# Photon spins from Radio Antennas

I’m a radio amateur (ham). Let us consider a half wavelength horizontal dipole antenna in free space – say at 10MHz and look at its far field.

Conventional high school physics says that because the antenna is horizontal, the electrons are accelerated horizontally and the resulting electromagnetic (radio) wave is horizontally polarised.

(Incidentally, this appears only to be true when broadside on. Due to simple 3D geometry, a vertical component creeps in when off-axis (see the plots from an antenna modelling program eg MMANA-GAL).)

However we know that linear polarisation (eg horizontal) can also be described by the combination of a right hand circular polarised wave with a left hand one. Obviously the amplitudes must be equal and for horizontal the two circularly polarised waves need to be in-phase.

We also know that circular polarisation of a wave is the manifestation of photon spin. Two photon spin states are permitted, and so its angular momentum can be either right hand, or left hand. Hence RHCP and LHCP waves.

Question:

So how can the above antenna be described to explain the production of right hand and left hand spin photons which when combined produce linear polarisations?

There appear to be two obvious possibilities. Either:

A) Production of opposite spin photons happens as a concerted process throughout the antenna such that all parts of the antenna produce equal numbers of RH and LH photons?

Possibly this might be due to the photon’s spin resulting from spin of the electron which has been accelerated. Because electron spins are evenly distributed throughout the antenna wire, so the resulting photons are evenly spread out in the resulting wave.

B) Opposite photon spins are produced simultaneously, but from opposite ends of the dipole antenna.

This might be due to the direction of current flow (ie electron flow). It flows into one arm of the dipole producing one spin, but at the same time electrons flow out of the other limb, so producing the opposite spin at the same time ie from positive and negative accelerations in the direction of electron flow.

• As an FYI, there is a Ham site here on Stackexchange. You might want to stop by. Mar 21, 2018 at 13:38
• Many thanks for the pointer! I wasn't aware of ham.stackexchange.com. Have just taken a look and sadly it is lacking the rigour that I'm enjoying here on physics.stackexchange. Its not only knowledgeable, but also a supportive community. My apologies for lower the tone of its debate with lowly ham radio! Many thanks! Mar 23, 2018 at 9:13
• Oh, I wasn’t worried about lowering the tone, just making sure a newish user found it! Mar 23, 2018 at 14:34