatory chambers are much newer and I was not aware of them until quite recently.  They are only possible because of computer data collection.

The concept is magnificently simple.  The chamber has completely reflective walls walls.  Thus the signal bounces around for a long time.  In a simple box this would go on forever if the reflections were exactly |1|.  This makes it impossible to distinguish direct arrivals from reflections.

I don't recall who came up with the idea, but by placing  reflective paddles which can be rotated about a single axis in the chamber the reflection paths vary depending upon the paddle orientation.  The method works because if you place the paddles in random positions and take many measurements and properly sum them, the direct arrivals grow stronger relative to the reflections as the different reflections cancel each other.  This is the 1/sqrt(N) suppression of random Gaussian noise concept in different clothes.

Academics build reverberatory chambers in ordinary office space and then disassemble them when they have completed their testing.  The materials are cheaper and the construction simpler.

For an amateur effort the reverberatory chamber (RC) seems far preferable to an anechoic chamber (AC).

Make a metal box and mount the source and sense antennae (proper Latin plural) in the box.  Mount 2-3 shafts woth metal plates on them to form the paddles.  Then randomly shift the paddle rotations and measure the response.  Do this *many* times and record the data for later processing.  Stepper motors are cheap and easy to program with minimal computer software.

Randomness is essential to the method working.  So either a true analog random noise source or a very high grade (e.g. Mersenne Twister) algorithm is needed for each paddle step.

A simple steel box made from flat galvanized steel sheet with fiberglass shafts mounting small sheets of the same material driven by stepper motors, an RF source and detector and software is all you need.  What you are doing is trading data acquisition time for expensive materials and construction.

I am going o stop here as this is the limit of what I can recall off the top of my head.  I'll post figures and relevant text later.

Have Fun!
Reg

[mr ed]

I know of one fairly large company who  used an outside test area. It was a field and surrounded by crops. It was also FCC certified. Turntable etc. Think simple first.

[wn1fju]

It also depends on what you are trying to accomplish with a shield room.

At my job, our company built (or should I say hired a shield room contractor to build and install) a 20 foot x 40 foot x 12 foot RF shield room.  The inner lining was all steel plates with joints very carefully shimmed to prevent leakage.  It took several weeks to painstakingly close up all of the leaks.  The purpose of the room was to stop any outside signals from getting in, and more importantly, to stop our inside signals from getting out (it was a secure, classified facility).

The room worked very well, almost too well.  As the original poster described, there were reflections everywhere.  You could put a directional transmitting antenna at one end of the room and another directional receiving antenna at the other end.  It pretty much didn't matter where you pointed the receiver, the signal strengths were just fine in every direction.

We ultimately decided to convert the room into an anechoic chamber which involved coating the steel insides with ferrous tiles and then gluing RF absorbing pyramids everywhere.  But even that wasn't perfectly satisfactory.  What happens with these pyramids is that they really want to see signals coming in at perpendicular angles.  With the low 12 foot ceiling height, the shallow angles of reflection seriously degraded the RF absorption capability.  We were warned about this by the various contractors bidding on the job, but we weren't about to raise the ceiling.

Anyway, given the amount of reflections we saw with the original room, without the anechoic additions, I would think we would have needed a whole lot of reflective paddles.  It's a cute idea and I hope someone perfects it.

[xmo]

RF is everywhere - in almost everything.  Even devices that don't use RF may need to be tested for RF immunity.  Consequently, there are many testing labs.  Here is a list of services that one lab offers:

EMC Emissions and Immunity Standards
•   IEC 61000 3-2, 3-3, 4-2, 4-3, 4-4, 4-5, 4-6, 4-8, 4-9, 4-11, 6-1, 6-2, 6-3, and 6-4
•   IEC 60601-1-2
•   IEC 61326-1
•   CISPR 11, 12, 14, 20, 22, 24, and 32
•   ISO 14982-1 Agriculture EMC
•   FCC Parts 15, 18
•   Industry Canada ICES-001, 003
•   IEC 61000 3-2, 3-3, 4-2, 4-3, 4-4, 4-5, 4-6, 4-8, 4-9, 4-11, 6-1, 6-2, 6-3, and 6-4
•   IEC 60601-1-2
•   IEC 61326-1
•   IEC 61010-1, 60950-1, 62368-1, 60204-1 and 60335-1
•   CISPR 11, 12, 14, 20, 22, 24, and 32
•   ISO 14982-1 Agriculture EMC
•   FCC Parts 15, 18
•   Industry Canada ICES-001, 002, 003

Wireless / RF Standards
•   ETSI EN 300 328, 300 220-1, 300 440-1, 301 489-1
•   FCC Parts 15, 18, 22, 24, 25, 27, 74, 80, 87, 90, 95, 97
•   RSS-Gen, RSS-210, and RSS-247
•   ANSI IEEE C63.10, C63.26
•   Australia/New Zealand AS/NZS 4268
•   Japan Radio Tests Radio Law No. 131, Ordinance of MPT No. 37,
•   1981, MIC Notification No. 88:2004, Table No. 22-11


Here's their Anechoic chamber.  The vehicle is on a turntable.

[rhb]

Anechoic and Reverberation Chambers
Qian Xu and Yi Huang
Wiley/IEEE 2019

[rhb]

If I manage to get my shielded, soldered seam 28 gauge steel room built I plan  welding studs and mesh to support spray on mixtures of sheetrock mud, charcoal, pearlite and powdered iron or steel to a thickness of 1-2 inches to suppress internal reflections.  With some chopped fiber added and paint it should make a good lab wall.  Far from perfect, but better than nothing and not especially expensive DIY.  Certainly cheaper than ESD flooring.

It's messy, but the spray gun and compressor are cheap as is the light weight mud with pearlite filler.  That leaves powdered iron or steel as the major expense.