I got around to doing a range test with two RFM69HW modules. If you haven't come across them, they're pretty similar to the nRF24L01, but they come in 915MHz frequency instead of 2.4GHz. The -HW chip is the high power 20dB version. One complication is that the RFM69HW is not 5V tolerant. But, the cool thing about the RFM69 chips is the antenna can just be a hook-up wire you solder onto the chip, so it's very customizable. Knowing nothing about antennas, I just use a 6" section of hookup wire shaped in a spiral. These modules seem to have much greater distances than even the PA version of the nRF24L01. I also like that they use a different, less congested frequency than wifi.
The GPS chip is the Ublox 6M. The GPS Arduino has this Ublox GPS and the RFM69HW wired to it, transmitting every second. But the GPS location is only requested every 3 seconds. Not sure if this makes sense...I wanted more transmissions of the signal than GPS requests.
The base station arduino is connected to my computer at home, via USB, with the serial monitor displaying every time it receive a location transmission from the GPS Arduino. The struct from the GPS Arduino is sending the time stamp, longitude, latitude, and the basestation also prints out the RSSI.
You can see from the map image that the farthest pings are over 700 feet. This is a pretty standard suburban lot, so the signal is going through trees and houses. My computer is next to a window, so that helps a little. From the time stamps, I can see a couple of missed transmissions when the RSSI is in the -90's. Above -90's (that is, -80, -70, etc...), the reliability is very good. When the two Arduinos are in the same room, the RSSi is in the -20's.
I have plans to make a dog tracker, where the GPS unit would be on the dog collar transmitting to a hand-held unit with it's own GPS. The hand-held also has a GPS receiver, and calculates the position of the relative dog using the two GPS coordinates. I might use one of those square LED matrix to indicate the direction of the dog.
That is very cool. For the dog tracker, how much current do you need and what power source will you use?
A few years ago, I bought a "tagg" GPS collar for our cat. With it, you can set boundries and alerts if animal gets past that point.
I suspect once you get your project going on the dawg, you can add stuff like that.
Thanks for your report and will be looking forward to future reports.
Steve
Pete999:
That is very cool. For the dog tracker, how much current do you need and what power source will you use?
Steve
For the test setup I used a USB battery pack and Arduino Unos. For the prototype dog tracker, I'm planning on using 4xAA batteries eventually, so 4-6VDC, into an MCP11702 regulator which would power everything - ATMEGA, GPS, and RFM69HW. I'm not sure exactly on the mA consumption, but something like this:
GPS: 38mA continuous
RFM69HW: 130mA every half second or so? So let's guess 60mA continuous
ATMEGA and LED indicator: 20mA continuous
So without putting any effort into sleeping the GPS, wireless, and microcontroller, I'm guessing 188mA. Using 2Ah AA batteries, that's maybe 10 hours of run time on a set of batteries without doing any power saving features.
Other people have gotten much farther distances than I have. I don't know if it's because of their antenna. I'm just using a plain old wire sticking out. Other people attach the wire as a dipole antenna (so attached in the mid-point of the wire, with each half being 1/4 of the 915Mhz wave length, or about 3 inches.
thanks for the report, I'll add my own measurements as soon as I have them. You say you just use a piece of wire as antenna. I checked with someone who used to work in telecommunications, he told me they had to clip wires for antenna's up to 1/10 of a centimeter within a 1/4 wavelength to get best reception. I have no clue how accurate one has to be. To be sure I ordered a premade antenna and will add it to the module. I have no GPS module so I will have to dash x's on a map while walking.
@TerryKing: RadioHead library is a good start for a lot of RF modules
If there are problems with the RF module range, check the power setting. This post has charts of the numeric value vs the measured power: RadioHead: RH_RF69 Class Reference
