Turnipsock, from thenakedscientists.com/forum asked:
My satellite dish is full of holes. Wouldn’t it be easier and cheaper to take all the holes out and just make a smaller dish?
Sarah - Before Chris scampered off on his holidays he managed to pen an excellent answer to this on our forum and he says that basically the dish is full of holes to reduce the weight and the wind resistance aka the likelihood of it being blown away from your wall. The dish works fine as a reflector despite it being full of holes because it’s picking up relatively long wavelength radio waves and microwaves. Because the waves are longer than the diameter of the holes, they’re easily reflected by the mesh in the dish because the waves won’t fit through the holes. What the waves see is effectively a smooth, flat surface which focuses the waves onto a point where the detector is positioned. By doing this, gathering lots of waves over a large area and focussing them to a point you can get a much larger signal going to your TV.
Ben - This is exactly what we did with our parabolic mirror in last week’s kitchen science. What we were doing though, instead of focussing satellite signals, is actually focussing light and heat down onto a point. You can actually use it the other way around and put the point source as your heat and use the dish to beam a jet of heat directly forward. Dave did this to me in his garage and it really was quite uncomfortably warm. Isn’t this the same reason why you have a gauze in the front of your microwave?
Sarah - Yes, you know when you turn on your microwave you can see the light on inside but obviously the microwaves aren’t coming out at you. It’s because the wavelength of the microwaves is much longer than the wavelength of visible light. Although the light can get out through the mesh in the door the microwaves can’t.
Ben - And this is why you don’t cook your nose while you’re watching your food cooking!