Why can radio telescopes use wire mesh as their parabolic dish?

Radio telescopes use wire mesh to save cost. How does that work? Won't the waves go through the gaps?

Answer

Depending on the source, radio waves are said to have frequencies from as low as 3 kHz up to 300 GHz. The corresponding wavelength $\lambda$ can be calculated via

$$\lambda = c / f$$ where $c$ is the speed of light and $f$ is the frequency. This gives us $\lambda = 3 \mathrm{m}$ for $f=100 \mathrm{MHz}$, a typical FM radio frequency. If a wave encounters a material with holes in it, like wire mesh, it doesn't necessarily "see" those holes. Because they are much smaller than the wave itself, the dish might as well be solid to the wave.

You can see this very nicely with your home microwave oven. Microwaves are roughly a few cm in wavelength, and the mesh of the viewing window has holes a few mm in diameter. Visible light is much smaller than that, so it can get through (meaning you can see the inside of the oven), microwaves "see" a solid wall they get reflected off from.

Now, since wire mesh is basically the same as using a solid dish, it's really obvious to use it, because it's much lighter and thus allows you to build larger dishes that can also be moved quicker.

This is the "nice and easy" answer.