Ok, since there are some radio wizards in this thread: if I want to log low bandwidth sensor data (so < 1kbps) on a unidirectional (if that helps) link over ~ 500m, almost clean line-of-sight with some trees in there, what are my options? I'd prefer to spend little money and use little power on the sensor end. I'm considering an ESP8266 (so 802.11) with a high-gain directional antenna. Sender and receiver are fixed locations.
Here are two low cost options (I recommend #1 for most people and #2 if you have an impressive beard):
1. The esp8266 like you have already mentioned. Get one with a u.fl connector and make your own cantenna to attach to it. You can use the microcontroller in the esp8266 to take the sensor readings (it even has an ADC to read analog sensors). This is the new hotness and there are tons of examples.
2. Even cheaper but less user friendly is a very low tech 433 MHz RF transmitter like you can get on eBay. You then need an external microcontroller to run it so you might actually be less cost effective overall and these things take quite a bit of effort to implement a reliable comm system because of the on-off keying system and zero built in error correction. There are libraries like VirtualWire or RadioHead that take quite a bit of pain away. You can get other frequencies if your country doesn't allow unlicensed 433MHz transmissions. Example of what I'm talking about: http://www.ebay.com/itm/like/140719918135?ul_noapp=true&chn=...
ESP8266 is worth checking out because its dirt cheap, but it really depends how "clean" your line of sight is. 900 MHz radios will have better propagation. Might want to check out something like this: http://rpi900.com/hardware/introduction.html
I'm not sure what you mean by "little money" but for less than $100 per location, you could have a 100+ Mbps link using Ubiquiti NanoStations. You can use Wi-Fi or their proprietary protocol and the radios are available on 2.4, 3.65, and 5 GHz. These little things will cover your 500m link with ease but might very well be extreme overkill if all you really want is <= 1 Kbps throughput.
Yeah, that's definitely overkill, but those are cool for applications needing the bandwidth. For comparison, the ESP8266 costs ~ $5, so that's "little money".
