# Why Don’t We Use Radios for Propulsion?

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?

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.