RF Materials Penetration Testing

Taking measurements for determining WiFi Signal Penetration through various building materials

Introduction and Scope

For various WiFi projects, I have had to ascertain attenuation values due to the presence of materials found standing between APs and Client Devices. These are simple structures such as walls, windows, and structures, that are located between transmitters and receivers, such as an AP and a Client Computer or between two Bridged APs, or barriers that lie between APs that form a Mesh.

This writing shows you the methods and thinking behind this testing in anticipation of assisting the reader pursuing best possible RSSI values for their WiFi deployment.  At a minimum, this paper should help you understand how barriers impede WiFi performance and affect good signal quality.  The reader will learn to calculate these attenuation values and determine the Path Loss due to various materials as well as open air.

Motivation

When deploying APs in emerging markets, it becomes necessary to economize by using cheaper products. In these initial markets, we were looking at a cost model where WiFi signals could be shared between tenants in housing structures made of various materials such as: Concrete, Steel Reinforced Concrete Block, Clay Brick, hollow Clay Block, Block with Concrete Plaster, Wood, Tin, Sheetrock, and Glass, just to name a few. To verify the viability of sharing WiFi signals across barriers that utilize these materials, I performed RF Penetration Testing to provide measurements showing the effective signal RF signal strength. For the final reports, all of the aforementioned materials were measured using US standard thicknesses, and I did not homologate across all thicknesses available. More complete testing of this nature has already been performed and results have been published by the National Institute of Standards Testing, or NIST, so my goal was to qualify the numbers I saw against their more complete testing verifying that my data was reliable.

In these projects, the cost of flexible APs was a paramount factor. Some of the models considered for use were IgniteNet's Spark2 AC Access Point that comes in around $80-$99 US and MikroTik hAPs Access Points that sell between $49 and $69 US per unit.  These units were selected for their accessible pricing and quality. One of the challenges to selecting these cheaper units is that no one has published extensive test results which makes RSSI and Attenuation information difficult to validate through comparison. 

To remedy this for companies that I consulted for, I came up with some simple methodologies to use in testing.  The reader will find these in the section below titled Methodologies. 

Before diving into measuring each and every type of material out there, I came across sources of this information that should be used to consult and verify the numbers you see in your testing.  As mentioned above, probably the most reliable of these resources are measurements performed by NIST. Whenever taking measurements, I was sure to compare my numbers to the numbers gathered by the researchers at NIST.  The numbers in that guide; however, do not show attenuation values for the 2.4GHz spectrum.  The values in their guide were arrived at before the 2.4GHz spectrum came into unlicensed usage. Today 2.4GHz is used less and less in commercial situations, and the lack of those values in their seminal work doesn't make their findings any less relevant. Their work shows solid testing in the 5.0GHz and 6.0GHz frequency ranges,  more commonly used today. At the time of NIST's testing, these bands were dedicated to other resources such as Citizen's Band, and Walkie-Talkie radios, and they continue to be important to the consumer.

In this guide I also talk about the Portable Test Configuration that I built for this testing.  This "Test Rig" made it possible for me to locate materials in actual use, such as different types of Windows used in Commercial and Residential buildings.  This made it so much more practical for my testing and permits me to present data that is gathered in "real-world" scenarios.  In this guide, I'll show you how to obtain the parts and build your own Test Rig, as well.

Methodology

After we explore a common mistake in taking measurements and measuring correctly, we'll discuss some of the Math involved.  Then, we'll jump into the measurements you should be taking. 

When measuring, you will

1. Measure Open Space

2. Measure Through a Material You Know

3. Compare to Known Values (NIST documentation)

4. Measure Signal Reflection 

And finally, we'll discuss how to delve into building your Test Apparatus to take measurements of your own.

Measuring Correctly

The Basic Formula

When taking measurements on signal strength, the approach that most of us take is to put our client device next to an AP to commence measuring the RSSI strength of the WiFi signal.  Then, we move away from that device and record the diminishing signal.  This not only seems right to us, but is the same method that many engineers resort to when doing Rate-Range testing.

Figure #1 - Erroneous Starting Point

The problem with this type of scenario is that the speaker and listener start too close to each other.  This method introduces uncertainty into the process.  The closer you are to the energy source, such as the AP, the more you are affected by even the slightest movements.  

