Faraday Bag testing in a reverberation chamber – a preliminary study

Alistair Duffy

Overview
In the previous issue of the newsletter, I wrote a small piece to introduce the fact that some tests are being undertaken at De Montfort University on Faraday Bags.  This has clearly generated some interest from the Industry as a more general problem looking for a solution.  This article presents some of the preliminary results on Faraday Bag testing, including a short introduction to the mode stirred reverberation chamber.

Faraday Bags
The ‘Faraday bag’ is intended to shield a mobile phone or similar small device to prevent unwanted applications being invoked remotely, such as wiping the memory, or to prevent possible problems with veracity of evidence.  It is, therefore, of interest to the police and security forces. The key to the quality of the bag is the quality of the shielding.  Good shielding reduces the signal strength to a point where the ‘phone and the base station cannot detect that they are each there.  In order to determine how good the bags are at this, it is important to test the bag as a complete unit and not just the material itself.  This article addresses testing the bags in a mode-stirred reverberation chamber and we are indebted to Disklabs (http://www.faradaybag.com/) for providing a Faraday Bag for us to test and publish the results of those tests.

The Mode Stirred Reverberation Chamber
Electromagnetic Compatibility (EMC) measurements often require the measurement of emissions from a device / system or the illumination of that device / system from an external source.  In many ways, an ideal facility is somewhere in deep-space away from anything liable to interfere with those measurements.  The impracticality of this has lead to a number of recognised facilities being developed.  These include:

• The Open Area Test Site (OATS) which consists of a very flat conducting plane over which tests are performed, nothing that can influence the results is allowed within the area of the ground-plane.  The price that gets paid (compared with the fictitious deep-space facility) is the existence of the ground-plane and the resulting reflections which cause constructive and destructive interference with the direct path signal.
• Anechoic (or semi-anechoic) chamber, which consists of an electromagnetically isolated volume where radiation absorbing material (RAM) has been placed on the surfaces or some of the surfaces.  This is a step towards the deep-space ideal but is expensive.
• Mode-stirred reverberation chambers isolate the internal, test, environment from the polluted external environment by enclosing it in a metal shield.  Effectively, it is a Faraday cage.  A major problem with a shielded room such as this, without a stirrer or RAM, is that the field strength can vary by 40dB within a distance of a few centimetres due to standing wave phenomena.  However, this high field strength can be capitalised on by inclusion of a metallic ‘stirrer’ that changes the boundary conditions so those hot-spots move around – i.e. stirred – and bathe the object under test in a varying field.

There are advantages and disadvantages with all these facilities (and those that have not been described here).  However, the reverberation chamber does have a number of features that makes it attractive for shielding testing such as the tests being carried out on the Faraday Bags.  Firstly, the movement of the fields due to the stirrer makes the fields statistically uniform.  So, assuming adequate stirring, the mean field strength over one revolution of the stirrer, is the same anywhere in the working volume and, if we assume that the field at a point has components in the three orthogonal Cartesian directions (i.e. x, y and z), each of those components is the same value and 1/3 of the value of the overall field.  In simple terms, it does not matter where the test object is put or in what orientation it is placed, it will still experience the same fields. Secondly, the effect of this moving standing wave is to present relatively high field strengths at the receiver for modest input powers, which means that if the highest value of received signal is measured for all stirrer positions, a much lower input power is needed than if an OATS was to be used.  Thirdly, the test object will see a maximum field strength at all locations over its surface sometime during the full revolution of the stirrer. Hence, the device under test – in this case the bag with an enclosed receiving antenna – is placed in the chamber and illuminated by an antenna with a mechanical ‘paddle wheel’ stirrer rotating to move the electromagnetic hotspots round in the room to ensure that the device under test is illuminated by a worst case field strength from all directions and all angles of incidence over one rotation of the stirrer.

