Question

Can anyone give me some information about phased array antennas... Can you simplify the concept behind them? I've seen the wiki pages and all... Just looking for a simplified explanation in terms of high school physics...

Answer 1

It's a clever arrangement of signals such that destructive interference occurs in the directions you don't want the signal to go, and constructive interference occurs where you do want the signal to go.

Answer 2

Yakey is right. You broadcast signal from many antennas, and arrange it so that the signals construtively interfere only the direction you want the signal to go. The way you do this is by delaying the signal to each antenna by different amounts of time, depending on where you want the signal to go.

For example, consider an array with just two antennas, one a little bit north of the other. If the two signals are "in phase," meaning neither is delayed, then the combined signal from the antennas will be strongest along a line between the antennas running east-west. Towards the "side" of the array, you might say. That's where the construtive interference between the signals is strongest. Think of two rocks through simultaneously into a water, and the ripples they make: the ripples will constructively interfere along a line between the two impact points, shooting out to the side.

Now suppose you want the signal to go northeast and northwest instead of east-west. You delay the signal to the north antenna a little bit. Now, for the same constructive interference to happen, the signals from the north antenna have to go a shorter distance (hence spend a shorter time traveling) than the signals to the south antenna, which got started earlier. So the place of maximum constructive interference will be towards the north of the east-west line. Simiarly, if you want the signal to go southeast and southwest instead of east-west, you can delay the signal to the south antenna a little bit.

If you add more and more antennas, you can construct more and more complicated additions of signal, so that you can build up a very precise signal that goes exactly where you want it to. That's the idea. Phased-array antennas are often made up of dozens, if not hundreds, of individual antennas.

The advantage is that you can move your signal beam very fast, essentially as fast as your electronics can alter the signal -- microseconds. That's way faster than any mechanical movement of antenna, if you're using a single antenna and just pointing it in different directions. So this is greally great for radar signals that need to sweep the sky quickly, e.g. for missile early warning or military radar. (For civilian radar it tends to be overkill.) The drawback is that the electronics are much more expensive, and you need many antennas. For short wavelengths, like radar, where an antenna is just a few centimeters long, this is easy, but for long wavelength, like radio, it's a problem and expensive.

I should point out that ordinary broadcast radio often already uses a crude phased array system. You'll sometimes find AM broadcast antennas in sets of three, in a row, which makes the signal shoot out in one direction (perpendicular to the line of antennas) more than others, either for regulatory reasons (because the station is licensed that way) or to reach a particular audience better.