We’re getting a little behind in documentation for all the experiments we’ve been running, and it is time to figure out which direction we want to pursue.
The best results so far were obtained with the directional antenna, the 7mm antenna, then the spider antenna. The first and 3rd were home made. The second would be the easiest to incorporate into the current system.
Here’s a summary of the different tests we’ve conducted.
Onboard F-antenna: This is our most studied antenna. Here’s what we documented:
Face to Face at 0.5, 1.0 and 2m, power 10, 15 and 20. Conclusion – Things are going to be complicated.
Rotating motes at 1.5m, power 20. Conclusion – Orientation matters.
Pairwise compatibility (redo) (best)- facing, 2m, power 20. Comparison of signal strength of seven different motes. Conclusion – the motes are effectively interchangeable. The best test used 14 motes, 24″ apart and measured a between standard deviation of 3.7 and within of 0.64 RSSI value.
Mote orientation – horizontal motes, 2m apart, 30-degree increments, power 10 and 20. Conclusions – Large variation (30-60, 25-50 RSSI) in signal strength as a function of orientation.
Mote Power level – vertical orientation, 2 rotations, 2 heights, 0.5, 1.0 and 2.0m apart, power levels 1-31. Conclusion – the power level can be tweaked to separate various regions of the RSSI, such as providing greater separation between distances or orientations.
Obstacle Interferance (inside puck) – one horizontal, one vertical, distance .55m. various obstacles: person, wire, roomba (like a laptop). Conclusion — these obstacles didn’t have much impact on the signal strength (< 1 or 2), except for the roomba, when put close to the mote. In the inside versus outside puck conditions, we find that the plastic of the puck makes a small, but insignificant difference in the signal.
Second orientation test – one mote horizontal, the other vertical, .55m separation, vertical mote rotated in 30 degree increments about 360 degrees. Conclusion – in this configuration, the broadcast pattern is fairly reliable and smooth across a variety of power levels.
Angle of approach test – A worn mote versus the Purell. The mote is worn in front at distances of .2 and .5 m and behind at a distance of .2. Conclusion: When behind at extreme angles, it was difficult to separate the signals.
Full angle-height test (repeat with vertical transmitter) – Puck and badge orientation. The first test with full statistics at 2 heights, 4 distances and 10 angles between 0 and 180. The results indicate that it will be very difficult, if not impossible, to determine mote distance from the puck or vertical mote with a single RSSI reading at power level 3. Perhaps .25m at the high level can be distinguished from others, but that is it.
Spider Antenna, Horizontal
Full angle-height test – Fully crossed 2 heights, 4 distances, 10 angles. Power level 3. Conclusion: Some separation of .25 high straight ahead from others, but other levels indistinguishable. Probably power level too low.
Spider Antenna, Vertical (repeat at power level 10, repeat at levels 10 and 20, multiple powers)- Fully crossed 2 heights, 4 distances, 10 angles. Power level 7. Conclusion: With one threshold we could get about 90% of the .25 m readings, 70% of the .50m readings, and about 5% of the .75 and 1.0m values.
By changing the power levels, you can get separation among different distances, but these are different between the top and lower levels.
At power level 10, the cutoff between close and far is less pronounced. At power level 20 we get wonderful separation between .25 and .5 m, probably at the 95% confidence level, but almost no separation between .5 and .75.
Spider Antenna, 45 Degrees.
We did this test to see if we could angle the “sweet spot” of the antenna through the center of the target area. We did 2 heights, 4 distances and 10 angles. Conclusion: Significant angular dependence (not surprising). If you look only at the regions near the front of the transmitter, there is good separation between the .25, .5-.75, and 1 m ranges, but the middle two ranges were not well distinguished.
Quarter Wave Antenna, vertical
Vertical small antenna through SMA connector. Fully crossed 2 heights, 4 distances, 10 angles. Power level 7. Conclusion. Works well at the high heights, clear distinction. However, lower motes couldn’t get the signal. Probably the signal pattern cuts them out.
Quarter Wave Right Angle Antenna, vertical
Right angle, larger, 1/4-wave antenna pointing downward. Standard heights and angles, power 10. Exhibits surprisingly high directional sensitivity in the vertical position where we expected that it would be symmetrical.
7MM Antenna
This antenna is soldered directly to the board. Power level 20. Test through 2 heights, 4 distances, 10 angles. Conclusion — Amazingly smooth progression of data. The CDFs of the results suggest that a 95% confidence interval would be hard to find, but the results are so “sensible and well behaved” that this really seems like a promising antenna to pursue.
Purell Devices – Motes in standard orientation and rotation about Ted’s and our prototype Purells. Ted’s prototype has high signals for several orientations at the .25 level and lower values elsewhere, but there is not much consistency. With the newer prototype with the F-antenna there is more consistency at .25 and .5m, but there are still bad orientations.
Directional Antenna
Homemade helix, about 6″ long and 3″ in diameter. Tested with 2 heights, 4 distances and 10 angles. Conclusion: Crystal clear distinction among the different distances of .25, .5 and .75.
