Has anyone ever put a magnetic or electrostatic dipole on a rotating shaft, spun it and demonstrated reception of a propagating wave in the far-field? The question If I create a varying electric field and it will then create a varying magnetic field, so will it also create light? Will I see a light ray? got me thinking.
I'm pretty sure that if I could put a strong enough magnetic or electrostatic dipole on a shaft spinning sufficiently fast, I could make a low frequency radio wave that would propagate to the far field and receive it with a suitably low frequency antenna and radio receiver.
I'm curious if such a demonstration has actually been done like that.
I'm not asking for analogous demonstrations or "that's in effect what a radio transmitter does with a loop antenna" type answers, I'd like to know if such a practical demonstration has ever been successfully carried out.
There's got to be a real, mechanical rotating shaft and a real magnetostatic or electrostatic dipole, like a bar magnet or two charged spheres separated by an insulating rod for example, and an actual receiver in the far field recording propagating electromagnetic waves, not just some evanescent tail.
 A: It's very difficult. A "turnstile" antenna is effectively a rotating dipole, simulated electrically. For the usual half-wave turnstile, the tips of the effective spinning element are moving faster than the speed of light! If you make it smaller, thus reducing the effective linear rotation speed, its capability as a radiator declines rapidly. I don't know of a mechanical system that can rotate >1/100,000 the speed of light.
Edit: In response to comments, here's a quick and dirty engineering sketch of an experiment.
Reference Data for Engineers (E. C. Jordan Ed., 1989) tells me that the radiation resistance of a dipole antenna scales as length squared (Jackson, of course, tells me the same). The current is charge/time, so it scales as velocity for the same charge. Power is proportional to the square of the current times the resistance. So, rotating a dipole at 10^-5 c radiates ~10^-20 of the power that the turnstile radiates for the same amount of charge on the elements. RDfE tells me the natural noise on Earth at 10 kHz (unlikely to be practical as a mechanical rotation speed) is ~160 dB above the nominal thermal.
Nominal thermal is -204 dBW/Hz, thus natural noise at this frequency is -44 dBW. Let's imagine that we can transmit +30 dbW (1 kW) with our turnstile: then, our mechanical version would transmit -170 dBW. Even if our receiver could capture this all (impractical), our SNR is -126dB in a 1 Hz bandwidth. Thus, we'd need to integrate for ~10^13 seconds to detect a signal. I don't expect to live that long ツ
A: Physically rotating the magnet on an orbital motion around a point in space is the principle used Faraday induction, to generate electric current in electric power plants. The electric current is induced by the rotating magnetic field into electric coils.
"The RPM will depend on the number of poles in the alternator. In case of 2 magnetic poles the RPM would be 3600, for 4 pole 1800 for 6 pole 1200 and so on
The formula is RPM=120 x f/n
Where f is the required frequency and n is the no of poles in the alternator."
https://www.quora.com/In-order-to-generate-electricity-at-a-frequency-of-60Hz-a-generator-in-a-power-plant-must-be-operated-at-how-much-RPM
If you mean in your question just spinning very fast a permanent magnet around its own N-S axis, I don't think you will accomplish anything the field will remain still static and there will be no noticeable EM radiation unless the magnet has large dimensions.   The physically spinning magnet will additionally precess that would generate EM waves of $ω_{p}$/2π frequency in Ηz units:
$$
\omega_{\mathrm{p}}=\frac{m g r}{I_{\mathrm{s}} \omega_{\mathrm{s}}}
$$
where $ω_{s}$ the spin angular velocity in rad/s, $I_{s}$ the moment of inertia, m the mass of the magnet and g Earth's gravitational acceleration, $r$ the cross-section diameter of the magnet pole divided by two  and $ω_{p}$ the EM precession frequency.
To get a feeling of the above equation where Is=7.5x10^-10 Kg m^2 is the calculated moment of inertia  of a sphere ferrite magnet, ωs=47.77Hz, m=0.3gr, g=9.81, m/s^2 and r=2.5mm the radius of the sphere magnet. The value obtained of $ω_{p}=5.2$  Hz units radiated EM waves corresponds to a value of no less than 312 rpm, Newtonian precession rotations. Imagine now try to spin on its N-S axis a large 10 cm magnet at 312 rpm! The mechanical stress would break the magnet apart.
If you mean to spin the magnet on an axis perpendicular to its N-S axis then yes it will radiate EM waves of frequency proportional to its spin rpm.
As for related experiments of what you are asking please see my comments on your question.
Here is an example of a rotating magnet ELF-ULF radio transmission antenna, and reception of the EM waves in the near field:  https://www.jpier.org/PIERM/pierm72/14.18070204.pd
Reception in the far field is practically very difficult to be demonstrated remotely (i.e. receiver must be at least 2 wavelengths away from transmmiter) for such low frequency EM waves which can have wavelengths of many Km. Unless, you have access to a ULF reception antenna array station (300Hz-3KHz) (wavelengths 1Km to 100Km)! en.wikipedia.org/wiki/Project_Sanguine or maybe HAARP https://en.wikipedia.org/wiki/High-frequency_Active_Auroral_Research_Program .
Of course one could try far reception with a sensitive ULF-ELF electrical short antenna like these magnetic antennas here: aaronia-shop.com/products/antennas-sensors/magnetic-antenna
Otherwise, as far as I know, there is no demonstration or reference currently to be found of such thing you are asking thus if someone has demonstrated reception specific in the far field of ULF or ELF EM waves generated by a mechanically rotating EM charge.
