Question

Laser beams are said to have high "spatial coherence". This means that the beam is highly concentrated even at long distances (low spread).

Can this be achieved with radio waves (much longer waves) or is it due to laser's stimulated emission?

Answer 1

Laser light is spatially and temporally coherent. The stimulated emission is mainly responsible for the temporal coherence.

So the answer is yes, you can create an electromagnetic beam that is spatially but not temporally coherent by placing a pinhole close to the source, and then another pinhole in the far field of the first pinhole. This beam will not spread out very much. (But also remember that laser light does spread out.)

Note that for RF frequencies, a "pinhole" is probably several meters in diameter. The far field distance is given by this inequality: , where is the distance, a is the diameter of the hole, and ? is the wavelength.

However, creating a RF pencil beam is probably not practical. The term "pencil beam" mentioned in the Wikipedia article is explained as being diffraction-limited. The size of a diffraction-limited beam gets larger with, I believe, the square root of the wavelength. It would be more like a gas-pipeline beam than a pencil beam.