We’ve all heard the idea of laser propulsion before, but why can’t a high powered RF source be used to propel a body? A simple high current coil with an AC signal applied to it to it could produce 100s of watts if not kilowatts of EM power. So why not use these instead of our underpowered 10W Red lasers?
What makes RF propulsion energy inefficient?
In principle the thrust-to-radiated-power ratio of all electromagentic radiation drives is identical.1 Frequency doesn't enter directly into the issue.
So it is tempting to say "use the band with the highest ratio of radiated-power-to-supplied-power". Good idea.
But then there is the question of collimation and the influence of diffraction. However you produce it, your beam is going to try to spread out. Applying acollimating system is generally a good idea, but diffraction limits the degree of success you can have based on the size of the collimator (big is good) and the frequency/wavelength of the EM signal (high-frequency/short-wavelength is good). Qualitatively you should be thinking of the opening angle $\theta$ (expressed in radians) as being given by $$ \theta \propto \frac{\lambda}{D} $$ where $D$ is the size of the aperture and $\lambda$ is the wavelength the constant of proportionality is geometry dependent.
The main problem with using radio-band (aside from the very low thrust of photon drive in general) is that without a large and therefore massive collimator you will lose non-trivial thrust to diffraction.
1 Identically tiny. You get about 3 micro-newtons per killowatt.
