There are various transmitter modules like the FrSky R9M system which operate at around 900MHz rather than the more commonly seen 2.4GHz. Why is 900MHz better for long range drones, and why is 2.4GHz so common if 900MHz can provide better range?
The strength of an electromagnetic signal will attenuate (decrease in strength) over long distances between the receiver and the transmitter due to the inverse square law, but this phenomenon applies equally to all wavelengths/frequencies.
The main difference in transmission distance capability between long-range RC systems like TBS Crossfire and FrSky R9M (which use the 900 MHz ISM band) and run-of-the-mill RX systems that use the 2.4 GHz band is due to the diffraction (and reflection) of the signal by obstructions in and around the line of sight between the transmitter and receiver. This process is explained well in this Electronics.SE answer. Essentially, lower frequency (longer wavelength) signals are less susceptible to signal strength losses by reflection and diffraction because they require larger objects to affect the signal in the same way.
As the Electronics.SE answer points out, this becomes easier to conceptualize if you look at two different frequencies like 1 MHz (wavelength of 300 m) and 100 MHz (wavelength of only 3 m). If there's, for example, a house or other large object in the vicinity of the path between the transmitter and receiver, the 1 MHz signal with a wavelength of 300 m will find it a lot easier to avoid being disturbed and reflected than the 100 MHz signal with a wavelength of 3 m for the simple reason that it is just so much larger in comparison to the house.
At extremely long ranges (like over the horizon), other effects also come into play like the ability of lower frequency signals to more easily reflect off the curved surface of the earth and the ionosphere, but these kinds of transmission distances aren't an issue for RC aviation for more practical reasons. :)
2.4 GHz RC solutions are far more ubiquitous because the antennae required to transmit and receive signals at a frequency of 2.4 GHz are far smaller than those required for 900 MHz. This is due to the change in wavelength between the two frequencies and is evident in the size difference between 2.4 GHz receiver antennae and those for 900 MHz:
- TBS Crossfire Immortal-T antenna: (The active section runs horizontally in the picture)
- Futaba 2.4 GHz receiver and antenna: (The active section is silver/white in the picture)