How to create an anechoic shielded box yourself

Testing and evaluating the quality of a radio device is still fun. For such laboratory studies, specific equipment is sometimes required, which costs a lot of money. Including - anechoic shielded boxes.

My name is Andrey, I work in the Yandex hardware development team. Today I will share my personal experience and tell you how to assemble a shielded box several times cheaper than analogues.

Many modern high-tech devices have long been equipped with wireless communications. In order to ensure the normal functioning of the radio part of such devices, it is necessary to examine in detail all the radio parameters and conduct a qualitatively-quantitative assessment of the characteristics. Especially in a situation where the radio is the main connecting link of the device with the outside world.

An expression is used in specialized communities, which shows that the accuracy of measuring any qualitative or quantitative characteristics depends on a huge number of conditions: in any tests related to RF and radio, everything depends on everything. In order to conduct at least some sane and representative research on a radio device, it is important to reduce the influence of external factors to the maximum.

Under all other circumstances, there are three main enemies of any engineer who tests and measures the parameters of radio devices:

1. Interference from other sources of electromagnetic signals.
2. Reflected radio waves of the source itself.
3. Conducted (propagating through conductors) interference.

Typically, specialized rooms called anechoic shielded cameras are used to solve these problems.



Such cameras have shielding properties to separate the external electromagnetic environment from the internal. It also implies protection against electromagnetic interference and radiation of various nature. In addition to the general shielding contour, a specialized radio-absorbing coating is placed inside the chambers, which takes part of the power of electromagnetic waves in a given frequency range to reduce the degree of reflection of its own radiation.

Often such premises occupy a significant area and cost a lot of money. Therefore, as an alternative to large anechoic chambers, desktop boxes can be used.



The principle of operation in them is approximately the same as in their large versions: it is based on the fundamental mechanisms of interaction of the electromagnetic field and matter. A completely tested electromagnetic screen with the required screening coefficient is arranged around the device under test, and a radar absorbing coating is placed along the inner perimeter of the screen, which allows to reduce the reflection of the device’s own radiation.

If the equipment for such studies is urgently needed (and the delivery time of shielded boxes is often calculated in weeks and months) and with a minimum of costs, directness of hands, theoretical base and, most important, experience gained during the work come to the rescue.

During the time until the professional kit for such studies comes from the supplier, you can manage to make the boxes yourself! This is what I happened to do, so today I will share my experience with you.

So, we have to make a shielded anechoic box that would isolate the device under test from external electromagnetic signals and attenuate them by about 40-50 dB in the range from 2 to 6 GHz. In addition, it is important that the reflected signal of the device itself is attenuated in a special material and does not affect the test results.

Knowing the general principles for constructing shielded anechoic chambers and boxes, we can estimate what we need for its manufacture. It turns out approximately the following list:

1. Telecommunication box to be used as the basis of our box.
2. Shielding fabric - material for creating an external screening contour.
3. Radio-absorbing material for the sticker on the inner plane of the box.
4. Glue for mechanical fastening of the screen and the radar absorber.
5. Conductive glue for electrical connection of fragments of wallpaper pasted on a box.
6. Tools: glue brushes, scissors for cutting the absorber and shielding wallpaper, a screwdriver for removing doors.

We look at what materials are on the market, analyze their characteristics and get the following picture:

1. Telecommunication box. It should be suitable in size and have sufficiently tight joints between the door and the housing. I took the telecommunication wall-mounted anti-vandal cabinet EC-WS-075240-GY 19 ”from Netlan.
   
   
2. Shielding fabric. For the screen, Aaronia X-Dream wallpapers were found, the manufacturer of which claims a very high shielding efficiency: up to 90 dB in the range from 2 to 6 GHz.
   
   
3. Radar absorbing material. As a radar absorber, I chose MOX-P-70, which provides a reflection coefficient in the range from −40 to −60 dB.
   
   
4. Conductive glue. The best option turned out to be conductive glue on Kontaktol-Radio graphite.
   
   
5. Glue for mechanical fastening of all components. I have ROGNEDA Metall 88 glue in 750 ml cans.
   
   

I calculated the amount of materials for two boxes: in one of them is the device under test, in the other - the signal source. The total cost came out like this:

Component	Qty	Price / piece (rub.)	Total (RUB)
Telecommunication box	2 pcs.	4162	8324
Shielding wallpaper	5 sq. m	5042	25 210
Radar absorbing material	3 sq. m	10 190	30 570
Conductive adhesive	16 pcs.	200	3200
	3	338	1014
		:	68 318

It turns out that we spend 34,159 rubles on one shielded box . Offers of Russian, Western and Chinese suppliers for boxes with the same volume of the camera and the degree of shielding start from $ 1,500 excluding shipping costs.

After purchasing all the materials you can begin to create our anechoic boxes.

First of all, we remove the doors and dismantle the locks and the welded fastener from them in order to seamlessly stick the screen and the absorber.



Then measure and cut the wallpaper. Glue them along the contour of the box, completely connecting all the joints between the parts.











After that, we fasten the door and check for electrical contact between the components of the box and parts of the wallpaper. Resistance should be less than 1 ohm.



Next, measure and cut the radar absorber and glue it on all the inner planes of the box.





I strongly recommend that you carry out all work with gloves: the absorber is made on the basis of graphite and leaves traces, like from a pencil lead.



In total, it took me about 6 hours to create two boxes, taking into account the time to dismantle the lock, all the necessary measurements, calculations and cutting of material.

After the box is assembled, it must be ventilated and wait for the glue to completely dry. The glue dries quickly enough, and the next day the boxes can already be used for their intended purpose. For express verification, we put a cell phone inside, close the box, connect the ground and try to get through.

As expected - the call does not go through, the connection with the Bluetooth headset is disconnected, the phone disconnects from the Wi-Fi router. Based on this, it can be assumed that we have managed to some extent with the task.

It would be great to conduct an optional comparison of professional boxes with known parameters and those that I got in a specialized laboratory - to get specific numbers and indicators on the degree of shielding in the working frequency range and reflection coefficient. But for now, due to the situation with coronavirus, this will not work. As soon as the opportunity arises, I will conduct a study and write about the results in a new article - I hope this will be interesting to readers!