Although radio waves are usually considered to have frequencies between 10 kHz and 300 GHz, there is no reason why astrophysical processes could not generate electromagnetic waves with lower frequencies (or higher wavelengths). The technical upper limit for the wavelength of a photon is constrained only by the width of the universe, as the wavelength obviously cannot exceed that size.

The Earth's atmosphere tends to block out most electromagnetic radiation originating from interstellar space with wavelengths greater than 20 meters or so (corresponding to frequencies less than 10 MHz), making them very difficult to detect.