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The energy of EM radiation depends on its frequency, and the lowest frequency corresponds to the lowest energy. Radio waves are at the low end of the energy and frequency spectrum.
The so called visible spectrum. (from red [low frequency] to violet [high frequency].
The audible frequency spectrum in humans ranges from about 20 Hz to 20,000 Hz.
The lowest frequency is effectively zero and this would be in the extremely low frequency (ELF) band in the sub-radio range of the electromagnetic spectrum.
I think in the ultraviolet region of the electromagnetic spectrum.
A spectrum goes from red at the low frequency end to violet at the high frequency end.
There are 525 center frequencies available in the JTIDS/MIDS frequency spectrum.
9kHz to 300GHz.
Gamma rays
If by greatest frequency you mean highest frequency, we know that the upper end of the electromagnetic spectrum has the greatest frequency. This is the home to the gamma rays. They have the highest frequency, shortest wavelength, and the greatest energy.
Radio waves have a lower frequency than microwaves in the electromagnetic spectrum. They have the lowest frequency and longest wavelengths of all the electromagnetic waves.
Extremely low frequency (ELF) radio waves. They can have wavelengths of over 60,000 miles (100,000 km) and frequencies from 3 to 300 Hz.