Figure #2 - Distance vs. Path Loss Chart

Generated from actual data, the chart shown above typifies pathloss measured against the distance from the AP.  At 0.001m (or 1 centimeter), the pathloss is low, but as the client device is moved away, the loss is significant for distances very close by:  2 cm, 5 cm, 25 cm, 50 cm, 75 cm.  Once the client moves to a distance of 2 meters, the losses start to flatten out.  At 2, 3, 4, 5 … meters distance the difference is minimal. 

Summarizing the data above, the client sees large losses in the first 1 meter of distance from the radiating energy source.  A difference of 1 cm drastically changes the value.  Now look at the same curve at 4 and 5 meters distance. A difference of 1 to 2 cm is negligible.  The loss of signal is minimal in comparison.  

To put it into the words of the respected WiFi consultant, Nigel Bowden, "If we relate free space loss characteristics to taking RF measurements, the closer we are to the RF source, the greater is the impact of any errors in distance measurement  in our RF readings." He continues, "if we are inside the first meter between the RF source and our measuring point, a distance measurement error of just a few centimeters could give us a significant error of several dB in our RF signal measurement."

Nigel's observations confirm what you see with the data, above.  The closer you are, the less tolerant you are to change.  If you set your equipment apart by 4 to 5 meters, you see minimal change if they are off by a few centimeters.

Figure #3 - Proper Initial Configuration

With that said, you should choose to begin measuring path-loss by setting your Transmitter and Receiver at 5 meters apart. 

The Math

You may have already deduced that the way to properly measure RF signal loss is to use a formula such as:

Loss-due-to-Barrier = 

  <Open-Space-RF-value-at-5-meters> - <RF-value-through-the-barrier>

In other words, you need to take measurements in free space, with nothing but air between your transmitter and your receiver.  Then, take the same measurements, using the same distances, and transmission settings with the barrier found between the transmitter and receiver.  Subtract the second value found from the first, and you now have your "Pathloss Due To Barrier" number. In other terms, you have now found your attenuation factor due to the presence of an RF Barrier.

Let's work through a simple example.  In the diagram below, the RSSI value measured was -66dBm.  

Figure #4 - Open Air Measurement

Now, with utilizing the exact same equipment, settings and distances, we arrange the testing rig around the WiFi barrier to be measured.

Figure #5 - Same Measurement with Barrier

So in our example, if we obtained a free space measurement of -60dBm, as depicted in Figure #4 above and then we obtain a measurement with the barrier at -66dBm, as depicted in Figure #5 above, then our loss due to the RF Barrier is:

-60dBm  -  -66dBm = 6dB loss due to the barrier

Measure Open Space

As explained in the previous section, you need to start by  measuring the signal that is lost due to simple attenuation in Free Space.  We call this "Free Space Path Loss" or FSPL.  

To get this number, set up your configuration with the client device sitting 5 meters away from the transmitting AP.  You'll need to use software to measure the Received Signal Strength Indicator, or RSSI.

For Mac, you can use the Beta version of Inssider.  Download it at:

https://www.metageek.com/downloads/inssider-mac/

The Windows version has been on the market for a while, you can download it at:

https://www.metageek.com/downloads/inssider-win

You'll have to set up a free Metageek account to get access, but it's worth it as the tool permits you to receive and graph constant SSID output.  

Figure #6 - Inssider output

As you run your test, and the AP is transmitting, you'll see the RSSI rate displayed as a moving line.  Once you maintain a consistent number, note it down.

Measure Through a Material You Know

When performing Materials Penetration Testing, start with Materials with known numbers.  The NIST document provides an excellent starting point.   In my effort, I started with a block wall that was built outdoors using hollow concrete block.  I moved on to brick walls and finally to wood structures. My measurements were extremely close to those gathered by NIST.  I'll speak to that in the comaprison section below.

On a customer sponsored trip to Kenya, and on numerous trips throughout Brasil, I noted that the most commonly used building materials are Concrete, Steel Reinforced Concrete, Masonry Block, Clay Block, Brick and in the case of Kenya, many roofs are fabricated from Corrugated Tin.  It is my believe that the majority of Central, and South American countries share these same building materials. 

