DEEP TECH

A TV satellite dish uses a parabolic reflector to focus the electromagnetic waves which are the TV signals sent from broadcast satellites orbiting the Earth at an altitude of 35,000 kilometers. TV satellite dishes only receive TV signals from space, they can't send data.

Dishy, however, both sends and receives internet data from a Starlink satellite orbiting 550 kilometers away. While the Starlink satellite is 60 times closer than TV satellites, its signal needs to be focused in a tight powerful beam that’s constantly angled or steered to point at one another. Compare this to TV broadcast signals which come from a satellite the size of van whose signals propagate in a wide fan that covers land masses larger than North America.

10,000 or more Starlink satellites orbit at incredibly fast speeds in a low earth orbit and are required to provide satellite internet to the entire earth.

Let’s now open up Dishy McFlatface. At the back, we have a pair of motors and an ethernet cable that connects to the router. Note that these motors don’t continuously move Dishy to point directly at the Starlink satellite; they are used only for initial setup to get the dish pointed in the proper general direction. Opening up Dishy, we find an aluminum structural back-plate and on the other side, we find a massive printed circuit board or PCB. One side has 640 small microchips and 20 larger microchips organized in a pattern with very intricate traces fanning out from the larger to smaller microchips, along with additional chips including the main CPU and GPS module on the edge of the PCB. On the other side are 1,400 dish copper circles with a grid of squares between the circles. On the next layer, there’s a rubber honeycomb pattern with small, notched copper circles, and behind that, we find another honeycomb pattern and then the front side of Dishy. Well, in essence, we have 1280 antennas arranged in a hexagonal honeycomb pattern, with each stack of copper circles being a single antenna controlled by the microchips on the PCB. This massive array works together in what’s called a phased array in order to send and receive electromagnetic waves that are angled to and from a Starlink satellite orbiting 550 kilometers above. Let’s see how a single antenna operates.

Here we have an aperture coupled patch antenna composed of 6 layers, most of which are inside the PCB. It looks very different from the antenna of an old-school radio, and is honestly, incredibly complicated, so We’ll step through the basic principles of how we generate an electromagnetic wave that propagates out from this antenna. To start, at the bottom we have a microstrip transmission line feed coming from one of the small microchips. This transmission line feed is just a copper PCB trace or wire that abruptly ends under the antenna stack. We send a 12 Gigahertz high-frequency voltage or signal to the feed wire which is a voltage that goes up and down in a sinusoidal fashion, going from positive to negative and back to positive once every 83 pico-seconds, 12 billion times a second, or 12 Gigahertz. Note that high-frequency electricity works differently from direct current or low frequency 50 or 60-hertz household electricity.

For example, above the copper feed wire, we have a copper circle with notches cut into it called an antenna patch. With DC or low-frequency alternating current, there wouldn’t be much happening because the patch is isolated, but with a high-frequency signal, the power sent to the feed wire is coupled or sent to the patch.

How exactly does this happen? Well, as mentioned earlier, a 12 Gigahertz signal is applied to the copper feed wire. When the voltage is at the bottom of its sinusoidal, or trough, we have a concentration of electrons pushed to the end of the feed wire thus creating a zone of negative charge which corresponds to the maximum negative voltage. This concentration of electrons on the tip of the wire repels all electrons away, including the electrons on the top of the patch, and as a result, these electrons are pushed to the other side of the circular patch. Thus, one side of the patch becomes positively charged, while the other becomes negatively charged, thereby creating electric fields between the patch and feed wire like so. However, when we reverse the voltage to the copper feed wire 42 picoseconds later, we have a concentration of positive charges, or a lack of electrons at the end of the wire, and thus the electrons in the patch flow to the other side, the voltage in the patch is flipped, and the direction of the electric fields are also flipped.

Because the feed wire voltage oscillates back and forth, 42 picoseconds between one peak and trough, the electric fields in the patch will also oscillate as the electrons, or current, flows back and forth. If we pause the oscillation we can see some of these electric field vectors, or arrows, from the patch, are vertical, and because they are equal and opposite, they cancel out. However, other electric fields are horizontal in the same plane of the patch and are called fringing fields. These fringing fields are in the same direction and thus they add to each other, resulting in a combined electric field pointing in this direction. At the same time, electrons flowing from one side of the disk to the other, which is an electric current, generate a magnetic field with a strength and direction, or vector, perpendicular to the fringing electric field vector. As a result, we have an electric field pointing one way, and a magnetic field pointing perpendicular to that.

