The ThingsNetwork has a good document on frequencies by country… as does the Lora Alliance in PDF format. In the USA only the 902 Mhz - 928 Mhz frequencies are legally set aside to use freely… I believe that you must have a HAM Radio license to use the 433 Mhz frequency.
I will let others answer your “reliable place to purchase” question. I elected to buy directly from China through Banggood (my first purchase on that website so we will see how that goes).
I think the 915/928 depends on the antenna, and a config change. So you can use either, (oscillator can do either 915 or 928 easy) If you have the flexibility to use either, I’d see if the band is congested around either of those points… Some thing like the RTL-SDR for band surveying…
Each antenna is “tuned” to a specific frequency to which it will naturally resonates. At that frequency you will have the most power and will get the greatest distance. You can transmit on other frequencies but with much less efficiency and therefore less power / less distance.
So the device has to be configured to tell the chip the frequency you want to transmit at and you should tune the antenna to get the best performance.
No they should be tuned to the frequency that you have purchased, however from what I have seen, they are cheaply made and usually only within a few Mhz of the stated frequency. Which is good enough if you are close but if you want longer distances you should consider tuning the antenna by using a VNA (Vector Network Analyzer.
If you are just playing with the technology then don’t worry because it should be good enough…
I would also love to have an answer to this question based on real-world tests.
Advantages of 868 would be shorter antenna lengths and theoretical advantages of 433 would be better penetration power and therefore less depenadance on line of sight. One disadvantage of 433 might be more interference because the 433 spectrum is very crowded in Europe.
I would love to see a VHF version that worked on the MURS frequencies here in the US. It’s criminal that TTGo doesn’t offer one! It would take a longer antenna, sure, but you would get much better range in non line of sight situations (like every urban area on earth, mountains, etc.), and in the US MURS is still wideband, so that would increase your range as well.
Lower frequencies have better range because they’re less likely to interact with the stuff they’re traveling through compared to higher frequencies. All electromagnetic waves (radio, microwave, light, etc) have a reflective and refractive index specific to whatever material their interacting with:
The main advantage of less than 1Ghz frequencies for range is they’re less susceptible to absorption by rain; the average rain drop is about 0.5 mm and that attenuates any frequency above 600 MHz heavily with higher frequencies getting extra attention. If I remember correctly the probability of a photon interacting goes up by a factor of r^2 as it moved through a material and for 2.4 GHz signal a water droplet would be 4-5 times larger than the photon’s wavelength making it 16-25 times more likely to interact with a water droplet than a 600 MHz photon.
Additionally water as a general molecule has a huge absorption coefficient for much of the EM spectrum. So GHz will always have less range and faster dropoff than MHz or lower frequency communication as the humidity rises.
That’s why I can’t wrap my mind around tt go not having a MURS version for the US and Canada. Now that I ordered mine in 915, though, I’m sure they’ll release the MURS version any day now. You’re welcome!