We started BeaconGrid with the notion of developing integrated electronics hardware & software that will fit into a standard single gang duplex wall outlet. The ultimate goal was to create a sensor outlet (presence first) that would be secure, scalable, and reliable.
The BeaconOutlet (as we now call it) was designed to include BLE (Bluetooth Low Energy) for gathering nearby presence data and engaging with mobile devices. The outlet additionally featured Wi-Fi and additional sensors, which enabled wide-area communication for true remote management.
First, we considered the enclosure. The electronics could be packaged in two ways: a separate unit that attaches under the terminals of a standard duplex outlet or integrated into the housing of an outlet, requiring the design of a new outlet.
We ultimately decided on a new outlet design.
The concern with “piggy backing” an electronic module onto an existing outlet will be its compliance with the National Electric Code (N.E.C).
The N.E.C has strict rules about bonding, or the connections of conductors. Additionally, the N.E.C has strict rules about the Box Volumes Limit, or simply the amount of occupied vs. available space inside an outlet box. We felt that an integrated approach would yield a smaller, more compact design that would satisfy our requirements and the N.E.C.
Radio antennas come in many sizes, strengths and packaging forms. In this application, the size and strength of the antenna is critical. Antenna performance is severely degraded by the presence of metal walls, and for that reason, we performed preliminary testing to understand what style and size of antenna would perform best.
The following test provided us with a relative comparison between antennas. These tests helped us narrow the selection field for an antenna, and provided inital insight into design considerations for the final form of the outlet.
To test various styles, sizes, and positions of antennas, we created a mock steel stud wall with a ½” drywall wall panel. We choose these materials because we feel they best represent the most common construction practices, and the worst case for signal degradation.
Mock wall assembly (front view) with outlet installed.
Mock Wall Assembly
The outlet was installed with a 3D printed plastic bracket to support a PC board and internal antenna.
Side view prototype beaconOutlet and PC board.
The PC board with an internal antenna represents one of the 2 distinct styles of antennas we tested. The advantage of this antenna is its compact size and limited need of additional components. The second kind of antenna we tested was an external antenna with a thin wire connector.
External Antenna taped to front-end of the outlet on dry wall.
We tested all external antennas by taping them with masking tape onto the drywall. This was to simulate the antenna being taped to the inside of a plastic wall plate. We initially envisioned an external antenna option requiring a custom wall plate to which the antenna is affixed, with a receptacle and plug of sorts that makes the connection between the outlet and wall plate.
The advantage of an external antenna is the added signal strength that it is capable of broadcasting. This is because it is not susceptible to the interferences created by having the antenna inside the outlet box. But as it turns out, this was not an issue.
To relatively compare the signal strength of each antenna, we placed the mock wall unit on a counter, and paced off and marked data collection points. We recorded data on a 12 foot radius away from the outlet, at 10, 45, and 90 degrees from the front face of the outlet. Additionally, we took a 4th data point, 30 feet away from the outlets face, at 90 degrees.
Antenna Testing Measurement Layout
We then measured the signal strength of the beacon under various conditions. The list below shows the evolution of antenna testing. We collected data points for all 4 measurement points in each instance:
- PC board – internal antenna -without mock wall present
- PC Board - stub external antenna- without mock wall present
- PC board – internal antenna- inside mock wall- no wall plate
- PC board – internal antenna- inside mock wall- with wall plate
- PC board – internal antenna- inside mock wall- with wall plate- vacuum cleaner running
- PC board- FXPT3 external antenna – vertically oriented
- PC board- FXPT3 external antenna – horizontally oriented
- PC board- FXP70 external antenna – vertically oriented
- PC board- FXP70 external antenna – horizontally oriented
- PC board- FXP70 external antenna – horizontally oriented- inside box
We ran one test to determine whether running a high inductive load would affect the signal strength.
In part 2, we discuss the results of our measurements. Stay tuned!
(If you can't wait, email us @ email@example.com)