Well I thought I was done! Someone asked if a Nordic 2.4GHz transceiver could be used instead of the 433Mhz RM22 transceiver that I'm using. So it was back to the parts stores - and the drawing board.
The Nordic nRF24l01+ transceiver is just a tiny chip, so there are various breakout boards available with it.
The low end boards on eBay have a built in antenna on the board. They are small and cheap, but have a significantly shorter range than "high end" breakouts. (There's a middle range with a spring antenna or a chip antenna which I don't know much about.) The high end (~$20) boards have an SMA antenna connector and signal amplifiers (PA & LNA) that make a big difference in the range. However, it appears that the amps are always powered, and still draw about 6.5mA in power down mode. I wanted good range, but low power for the sensing unit so I will try the no-amped version on that side, and the high end board for the display station.
Please note that the sources linked to here are just examples. You will find lots of options for these if you search.
There are Arduino libraries that make it easier to integrate to the device. I choose "maniacbug's" as it was the most full featured, current, and has good documentation. The data sheet for the Nordic nRF24l01+ is here.
I spent the weekend making test programs and eventually porting my RF22 client and server sketches over to this device.
So what's the difference? A big difference for me is the form factor. The 433MHz RF22 really needs either a breakout board (which makes them expensive) or a PCB which also has the rest of the components. I made PCBs to hold the RF22s for the client and server as described in the page above. However, the 2.4GHz transceivers are more friendly to work with. This means it's much easier to make a wireless radiation monitor on your own - without buying a custom PCB (which I was going to sell!). For the server (display station) it's easy to use the Adafruit Logging Shield on an Arduino. For the client (sensing unit) it's also easy to plug the transceiver into the Geiger Kit (which I still sell!).
The calculations for the power budget of the solar powered sensing unit with the non-amped nRF24l01+ are as follows:
The picture below shows both units communicating. On the display station the Adafruit Logging Shield provides the SD card and the RTC.
Software-wise, after a bit of a struggle with the RF24 library, I have all of the functionality I had with the RF22, and I added the DHT22 temperature / humidity sensor.
You can download the current versions of the client and server software here. (requires Arduino IDE Rel 1.0.x)
You can also download a package of the (1.0.x) libraries used to compile the sketch - here
I really wanted this to work, and was planning on gutting my enclosures with the 433MHz version and replacing them with this 2.4GHz version. However, in my situation (backyard to basement), I get much more dependable communications with the 433MHz version. Here are some of my observations:
So if you don't expect to need the penetrating power that I did, these 2.4GHz transceivers are probably the way to go.
BTW, I found that it was easy to modify the display station boards I made to support the 2.4GHz transceivers, so if you're interested in using that "all in one board" instead of an Arduino with an AdaFruit Logging Shield, let me know, I have a few left. Here is a pic with the 2.4GHz transceiver on that board . . .