As part of an ongoing project I have been trying to get multiple Raspberry Pis to talk to each other across a city centre. First, they used wifi for ad-hoc networks. In some cases I’ve used 3G dongles to connect over the Internet. Both have their downsides. Now, I am looking at radio communications to build a mesh of devices.
After a bit of investigation I’ve purchased three Ciseco Slice of Radio devices from Pimoroni or Ciseco themselves. These are fairly straightforward devices that enable the Pis to communicate over radio via the serial port.
There’a a fair amount of information out there about configuring them (although you need to look for the chip itself – SRF – rather than ‘Slice of Radio’ in Google) – this is the best resource I found (with handy links at the bottom).
These devices send 12 byte packets by default, so I’ve written a low level protocol to support a mesh-style network. This will allow the individual devices to relay messages through the network and eventually communicate with a server on the Internet. More on that soon.
Meanwhile, the range of the devices as bought is somewhat limited. I have two devices successfully talking to each other over radio, from the front of the house to the garage in the back garden. Straight-line this is going through two internal walls, an external wall and a metal garage door – so not too bad.
In production this is likely to be more challenging, so I will need to try adding an antenna to the device. There are two types I can use – u.FL and a simple (vertical) wire.
I’m an absolute novice with radio so please excuse/correct me if something is wrong! These are more notes than anything concrete!
u.FL supports the ‘standard’ coaxial aerial found on wifi hubs and – as far as I can tell – this is the connector (presuming u = micro). One can then get a u.FL to SMA (bigger) connector and start adding standard antennae.
Option two is to use a ‘wire whip’ or ‘whip antenna’ connected directly to the board. For an 868Mhz device this needs an 82.2mm vertical wire.
In terms of aesthetics and practicality I’d rather not have a vertical wire coming directly out of the Pi – as shown here – and need to figure out the options.
For the antenna, then it looks like I’ll need the following:
u.FL to SMA cable – £3.33
SMA ‘rubber duck’ aerial. – £4.58
Total material cost of adding the aerial is therefore £8.91 + VAT
There are various other issues to deal with, but this seems like it’ll give a significant signal boost for the devices. As I explore further, expect more updates!