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Well, antennas are important in any wireless link. If you've ever tried to get a black
and white TV to work, you know how moving the rabbit ears will change the signal from
snow to a clear signal. So having a good antenna, and having it pointed in the right direction,
is really important to getting a clear signal.
One of the problems we have to deal with is that the balloons don't stay in a fixed orientation,
they rotate. That's a challenge because the antennas on the balloon are polarized. Polarization
means the direction the electromagnetic waves are oscillating, so the electric field is
going back and forth like this or like that. There's a certain direction around which the
electromagnetic field is oscillating, and if that polarization doesn't line up with
an antenna on the ground, you don't get any signal at all.
This issue of cross polarization is probably familiar to anybody who wears polarized sunglasses.
If you look at any LCD screen the wrong way, it turns black. And that's because the polarization
of the sunglasses and the polarization of the screen don't line up.
What we did to address this problem is actually build two antennas in one. You can see there
are two cables coming out of it and they're both connected to the same patch elements
here, but they're actually driving it in two different directions, so it's a dual polarized
antenna. It can emit or receive radio signals that are polarized like this or like that.
So no matter how they're oriented, one or the other will get signal from the antennas
on the balloons, which are also cross polarized.
The design also lets us get more capacity and more throughput out of the same spectrum.
We're using it in two different ways, and we can get twice as much bandwidth through
it by using two different polarizations.