In terms of measuring shielding effectiveness, the reverberation chamber has an advantage over other possible methods.  The traditional method is to place the material being tested between two antennas and take the difference in received signal with, and without, the material in place.  There are many variants of this approach including the use different antenna types and waveguides.  Unfortunately, this generally measures the bulk shielding effectiveness of the material and not the finished product.  Of course, the finished product can have inherent weaknesses due to its manufacture or design which a bulk measurement will not be able to test (unless the designer creates specific tests to explore these design or manufacturing issues).  However, considering the points made previously about the reverberation chamber, an object placed in a reverberation chamber can have its total shielding effectiveness tested because all locations over the surface of the product will see a high field strength thus avoiding the case where an unexpected weak-spot was not identified until the product failed in actual use.  Similarly, the power required to test to an adequate level of discrimination is relatively low.

Figure 1 gives some dimensions of the chamber at De Montfort University (the input and output points give an indication of where the illumination and the test object are placed).

Figure 1 an indication of the dimensions of the DMU reverberation Chamber.

Thus using the reverberation chamber to test complete Faraday Bags has some potential advantages.  The next section describes the tests.

Setting up the test
The tests undertaken on the bags are, initially, to determine how much electromagnetic shielding the Faraday Bag provides the mobile phone.  In order to set up the tests, a receiving antenna was placed in a mobile phone body – actually this was a clear protective cover – which will ensure that the bag material will be kept at the same distance from the antenna as it would be in practical use.  The antenna was a simple monopole tuned for resonance at approximately 1800 MHz rather than a multiband PIFA or similar antenna.  This was chosen as a simple way to undertake tests at a required frequency.  The measurements are made by comparing the insertion loss with and without the bag between the transmit and ‘phone antennas using a network analyser.  One of the difficulties with the test is that the antenna in the Faraday Bag needs a cable running through the bag itself.  While this is not the recommended use of the Bag, the potential deleterious effects of doing this were ameliorated as much as possible by folding over the top of the bag as much as possible, ensuring that the seal around the cable is as tight as possible and using ferrite clamps on the cable itself, close to the bag entry, to attenuate any interference due to currents on the shield.  Figure 3 illustrates the test-phone construction.

Figure 3 Test phone construction

Results
Testing was undertaken over a range of frequencies (1500 MHz – 2100 MHz).  Over a complete revolution of the stirrer, the maximum value of coupling at all frequencies was determined for the case of the bag not being in place and with it being in place.  Then, the differences between the two sets of results were taken to give a measure of the shielding effectiveness of the bag as a whole.  Figure 4 presents the shielding effectiveness results for the Faraday Bag.  The results were obtained using 200 stirrer steps per revolution and 401 points across the frequency range.

Figure 4Faraday Bag shielding effectiveness at 1800 MHz +/- 300MHz

 

The best way to interpret the results is to look for the minimum shielding effectiveness in the region of interest. Around 1800 MHz, this is 30dB.  The excursions below this are relatively minor and the accuracy can be improved by increasing the data-point density, i.e. the number of points used in the measurement.  These figures agree well with tests published on the manufacturer’s website http://www.faradaybag.com/faraday_bag_testing.html which state that the attenuation at 1800 MHz and 2100 MHz is 30 dB (these were single frequency measurements)

Discussion
This article has provided a fairly gentle introduction to the application of reverberation chambers to testing Faraday Bags and has presented some preliminary results from one bag.  Despite the initial shortcoming of the cable egress from the bag itself, the results suggest that the bag tested performs well around 1800 MHz and this method of cable entry is possibly not a serious effect.

One question that is reasonable to ask is whether there is any real benefit in buying a Faraday Bag when foil trays (the sort used for home baking or take-away meals) are nearly two orders of magnitude cheaper.  In order to provide some evidence to answer this question, the same test was performed using a foil tray with a foilised lid (with the lid being placed foil-side outwards to improve the contact).  The comparative results are given in Figure 5.

Figure 5 Comparison of shielding provided by a Faraday Bag and a foil food container.

It can be clearly seen that there is a difference of around 10 dB at 1800MHz, with the Faraday Bag being better.  The foil container shielding may be improved by using some foil for the lid rather than the foilised cardboard lid! Of course, it is up to the user to decide if a shielding effectiveness of 15 – 20 dB for a few pennies is cost effective when 30dB can be provided for a few tens of pounds.

Further work is underway to look towards improving the test methodology and ensuring the accuracy of the results, as well as testing over both narrower and broader frequency ranges.