Long Range RF communication

Question

After playing with Arduino and different kind of sensors for my garden, now I'm starting a new project for my free time.

I want to work with RF communications because I need a long range device for my application, about 2 Km distance.

The idea is to make only an identification of which unit is, as a long range RFID but without RFID.

I mean some devices/units are placed somewhere, and after a time, someone could move them to another place, so I want to know where they are, only reading information send by them via RF. I don't mind their real position (GPS) because I going to be able to see them where they are from my top window. I only want to know which of them are.

1. I'm reading about 315/434 MHz, but it seems not be able to get this distance without a high power consumption.

2. What about a lower frequency (150 MHz)? It is above the license frequency band for AM/FM radios.

   • I live in a village - I have a lot of terrain to play with my experiments and a line of sight over 2 Km.

EDIT:

The @Hoppo idea is just what I'm trying to do. Also it lets me to get "energy harvesting" because the idea is transmitters go with a small battery.

Also the transmitters have to be small enough and without antennas for not disturbing and avoid dogs play with them.

In receiver side, it doesn't matter if I need a larger antenna or more power. It will go connected directly to a PC or power source.

Moreover as @Hoppo says, I only want to send a 'ping', a message with an identifier and maybe battery level, so data rates could be lower than 9600bps.

Answer 1

If you are able to see the devices, then we can only assume line of sight, 2km distance 433Mhz (70cm) should be fine with quite a low power solution. If you can't see them then that drastically reduces the transmit range at 70cm without increasing power consumption. As with all radio communication it can be power hungry. I have created similar projects with the arduino using a radiometrix NTX2 transmitter at 434.650Mhz. My solution to save power was to turn the transmitter on, send a location 'ping' and then turn off the transmitter again rather than constantly transmitting. Easily done with an arduino.

Answer 2

The article "Extreme Range Links: LoRa 868 / 915MHz SX1272 LoRa module for Arduino, Raspberry Pi and Intel Galileo" mentions a test of LoRa spread-spectrum modulation that sent data up to 22km (13.6 miles) line-of-sight, and up to 2km (1.2miles) in an urban environment going through buildings. The data rate apparently slows down "to few bytes per second" in difficult conditions.

The articles "IBM, Cisco Back Semtech's LoRa Radio for IoT" and "Long-range Wireless IoT Protocol: LoRa" mention a few other long-range, low-rate data protocols.

I hear that OpenRF and IBM LoRaWAN are open-source implementations of LoRa. Apparently LoRa and OpenRF are so lower power that some implementations are expected to "operate for several years using inexpensive off the shelf batteries."

Answer 3

In free space the path loss between two points is governed by what is called a Friis equation (http://en.wikipedia.org/wiki/Friis_transmission_equation). It is true only in free space, but provides a good starting point to estimate the real path loss. There are also a lot of more accurate models of various complexity (two ray model, etc). Generally, if you are trying to get the maximum distance, low frequency is your friend. Of course it comes at a price of larger antennas and lower data rates (which may not matter for your application). You also want to mount antennas as high above the ground as possible and get more directional antennas (e.g. Yagi-Uda).