I am trying to build a system that checks temperatures on certain units and transmits those temperatures (or signals indicating a temperature is above a threshold) to a particle photon over radio waves. Wifi is not available.
I am hoping that the device(s) that monitor temperature can send signals .25 of a mile to the photon (the endpoint).
What hardware should I be considering for such a system (radio modules, micro-controllers, etc) and would particle photons be a good candidate to use for hardware?
The nrf24l01 is worthwhile looking into. They have a pretty decent range, and are really cheap. The Photon can definitely be used for this.
What kind of environment are you in (densely built, or open line of sight)? You can consider using a more powerful (directed) wifi antenna to get a larger (more specific) range.
Check out LoRa (https://www.lora-alliance.org/). It is a platform for unlicensed transmitting data over substantial distances. You can buy breakout module tranceivers from places like Adafruit, e.g. https://www.adafruit.com/products/3072. You can also buy complete systems based upon LoRaWAN, if you need complete networking capability. However, it sounds like simple point to point communication is all that you need.
You could make LoRa temperature sensors with some temperature IC (e.g. DS18B20) and an ATTiny85 microcontroller or similar cheap device (e.g. https://www.adafruit.com/products/1501). Use this microcontroller to periodically read out temperatures and send out some text meassage over the LoRa tranceiver to a similar transceiver attached to your Photon.