There are many complicated techniques that can be employed to measure the attenuation values of various building materials to use within a predictive RF design. As fun and exciting as they can be, this post will explain a process to measure wall attenuation using the latest release of the Ekahau Survey v2.0 app for iOS. This technique can be used to measure walls, warehouse shelves, or any other building materials of which you can walk on both sides. Remember, if you care about it, measure both sides of it.
To use this method you will, of course, require:
- Any Wi-Fi access point – it does not have to be the model you plan on deploying, just a device that can generate Wi-Fi beacons on relevant Wi-Fi band(s)
- an iOS device (iPhone or iPad)
- Ekahau Survey v2.0 app – requires an Ekahau connect license, but free to download
- Ekahau Sidekick
- Micro-USB (Sidekick) to lightning/USB-C cable (depends on iOS device)
- Apple Pencil (version 2) – while not entirely necessary, I prefer using the pencil and recommend Pencil 2 as there are no parts (lighting adapter and connector cover) to lose and subsequently no reason to pay the “Apple tax” to replace those parts
- Wall or building material to measure – seems obvious, but the building or at least the wall/building material must exist in the real world and not just on paper/CAD
- Scaled floor plan of the site
With the release of version 2.0 of the Ekahau Survey app, it is now possible to configure the channels on which the Sidekick will scan. With previous versions, channel scanning was hard coded, based on the country code settings of the iOS device. Region settings could be modified by tapping Settings > General > Language & Region and then modifying the Region to match the regulatory domain in which you were interested. Another new feature in Survey 2.0 that makes this possible is the addition of RTFM (no, this is not short for the “Red Team Field Manual”).
1) Position an AP at least 5m away from the building material you plan to measure. Configure the AP with:
2) Launch Ekahau Survey in iOS to create a project using the Survey app or load an existing project from Ekahau Cloud / Sidekick.
3) Once the project is open, tap the “Sidekick” icon in the upper right corner, then tap the slider to disable “Scan all channels”.
4) Tap “2.4 GHz” and choose a single 2.4 GHz channel (ch 6) of a nearby radio. Repeat this process on “5 GHz” and choose a single 5 GHz channel (ch 153) of a nearby radio. Click the back arrow in the upper left corner to return to the previous screen.
5) Tap the Survey tab – middle tab along the top.
6) Choose Continuous Survey Mode. From left to right, survey modes are “Stop-and-go”, “Continuous”, “Autopilot”, and “GPS” (GPS mode is only supported on iPhones & LTE enabled iPads).
7) Start the continuous mode survey by tapping on your location on the floor map. After 5 seconds, click again on the same location and begin walking; the sidekick continually collects data along your walking path. The pause at the start allows time for the sidekick to cycle through all the channels at the starting location.
8) At the end of your walking path, tap the “End” button in the upper right corner. This will stop the continuous survey when you stop walking, to complete the survey path and save data to the project.
9) Position yourself on the other side of the wall at the same location from which you started the first walk. When ready to survey, repeat steps 7 and 8.
10) Analyze your measured results on each side of the wall (or building material of interest) by tapping on the “Inspect” tab along the top to access the RTFM view. Tap the ellipses and switch to “Radios”. Additionally, select the band that you want to analyze along the bottom – either 2.4 or 5 GHz. You can now click the points along your walking path.
11) Focus on your test radio by clicking on it and tap along the collected data. You will be able to note the RSSI values measured as you walked along the wall. Take the average of these values – call this value ‘x’ if you’d like. Repeat this process for the second walking path calculating the average RSSI values along the path – call this value ‘y’ or any random Greek letter. The final step is to calculate the delta between the ‘x’ and ‘y’ values. This delta (or difference) in the two signals represents the attenuation value of the wall or object. Depending on the environment you are measuring or length of walking paths, it may be better to calculate the delta values at each point along the wall and average the delta values to determine the attenuation value of the wall or material.
Which technique should you use? If the difference between the min/max values along a single walking path exceeds 2dB, use the second method as there may be material inside the wall (metal studs, covered brick/masonry, insulation, etc) which changes the attenuation values. Measuring the delta directly across from measured points is a better approach here. If the difference between the min/max values along a single walking path is <=2dB, use the first method and take the average values along the walking path.
12) Update walls in the predictive model with newly measured wall attenuation values.
Use the following tips to increase the accuracy of this technique:
- Walk as close to the wall/attenuation object as possible along both sides.
- Walk in the same direction along each side of the wall to ensure to additional sources of attenuation (like a bag of water) are not introduced between the Sidekick and the AP, as you only want the building material (and a very small physical distance) to be the delta between the two measurements.
- Specific values for RSSI of the ‘x’ and ‘y’ measurements do not matter, what matters is the difference between the two values. For example, ‘x’ does not need to match your primary RSSI design target; it could be measured as -88dBm. If you also measure a value of ‘y’ as -92dBm, for example, you can calculate the delta as the value of the wall attenuation – in this case 4dB.
- Lock the sidekick to scan a single channel on each WNIC to increase accuracy.
- Units of ‘dBm’ (dB referenced to 1mW) are absolute values. Wall attenuation values are measured in units of ‘dB’ which is a relative value reflecting the difference between two measured points.
- Click accurately where you walk – specifically click where the Sidekick is when you walk.
- Keep the Sidekick in the same orientation during the entire process.
- You don’t have to measure every wall, but it’s a good idea to measure multiple walls of the same type to confirm consistency and average those wall attenuation values.
- Remember the THREE RULES for “clicking” during a continuous mode survey:
- Click when you start walking
- Click when you stop walking
- Click when you turn
- Carry a protein or energy bar in your survey kit – no one likes a hangry surveyor.
- Walk a path perpendicular to the AP (as shown in blue) and NOT towards/away from the AP (as shown in red).
Ekahau Survey v2.0 application for iOS
Survey 2.0 feature highlights video