Alright, for transmitting. Here’s the simple version: Lets go back to the pond. If you drop two stones in the pond and observe the pattern, do they not interfere with eachother? The waves shift a bit when they hit eachother. Mathematics can calculate this interference to produce precise controlled pulses using hundreds of little stones.
Here’s the longer writeup. OK, first of all, to understand transmission, you need to understand diffraction. The easiest way to think of diffraction, is that if you send a radio signal through a slit which is significantly smaller then the wavelength of the wave, it will produce a perfect, semicircular pattern. This is important – as mathematics exist for calculating this type of signal propagation pattern. (By someone much smarter then I).
Now, back to the phased array. Each individual antenna on a phased array acts as a slit – emitting a semicircular signal. Most importantly, the signals are either identical or close to eachother in frequency. This allows for combining the waveform. A phase shift between the T/R modules will allow for a small change in direction.
OK, back to the diffraction thing, you got the part where multiple emitters sending signals slightly out of phase with eachother will act as a single transmitter. Now, the problem is, how do we calculate which direction the signal is sent? This is where the mathematics of n-slit diffraction come in handy. You can treat the T/R modules as individual slits, and calculate it that way.
If anyone wants to look up the formula for it, it’s available on wikipedia. I don’t feel like typing it up here.
Edit: It’s been a while since I posted something like this, and I wrote something stupid/wrong tell me. Also bring-it-on, thanks, that saved me a bit of time.
Now, here’s the second part of the post. How phased array helps. To do this, I’ll first have to explain ECM in a semi technical manner; this will be a relatively concise explanation.
Radar works by bouncing a signal off something, the signal bounced off will be around a billionth of the total power transmitted by the radar. It will be a very weak signal, and very easy to Jam, if it was a pure signal.
However, it’s not a pure signal. A radar sends out waves in different packets and frequencies, making it hard for the other guy to Jam. A radar can differentiate between a signal sent out by itself and one sent by another source because it filters out all signals that weren’t the precise pulse it sent out.
However, modern ECM can counter this by finding the pattern of the radars signals, and sending this same pulses. Radar can counter that by making its pulses/frequencies/patterns random and having a narrow beam. This is where phased arrays with large amounts of T/R modules come in.
A single directional antenna being driven by a motor produces a signal spread a bit like this.
http://www.astronwireless.com/radiation_patterns.html – Awesome site about antenna radiation patterns and their usage.
This is a Yagi antenna’s radiation pattern, used by Japanese land radar in WW2, its a bit outdated, but the concept remains. The point is, even when the target is not in the scanning beam (main lobe) it can detect leaked radio waves and alert its RWR to the direction/type of enemy plane being used or prepare effective ECM countermeasures.
Enter the phased array. With a low module count, you’ll still have the signal leakage problem, but once you get higher and higher amounts of individual elements, you’ll have an very very well defined, very tight beam which looks considerably more like the expected cone then a bunch of lobes. This allows for tight scans which may potentially prevent the opponent’s RWR from alerting him, or allow his ECM systems to come up with countermeasures.
Edit: If you like these posts, please respond. I have a bit of time to write more information, including a planned semiarticle on passive and active phased arrays, and ways of signal processing/target identification.
I also want to write about how well designed datalinks work, but apparently it seems noone is interested.
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