On Sunday I spent the morning self-consciously traipsing around my brother's neighborhood with an apparently wierd collection of hand-held electronics and foam in the form or my APM, battery and a Hextronic 3DR knock-off 915MHz telemetry radio and Skyfun fin. We left a laptop on the top floor of the house running APM Planner with the base station radio connected directly to the USB port and using a standard "duck" monopole antenna.
The mobile unit used my experimental lightweight directly-soldered dipole and the contraption was powered by a standard Futaba NiCad receiever pack as pictured above. The aim of the new antenna design was to reduce mass and configure it so that it could be embedded entirely within the vertical stabilizer of the Skyfun.
Unfortunately, I don't have a functioning GPS at the moment, so I located our position relative to the base station by memory and adding a series of coded yaw and roll gyrations of the APM to help identify the approximate location by post-processing the tlog file and correlating with Google Earth measurements. The time-domain plot of the RSSI results looks like this:
What is immediately obvious is the elevated noise floor at the base station. I guess it could be laptop power supply related or some other component, but I think it begs some experimentation with USB cable extensions and remote antennae. I'd also like to make a dipole up to see if I can improve the efficiency.
From the post-processed position data (approximated by the ruler function in Google Earth) I have plotted an RSSI profile with radius. Note that we were not line-of-sight for a lot of the meanderings due to the geography and surrounding suburbia. There is also a huge steel girder railway bridge that we passed under during the process, which appears to have had some effect. The transmission dropout in the first graph appears to be due to terrain masking entirely.
During the log file post-processing in Excel, I concocted an error rate calculation to compare the data with RSSI. What you see here is probably of little absolute merit, but it does indicate a dramatic increase in the error rate as the RSSI value falls much below 100. The time-domain plot would seem by eyeball to indicate successful transmission with an even lower RSSI.
Since I had also wanted to see if my new-fangled antenna had made a difference, or at least no difference, I plotted RSSI against remRSSI to see if there was any bias. The answer is, if you squint a little, perhaps to a small degree. The base station appears to recieve a higher signal at range than the mobile station. Whether this amounts to improved signal transmission from the mobile station, it's hard for my RF ignorance to determine, but I will clutch at this straw for some feelings of feeble accomplishment.
So, it seems likely that the system as it stands will yield a reasonably line-of-sight data link to circa 500m. If I can improve the base station noise floor, this may be able to be significantly improved. I was actually hoping to reach the 1000m mark, but this looks like requiring more development and experimentation.
All of this is (for completeness) with the radios configured using mission planner with the frequency band from 918-928MHz, 30 channels, 20mW power. For some reason, this is the maximum power available on the selection menu when connected. There might be a firmware update to try.