I have a project that requires me to send data wirelessly in an open space over a decent distance (1 or 2 km). I found a module such as this: https://www.seeedstudio.com/2KM-Long-Range-RF-link-kits-with-encoder-and-decoder-p-321.html#reviews, which seems like it would work as a transmitter and receiver, but I cannot find them in stock anywhere. Is there another method of sending data between the Arduino and Raspberry Pi that you would recommend which allows for a range of 1 km?
I assume there is no internet available at both points?
I managed to get a decent range with an HC-12, but of course the bandwidth is low. Make sure you have line of sight.
some managed to get a significant distance even at 10mW. get real ones.
Yes, you are correct that there would be no internet available at both points. I will try out the HC-12 as it seems to meet the needs for my project. Thank you!
You will need a proper antenna at each end. Not that springy thing in the picture. 
There are LoRa modules with Serial front ends, in the same style as the HC12.
You say send data but not how much. If the HC12 has enough range (for the location) and data rate then fine.
But if it does not, consider the LoRa modules, for the same data rate (as the HC12) they will cover around 10 or more times the distance. And for the same distance coverage (as the HC12) the LoRa module could provide maybe 5 times or more the data rate.
Yes - Lora modules would do too.
The data we would be sending is just NPK, PH, temperature and humidity from soil. Would this be fine with the HC-12?
The springy thing is made to be stretched out straight. Makes a much better antenna that way.
Paul
Really? 
Yes. Those were right in the instructions for the two remote receivers I bought a couple of years ago. BIG improvement when I did it.
Paul
In general longer antennae will have substantially higher gain, at least up to half a wavelength.
But antennae also need to be tuned (resonant) for maximum efficiency, so a given length of wire needs to have a particular shape to be resonant. Just "pulling it out" may not be the correct shape.
An end fed antenna at circa half a wavelength has a fairly high impedance, so may not be a very good match if the transmitter output stages are tuned to expect a typical 50R.
if you watch the video they are testing different antennas on the receiver. They used the small springy thing on the mobile one. Quite decent results if you have line of sight.
Note their point that if you can be higher above ground, you'll get better results too.
I was being cautious about mentioning actual (wave)lengths. That was the whole point. "Tuning" by definition includes actually matching the feed to the antenna, only certain lengths (multiples of a quarter wave) will approach 50 Ohms and unless you can tune out the reactance of some random length, it will not be efficient anyway.
I think the assumption that the driving impedance is 50 Ohms is not documented for the device.
Paul
Indeed. With these micropower systems, the load is not critical to the operation as it is when delivering many Watts and you have a potential risk of overheating the output devices.
Actual impedances are also important when a transmission line is involved as a mismatch at the antenna can (will) be transformed to a completely different impedance at the other end unless the transmission line is an exact multiple of half wavelengths.
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