# How does an antenna emit photons at a specific frequency if the antenna is made up a specific metal (Al) for example?

My base assumptions...

• An antenna emits energy over long distances in the form of photons.
• A photon is emitted when an electron changes energy state from higher to lower levels.
• A photon's "frequency" is dependent on the photon's energy level. Lower energy = lower frequency.
• Every radio frequency is associated with a specific photon energy level.
• When a radio transmitter transmits a higher "power level" that equates to more photons at the specific photon energy level not photons at higher energy levels.
• Antennas can be made out of aluminum that transmit at frequency ranges from ~30kHz to 40+GHz range.

My question(s)...

How can we have an aluminum antenna operate at 1 MHz emit photons at a specific energy level equal to 1 MHz and have another antenna made of aluminum emit photons at a power level equal to 25 GHz?

Take the helium visible spectrum ([Helium Emission And Absorption][1]). There are specific photons that are emitted or absorbed by helium which in this case show up as visible light. If we had the same spectrum graph for aluminum would it not have a similar set of discrete slices which may or may not fall within the visible spectrum?

Radios can transmit down to the sub 1Hz frequency accuracy.. Assuming we have antennas that operate at 30 kHz up to 40 GHz and we use a 1 Hz frequency resolution that means aluminum is capable of generating photons at 40 billion different energy levels. How is this possible yet helium only has ~11 photon energy states?

• You are missing that the aluminium is a gigantic solid crystal at least. It is not an isolated atom, and is actually kind of closer to the classical idea of a conducting metal rigid body for this purpose than as a single quantum atom. Once you take into account that there are so many atoms playing together, then the many frequencies will make sense. Commented Aug 17, 2023 at 5:28
• Does this answer your question? How can current flow through an open wire (like a dipole antenna)? Commented Aug 17, 2023 at 7:02
• @Farcher: I don't think that Q&A addresses the OP's fundamental concern, which is how individual atoms have distinct absorption & emission lines but an antenna does not. Commented Aug 17, 2023 at 12:07
• @MichaelSeifert - indeed, that is the heart of the issue. In an antenna, the emitted wave is the result of the overall current distribution in the antenna, not (much) a function of what the antenna is made of to first order. The time varying electric and magnetic fields in and around the antenna couple into propagating E&M wave modes, resulting in the emission of the radio waves. The photon energy in HF/VHF/UHF is much much lower than any atomic transitions in the material. Commented Aug 17, 2023 at 12:28
• "A photon is emitted when an electron changes energy state from higher to lower levels." In an antenna, that is not the process by which photons are created. Photons are created by acceleration of charge. Commented Aug 18, 2023 at 4:53

## 4 Answers

A photon is emitted when an electron changes energy state from higher to lower levels.

This is what happens in LEDs and lasers, but it's not (at least the way you're thinking) what's going in RF antennas or incandescent optical sources.

If there's a state change involved in the emission from an antenna, it's between two states within the conduction band of the material (aluminum in your example).

How can we have an aluminum antenna operate at 1 MHz emit photons at a specific energy level equal to 1 MHz and have another antenna made of aluminum emit photons at a power level equal to 25 GHz?

Since there are a (near) continuum of available states in the conduction band, it's possible to produce a wide range of different photon energies from transitions between those states. Notice that even 25 GHz radiation has a photon energy of only about .0001 eV, a very small value compared to the gap between conduction and valence bands in typical semiconductors, or between the top and bottom of a band.

There aren't many systems where quantum mechanical effects are important in RF and low microwave frequencies. One where they are is the atomic clock. In the case of the cesium atomic clock, the level transition that is used to lock the clock frequency is a hyperfine transition; that is a transition between levels that are distinguished by the interaction of the magnetic field of the nucleus interacting with states that would otherwise have identical energy levels (they'd be degenerate). It's not a transition between states differentiated by the principal quantum number you learned about in freshman chemistry or physics.

