Hello,
I’m planning to build my own TTN Gateway using a Raspberry and cover my area by installing it outside. My city does not have any gateway ! 
I’m quite lost with all these available HATs we can found. Are all HATs compatible with TTN ?
Is there difference in term of performance, community help, configuration,… ?
I was thinking to buy RAK2245 (8 channels).
Thank you for your help.
Welcome,
LoRaWAN is a tightly defined standard, so any reasonable vendor offering will work with TTN.
There are some differences in performance as there are different chipsets used - for instance the RAK2245 uses Semtech SX1301 chip whilst the Pi0 gateway uses the RAK2246 which uses the SX1308 and the RAK2287 uses the SX1302 chip. You can have fun disappearing down the rabbit hole of Googling which is better and then post your results - get some popcorn and watch the major nerds here get in to a mass debate.
Different vendors come with differences in build quality. But they are too damn complicated to build for them to be knocked up on the cheap.
In terms of community help, browse the Gateways category to see what’s active and the sort of responses. But you should also review the support from the vendor as they should be the subject matter expert, it’s all best endeavours here and we encourage people to search first and ask questions later.
For configuration, vendors typically provide pre-built images for the Pi or some form of installer for a standard Pi image.
All that said, you’ll notice that RAK define all the Pi HATs as developer gateways - they aren’t weather proof and using an SD Card as main storage comes with some inherent risk in terms of wear & tear - which can be mitigated / managed - but consider that the majority of external gateways use Read-only Flash and runs everything in memory. However you can get good results with a branded SD Card, move the logging to RAM and check in on it monthly, a Pi gateway can run ‘forever’.
You will need an appropriate enclosure to put it next to its antenna or use a cable to reach it. The higher the better for the antenna.
If you take a RAK Pi based gateway and can follow instructions, you can go for unboxed to live gateway inside an hour. There isn’t actually that much to do once they are setup, they just sit there, being a gateway. The fun is finding use cases in your community for devices - flood warnings, feed levels, full rubbish bins, road ice warning, signal light failures and so much more.
I’d recommend reading most if not all of the TTN docs, not to memorise them, but so you get good understanding of all the moving parts.
I think that for a Raspberry Pi based DIY gateway RAK2245 (RAK2245 RPi HAT Edition) will be a safe choice.
Another option is to use a RAK2247 in combination with a RAK2247 RPi HAT.
Both configurations cost about the same.
I am unaware if one has important advantages over the other. Maybe other users here can elaborate on that. A RAK2245 based solution is probably more compact but I don’t know which of these two will run cooler.
I would skip the RAK2248. You only need a single gateway (not tens or hundreds) so no need to go for the little bit cheaper RAK2248 (except for having SX1302 if that would be important).
Like @descartes already said: for an outdoor gateway you will have to provide your own weather-proof casing and antenna.
As for SX1301 vs SX1302 the following RAK news article shows this:
Although a success in the IoT community, the SX1301 faced a few issues in running very high traffic load with long payload, and receiving packets from unknown devices.
Thank you for yours returns.
Indeed a Raspberry base gateway is surely not the best choice in term of reliability, or required some extra configuration to improve it, but it is a good way to better understand what is Lorawan by diy . Hope it will be reliable enough for the TTN.
RAK 2287 seems to be a better solution in term of power consumption (and temperature)
Concerning the placement, I leave in mountain, and I will face Antenna to the valley at about 200m over. I hope it will be a good place
Thank you for the documentation that I have already start and continue to read.
Next step, installing the gateway and test a device, and then, install it outdoor.
Thank you
Don’t get me wrong, they are OK to good and, as you say, with a few tweaks, they will just run & run. AND they are a great platform to get started on as it is so much easier to see under the hood.
Normal pole (omnidirectional) antenna’s aren’t really faced in a particular direction. Best thing, stick it up high and then take a device in to the valley and see what reception you get.
Good luck