# Radio frequencies, what are the limits?

What is the correct range of frequencies for radio waves? Wikipedia's article on radio waves states that all frequencies below 300 GHz are radio waves, while the article on the radio spectrum states the upper limit is 3 THz. Additionally, the article on microwaves states that microwaves have frequencies between 300 MHz and 300 GHz and radio frequencies are below 300 MHz. Why is there such inconsistency? What is the correct frequency range?

• Are these our "limits" we are pushing on nature? Does nature have it sorted? Commented Feb 12, 2023 at 9:25
• And 50 Hz is a "wave" ... point of view of "terrestrial" atmosphere ... 40 000 km wavequide (?). Commented Feb 12, 2023 at 10:20

Radio waves and light waves are both electromagnetic (EM) waves. Visible light is physically the same thing as radio, just at a different frequency/wavelength. All EM waves are a coupled electric and magnetic field propagating through space; it just so happens we can see some of them with our eyes, feel some as heat, get radiation sickness from others, and some pass through our body entirely unnoticed. Since they have such different effects which are dependent entirely on their wavelength, we categorize them into different groups.

Radio waves are EM waves we can create by oscillating an electric field in a conductor. Through careful control of the exact electric field at any given instant, we can encode and transmit information through these oscillations.

Visible light, along with infrared and ultraviolet, is produced electrically through high heat (incandescence), fluorescence, high voltage gas discharge, or more recently LEDs. We use them for lighting, heating, fiber-optic communication, disinfecting (in the case of UV), among other things, but with current technology we can't really control its frequency through modulation like we do with radio waves. Generally, work in these frequency bands falls under the umbrella of 'optics'.

And between them lies the terahertz gap. Above about 1 THz ('submillimeter wave', 0.3 mm) we lack the technology to reliably produce and transmit radio waves; below about 30 THz (10 μm), far infrared optical detectors are not really feasible.

(Far UV, x-rays and gamma rays, called 'ionizing radiation' as their high energy can strip electrons from atoms, can be produced through other means, like x-ray tubes, radioisotopes or high-energy particle collisions, but are not, to my knowledge, used for communication.)

In short: the maximum frequency for a "radio wave" is whatever we can produce using an oscillating, modulated radio transmitter. As technology advances, the maximum frequency will no doubt continue to increase.

• So if I had a radio transmitter and turned the frequency all the way up, it could in theory emit light?
– YPOC
Commented Feb 13, 2023 at 11:05
• @YPOC: yes, and not "in theory", but it would be exactly like that. If you have any doubts, please keep re-reading first paragraph of this answer. Commented Feb 13, 2023 at 11:22
• Literally light, if you can justfigure out the minor technical challenges underlying "turn the frequency up". Commented Feb 13, 2023 at 11:46

There is no standard I am aware of that defines what is considered the radio portion of the electromagnetic spectrum. There is no correct frequency range. There are only conventions that often exist only for historical reasons. Sometimes these conventions are useful because they tell us about the kinds of approximations that can be used in analysis and the enabling technologies that may be used.

For instance, microwaves and radio waves are frequently lumped together these days (as in the wikipedia article you linked) as the underlying technologies used to produce them are similar, most often solid-state oscillators. The optical spectrum is often talked about as separate, however, because thing's like lasers and LEDs are used to produce it, and the wavelength is often much smaller than the structures the electromagnetic radiation is interacting with (the same can certainly be true of longer wavelengths but it is a less common situation)

It is all electromagnetism. Any labels we put on the spectrum are arbitrary and different groups of people have labelled it in different ways.

If you read one line beyond the table where the claim that "microwaves have frequencies between 300 MHz and 300 GHz and radio frequencies are below 300 MHz" appears, you find:

In descriptions of the electromagnetic spectrum, some sources classify microwaves as radio waves, a subset of the radio wave band, while others classify microwaves and radio waves as distinct types of radiation. This is an arbitrary distinction.