Hi, I'm Mark Hickle from Purdue University
In Professor Dimitri Peroulis' research group
And today I'm going to show you a little bit
about how phased array antennas work
Now, throughout this discussion as I'm talking
about electromagnetic waves, I'm going to
draw analogies between those waves
and ripples in a pond using some slow-motion
videos I've made of water waves, because since
those two types of waves obey almost exactly the same mathematical equations,
we can use this as a powerful tool to visualize how electromagnetic waves work,
since we can't actually see them.
So, most types of antennas transmit about the same amount of power in all directions.
Kind of like a lightbulb - if you look at a bare lightbulb from any direction,
it has the same brightness.
You can also see from this video, the single stream of water droplets
produces a circular, uniform wave in all directions.
But sometimes that's not what we want.
Sometimes we want something more like a laser pointer
or an antenna that has a very narrow beam of electromagnetic waves
that we can point in any direction.
An example of this is, in RADAR systems they send out an electromagnetic wave,
and they listen for that wave to bounce off of an object and return to the RADAR.
They can use this to determine the distance to the object.
But if they send the same amount of electromagnetic waves in all directions,
then they can't tell if they're detecting a plane coming in for landing or just an office building down the street.
And that clearly defeats the purpose of the RADAR.
So in this case, what we want instead is an antenna that has a very narrow beam
of waves that it sends out, so that it can tell the distance to
and the precise direction of the object that they're detecting.
Now contrary to what I said earlier,
it's actually not difficult to design an antenna that has a narrow beam.
It turns out that the bigger you make your antenna, the narrower its beam is.
An example of this could be the big satellite TV
antennas that your parents had back in the day.
But the problem with these is they're big and heavy,
so if you want to turn it to point it in a different direction,
it's difficult to do, and it's slow.
So we need something better than this,
and that's where phased array antennas come into play.
So a phased array of antennas is essentially a group of antennas,
which could be our small light-bulb-like antennas, placed next to each other in a rectangular grid,
or in the simplest case just in a line.
Each of these antennas sends out the same signal, and we notice a very interesting result:
As the sinusoidal waves that the antennas send out travel outward,
they constructively and destructively interfere with each other, so that if we have designed
our array correctly, they all add together into a narrow beam in one specific direction,
but they cancel each other out in all other directions.
We can then look at this array as a single composite antenna which has a very narrow beam,
which is exactly what we said that we wanted. We can also see this in this video:
Here we have two streams of water droplets, representing two antennas.
We see that the waves they send out cause patterns of interference,
forming a main beam perpendicular to the drops,
and cancelling each other out in these other directions.
We also see these other beams to the side. These are called "side lobes", and although they aren't desireable
they can be suppressed in real systems, and here they're just an artifact of our somewhat limited setup.
Now in most practical systems you'll have dozens, maybe even hundreds of antennas,
which allow the beam to get narrower and narrower, and more closely approximate
the laser-pointer-like antenna that we talked about.
Now you might wonder what we've gained, since we just traded a big antenna
for a bunch of small antennas which probably add up to the same size.
Well, the answer lies in how easy it is to change the direction of the phased array antenna.
So we saw before that if we send out the exact same signal from each of the antennas,
that they add together to form a narrow beam perpendicular to the antennas.
But it turns out that if we add a slight time delay to each of the siganls that we send out
from each antenna, that direction in which they add together into that narrow beam changes.
And that new direction depends on how much time delay we add to each of the signals.
And time delay is really easy to do in digital processing, which is perfect.
Now we have an antenna that has a narrow beam and we can steer that beam back and forth
just with a little bit of digital processing, which is very fast.
And we don't even have to worry about moving this big, heavy antenna back and forth.
Now we can see this effect in our video.
Here, if we change the timing of the drops a little bit so they don't hit the water at exactly the same time,
then that changes the direction of the main beam.
We can see here that as we change the timing, we can actually steer that beam back and forth,
just like in a phased array.
So that's just a really high level overview of how phased array antennas work.
I hope it helped give you an understanding, and that it helped you visualize
how electromagnetic waves interact with each other. Thanks for watching.