>Is the 2.4 GHz IEEE 802.15.4 transmitter the best option for the price and distance or can someone recommend a different protocol/technology?
Answer: When you are in the US, use 900MHz because it is FAR AND AWAY the best band for the data that's being generated - small amounts of data that needs punch through ability in all weather. Signal strength counts and 900MHz beats 2.4GHz. 2.4GHz is great for file downloads and web pages, but process data is a couple integers or a floating point value so the bandwidth of 2.4GHz is just not needed. 2.4Ghz sacrifices signal strength for bandwidth.
>Is 900 MHz better for longer distances like this?
Answer: Absolutely yes.
>Outside the US, is 900 MHz a problem where 2.4 GHz is unregulated?
Answer: Very likely, 2.4GHz is widely used outside the US with the attendent loss in distance and signal strength. Depends on the county.
>Would you say the Wireless HART transmitter from Phoenix Contacts pn RAD-WHA-1/2NPT - 2900100 is the 'industry standard' and a good default option?
Answer: absolutely NOT for a couple reasons
- 2.4GHz
- need a HART gateway/wireless access point
- probably need a HART configurator to make the adapter run
- you can't get what want from HART - a HART gateway output is Modbus/OPC, not an analog 4-20mA, which is what you want
all of which are absolutely unnecessary unless you like expensive toys.
What you want is wired-in/wired-out. Three choices I've used below. All work.
Phoenix Contact used to have a pair of 1 Watt 900MHz one-way radios, so-called electricians radio, because even an electrician could figure it out.: the field transmitter is a NEMA 4 'beer can' with an omni antenna with a conduit threaded fitting and flying leads, the receiver is a DIN rail mount receiver on the other end. Wired-in/wired/out. 4-20mA in/4-20mA out. Obviously the radios need DC power as well as the field transmitter.
Banner has a pair of 1W 900MHz radios that are NEMA 4 but their plastic cases really need to go into a panel when outdoors. Bidirectional 4-20mA and a pair of DI-DO: an PNP transistor at the receiving end follows the state of dry contact DI at the transmitting end. The Pre-Mapped pair of radios does not require software, just some button pushing to tell which network layout to use. Excellent performers.
Precision Digital has a pair of 900MHz radios, integral omni antenna, in very nice EX housings (no FM approval) that are clearly weather proof. But the radio power is only 1/4W not 1W. I just used a pair as RS-485 modems for a 300 foot shot and they worked out-of-the-box at 19.2K baud. The radios have a bidirectional 4-20mA wired-in/wired-out and an absolutely useless DO at 3Vdc (like what do you do with that?). When both ends need to be outdoors those screw cover EX housings are tough as nails. Model PDW30 or something similar. For a mobile app, I'd probably stick with a 1W radio.
