Today, we're gonna talk about the DOPPLER EFFECT.
Maybe you heard about it on one of my favorite TV show, The Big Bang Theory.
Well, then you HAVE to watch this:
The most common example used to describe the Doppler effect is a siren:
Have you ever noticed that the frequency of a siren's wail changes, depending on how close it is to you? The farther away it is, the lower the frequency, and the closer it is, the higher.
Ever wonder why?
Let's say I'm that little red dot in the center of that diagram. And I'm a beetle, one of those amusing ones who bangs its foot to make noise to attract a potential mate. With me so far? So, when I'm thumping away, the sound waves travel out equally in all directions. Assuming potential mate A and potential mate B are equidistant away from me, my come-hither sound wave will take the same amount of time to reach each of them, and they will each hear it at the same frequency.
But suppose potential mate A, who is located towards the right side of the diagram, responds with a sexy sounding thump of his own? Well, natch I start hustling in that direction. (I may be a beetle, but I'm no fool.) As I move away from B, who is on the left, it will now take longer for the sound of my thumping to reach him. (I'm multi-talented; I can run and thump at the same time.) As I approach A, my sound waves take a shorter and shorter time to reach him. That is, they come at a higher frequency. And for poor he-who-hesitates-is-lost suitor B, the waves take longer, and are at a lower frequency.
So, what does that have to do with amateur radio, you may ask? Amateurs can communicate via satellite. Or with a space shuttle, or the space station. Which are all moving, right? So, the satellite's frequency changes as it moves across the sky, and amateur radio operators must track that change, and coordinate their radio's frequency if they want to make a successful contact. Cool, huh?
And no, we don't have to do it by guesswork, either. There are some terrific computer programs specifically designed to track satellites and calculate this information.
(But ya know, I still like Sheldon's explanation the best ...)