Let’s move forward in time to where the voltage on the feedline becomes positive, and now, we’re at the peak of the sinusoid, 42 picoseconds later. The charge concentrations, or voltage, as well as the current, is all flipped, and thus the electric and magnetic fields point in the opposite directions. Electric and magnetic fields propagate in all directions, and by creating these oscillating fields, we’ve generated an electromagnetic wave which travels in the direction perpendicular to both the electric and magnetic field vectors.

Because the two sets of field vectors are not all in the same plane, but rather are curved, the propagating electromagnetic wave travels outwards in an expanding shell or balloon-like fashion, kind of like a light bulb on the ceiling. Let’s simplify the visual so we only see the peak and trough or top and bottom of each wave and note that the trough is just a vector pointed in the opposite direction. Additionally, the strengths of these field vectors directly relate back to the voltage and signal that we originally sent to the copper microstrip feed wire at the bottom of the stack. Which means, if we want to make these electric and magnetic fields stronger, we just have to increase the voltage sent to the feedline. It’s like a dimmer on a light switch: more power equals a brighter light. Thus far we’ve been talking about this aperture-coupled patch-antenna as transmitting; however, it can also be used for receiving a signal. In this microchip, called a front-end module, we switch the antenna from transmit to receive and turn off the 12 Gigahertz signal.

When an electromagnetic wave from the satellite is directed towards Dishy, the electric fields from this incoming signal will influence the electrons in the copper patch, thus generating an oscillating flow of electrons. This received high-frequency signal is then coupled to the feedline where it’s sent to the front-end module chip which amplifies the signal. Thus, these antennas can be used to both transmit and receive electromagnetic waves, but, not at the same time.

Let’s see how a single antenna can be combined with others in order to amplify the beam to reach outer space. This single antenna is only a centimeter or so in diameter and using only it would be like turning on and off one light bulb and trying to see it from the international space station.

What we need is a way to make the light a few thousand times brighter, and then focus all the electromagnetic waves into a single powerful beam. Enter the massive Mr. McFlatface PCB, 55 centimeters wide with a total of 1280 identical antennas in a hexagonal array. The technique of combining all the antennas’ power together is called beam forming.

So how does it work? When we have two simplified antennas spaced a short distance away, one antenna generates an electromagnetic wave that propagates outwards in a balloon shape. At every single point in space, there’s only one electric field vector with a strength and direction and thus the two antennas’ oscillating electric field vectors combine together at all points in space. In some areas, the electric fields from the antennas are pointing in the same direction with overlapping peaks, and thus add together via constructive interference, and in other locations, they are opposite with one peak and one trough, and thus they cancel each other via destructive interference. We can now see that the zone where they add together constructively is far tighter, or more focused, than a single antenna alone.

When we add even more antennas, the zone of constructive interference becomes even more focused in what is called a beam front. Thus, by adding 1280 antennas together we can form a beam with so much intensity and directionality that it can reach outer space. Now you might be thinking that the strength of 1 antenna duplicated 1280 times over would result in a combined power of, well, 1280 times a single antenna, but you’d be mistaken. The effective power and range of the main beam from all these antennas combined is actually closer to 3500 times that of a single antenna.

Dishy McFlatface and the Starlink Satellites undoubtedly have some rather complicated science and engineering inside and to fully comprehend it all you have to be a multidisciplinary student.

The solution is to phase shift the signal sent to one antenna with respect to the other antenna and, as a result, the timing of the peaks and troughs emitted from one antenna is different from the other. These peaks and troughs propagate outwards, and the location of the constructive interference is now angled to the left with destructive interference everywhere else.

If we change the phase of the antennas again, the zone of constructive interference is angled to the right. Therefore, by continuously changing the phase of the signals sent to the antennas, we can create a sweeping zone of the constructive interference.

Far away from the antennas, the waves join to form a wave front that is a planar wave. Kind of like ocean waves crashing on a shoreline. Just as before, by continuously changing the timing of when each wave peak is emitted by each antenna, we can change the angle at which the wave front is formed, essentially steering the beam in one direction or another. And, if we bring in more antennas in a two-dimensional array, we can now steer the beam in any direction within a one-hundred-degree field of view.