I located structures, and built structures, to perform my testing.  Being the son of a Carpenter, I utilized learned skills that are common in the trade to construct and fortify these walls. 

Compare to known values (NIST document)

The Engineers and Scientists at NIST have done a tremendous job in taking and recording these measurements.  Their work stands to this day as accurate and useful in this field.  

The NIST team built a specialized framework for testing.  In almost all of the materials tested, they performed measurements through single, double and triple amounts of the material. That is to say, the Masonary Block would be single ,double and triple thickness.  NIST is very careful to define the exact materials in use, water content, and chemical makeup.  Composition greatly affects attenuation; however, in my experimentation, I had neither the time or resources sufficient to test at that level of precision.  Even though I may have used blocks and brick of slightly different composition, the numbers recorded were so close that I was able to draw sufficient conclusions and use the data to determine the type of APs and Antennae that I would go on to procure and use throughout projects.  The similarity in results promised the same level of satisfaction by both the client corporation and the customer base that utilzed the deployments.

Measure signal reflection 

There are challenges to obtaining exact numbers that the NIST team has provided. You'll notice that your numbers are close but definitely not exactly the same. There are several factors to take into consideration:

• Signal Reflection

• Signal Saturation around Barriers

• Models of Antenna used

• Models of Radios used

• Imperfect Materials

Signal Reflection 

Signal Reflection is an extremely common event where the RF Signal is bounced off of hard objects and reaches the receiver a bit later than the intended signal.  In the case of the diagram, below, you can see how a delayed signal will bounce off of buildings and structures and reach our intended target.

Figure #7: Signal Reflection

The anomaly of Signal Reflection causes ghosting and can offset the results of testing.  

Signal Saturation around Barriers

To say that a WiFi signal saturates an environment sounds like an exaggeration but it isn't.  Depending on the Radios in use and the Antenna, a WiFi signal emanates from the radio generally in a globular fashion. 

Figure #8: Sample Signal Saturation

The signal goes above, around, under and through objects in a very unpredictable manner.  Measurements taken to see how much of the signal is attenuated as it passes through a specific material is often complicated by signals that wrap around the barrier.  

Focusing this signal will help tremendously.  Also, setting up a test so that the signal is absorbed when it attempts to wrap around the material is ideal.  The NIST team worked to do just that. 

In my own testing, I obtained RF absorbing panels and mounted them above, below, and to the sides of the materials I tested. 

Figure #9 - RF Absorbing Foam Sections

This material is excellent for absorbing stray signals, and saturation.  When mounting, you should face the teeth towards the RF source.

Models of Antenna Used

To help avoid saturation and reflection problems, the team at NIST created a test apparatus that isolated the speaker and listener.  They also utilized specialized "Horn" antennae to focus the signal into a field of limited width and height.  This reduced the occurrence of reflection and produced trusted results.  You could utilize horn antennae but the cost can be a bit high. I've priced them between $1,700 and $3,500 USD each so it puts this type of testing out of my immediate reach.  You should have better luck if purchasing them used.  Horn antennae are metal and are known for durability.

Figure#10: Horn Antenna

In the NIST test bed, they utilized something like this diagram:

Figure #11: NIST Test Configuration - Side View

The Sender and Receiver both utilize the Horn Antenna to send and receive a very focused beam.  This has the effect of reducing reflection above and around the test configuration in the middle.

Measurement Wheel

For measuring your distances, I suggest you buy a walking measuring wheel.  You can find a unit online for around $30.  This lets you accurately measure the distances to place the stands you create.

You can use a can of spray chalk to mark the spots you measure for equipment location. The nice thing about the spray chalk is that it washes away in the next rain storm.  This might keep you out of trouble with property owners, but I cannot guarantee that.

References

Wi-Fi Planning, Walls and dB's - Measuring Obstruction Losses for WLAN Predictive Modelling

NIST Construction Automation Program Report No. 3 - Electromagnetic Signal Attenuation in Construction Materials

http://www.ets-lindgren.com/sites/etsauthor/WhitePapers/APM.pdf

RF Path Loss & Transmission Distance Calculations