The EM radiation from an antenna does not come from electrons changing their energy levels in atoms.

Any charged object will emit EM radiation when it is accelerated. This emission is described by the Larmor equation. In an antenna we apply an oscillating potential to the antenna and this makes the electrons in the metal of the antenna oscillate at the same frequency. The acceleration of the electrons as they oscillate then emits EM radiation, again at the same frequency. The end result is that EM waves are emitted at the same frequency as the applied oscillating potential.

There is a fundamental conceptual error in your mental picture of how RF antennas work.

To begin with, radio-frequency antenna design and operation is not modeled on the basis of photon emission, it is modeled on the basis of creating electromagnetic waves through the use of alternating electrical current flow. As such, the photoemissive character of the antenna material itself has nothing to do with the antenna's effectiveness in converting high frequency AC into electromagnetic waves radiating into space. What does matter is just the electrical conductivity of the antenna elements themselves.

Note that when an RF antenna is radiating EM waves, the electrons associated with the aluminum elements of the antenna are not transitioning between different energy levels at all. (In fact, a solid piece of metal does not even possess the electronic configuration of a helium atom, with its discrete energy levels.) Those antenna elements are simply pieces of thick wire that are conducting electrical current.

What are photons?

Photons get emitted every time when a body has a temperature higher 0 Kelvin (the absolute zero temperature). All bodies, surrounding us (except black holes) at any time radiate. They emit radiation into the surrounding as well as they receive radiation from the surrounding. Max Planck was the physicist who found out that this radiation has to be emitted in small portions, later called quanta and even later called photons. Making some changes in the imagination of how electrons are distributed around the nucleus, it was concluded that electrons get disturbed by incoming photons, by this way gain energy and give back this energy by the emission of photons. And photons not only get emitted from electrons. The nucleus, if well disturbed, emits photons too. Such radiations are called X-rays and gamma rays.

What is electromagnetic radiation?

EM radiation is the sum of all emitted photons from the involved electrons, protons and neutrons of a body. All bodies emit infrared radiation; beginning with approx. 500°C they emit visible light, first glowing in red and then shining brighter and brighter. There are some methods to stimulate the emission of EM radiation. It was found out that beside the re-emission of photons there is a second possibility to generate EM radiation. Every time, an electron is accelerated, it emits photons. This explanation helps to understand what happens in the glow filament of an electric bulb. The electrons at the filament are not moving straight forwards, they bump together and running zig-zag. By this accelerations they lose energy and this energy is emitted as photons. Most of this photons are infrared photons, and some of this photons are in the range of the visible light. In a fluorescent tube the electrons get accelerated with higher energy and they emit ultraviolet photons (which get converted into visible light by the fluorescent coating of the glass). Higher energy (with higher velocity) electrons reach the nucleus and the nucleus emits X-rays. As long as the introduced energy is a continuous flow, not one is able to measure an oscillation of EM radiation.

What are EM waves?

Using a wave generator it is possible to create oscillating EM radiation. Such radiations are called radio waves. It was found out that a modified LC circuit in unit with a wave generator is able to radiate and that it’s possible to filter out such a modulated radiation (of a certain frequency) from the surrounding noisy EM radiation.

So the wave generator has a double function. The generator has to accelerate forward and backward the electrons inside the antenna rod and by this the photons of the radio wave get emitted, and the generator makes it possible to modulate this EM radiation with a carrier frequency. It has to be underlined that the frequency of the emitted photons are with the typical wavelengths from their exited states. There is an optimal ratio between the length of the antenna rod and the frequency of the wave generator. But of course one can change the length of the rod or one can change the frequency of generator. This changes the efficiency of the radiation to the needed energy input only.

How can we have an aluminum antenna operate at 1 MHz emit photons at a specific energy level equal to 1 MHz and have another antenna made of aluminum emit photons at a power level equal to 25 GHz?

You have thought and deduced correctly. To conclude from the length of the antenna rod to the wavelength of the emitted photons is nonsense.