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It has been said that RF consists of IR photons (here (now deleted; screenshot) however opinions seem to differ):

The smallest energy for photons is in the range of the infrared radiation. Longer wavelengths are related to modulated EM radiation called radio waves and micro waves.

Is the above statement correct? What the spectrum of these photons looks like and depends on? Does it change if the emitter is cooled (to near 0K perhaps) or heated? Can we measure those individual photons to say something about the emitter (and/or the reflector) that we cannot say based on the radio wave as a whole?

A popular answer, suitable for anyone with engineering background, is preferable.

Emilio Pisanty
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Eugene Ryabtsev
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  • For a wave generator with given power make the antenna rod diameter smaller and at the end you will see the rod glowing (IR and red photons). Not to forget that a radar antenna could emit X-Ray photons. – HolgerFiedler Oct 06 '16 at 09:58
  • @HolgerFiedler - I'm trying to imagine a radar antenna emitting x-rays as a 'normal' thing. – Jon Custer Oct 06 '16 at 12:48

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We should start with the obligatory warning that a beam of EM radiation is not a swarm of photons. See What is the relation between electromagnetic wave and photon? for more on this.

With that out out of the way, there is no lower limit on the energy of a photon. Radio waves of arbitrarily low frequencies still gain and lose energy in units of one photon i.e. $h\nu$. For example the famous 21cm line is produced when a hydrogen atom transitions between two electronic states and emits a photon.

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John Rennie
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    If an EM radiation is not consist of photons (be emitted from accelerated electrons in the antenna rod) and if photons are only excitations of an overall existing EM field, how to connect this with the Standard model of physics, where photons are particles? Having a different model of quantum theories should negate the existence of photons? – HolgerFiedler Oct 06 '16 at 11:02
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    Why is it so often assumed on this site that light and all its phenomena cannot be explained with photons?? Whats strange about that it is that particles can explain all light phenomena and the wave theory can only explain some. In addition a light wave itself cannot even be described but a photon can. – Bill Alsept Apr 21 '18 at 09:57
  • Radio emissions are photons, as are microwaves, infrared, visible, ultraviolet, X, & gamma. Radio is EM radiation & falls into the EM spectrum. Just as microwaves excite rotational energies of molecules & infrared excite translational energies, radio has more to do with electrons. The notion that radio emissions are not of a photonic nature simply makes no sense to me. I think rather there’s a separation between transmission of photons versus electrons & their states, just like how heat propagates on its own after excitation via radiation. But I’m a biochemist & I don’t know quantum theory. – hepcat72 Apr 18 '19 at 13:29
  • @hepcat72 see my answer to this question to get an idea of what photons are. – John Rennie Apr 18 '19 at 13:32
  • I’m afraid that’s over my head. The only reason I ended up here on physics stack exchange was because I was curious to know what causes the emission of RF photons in an RF transmitter and why the antenna length is derived via the speed of light divided by the MHz frequency. I recently created a 433Mhz transmitter on my RPi and I was fascinated by the requirement that the antenna had to be a harmonic length. I get the goal of getting resonance of electron flow, but was stuck on what emits the photon. I know that an electron falling to a lower orbit emits a photon, but I don’t think that’s it. – hepcat72 Apr 18 '19 at 13:53