In 1982 ET phoned home using an umbrella, a speak and spell toy, and some coax cable.
But when we want to talk to our own starships we’re going to need something quite a bit
bigger.
Today, I’ve come to the Carnarvon Satellite Station to find out more about how to speak
to the edge of the universe.
We talk to spacecraft using radio waves, which inhabit the lowest energy part of the electromagnetic
spectrum.
Where visible light has a wavelength of a few hundred nanometres [a few millionths of
an inch], radio waves have wavelengths of seven centimetres [3 in].
Now, we use this particular part of the spectrum because it's long enough to transfer through
the atmosphere with minimal interference, but at the same time is short enough to still
contain lots of important information.
To generate a radio wave we need two electric charges; one positive and the other negative.
We'll draw field lines which link them together.
To keep things simple we’re only going to be considering one of these lines.
As we bring the charges inward the field begins to buckle and then breaks when the positive
and negative coincide.
This free bubble of electric field propagates outward.
As the charges continue to move, a new field- but one pointing the other direction – is
formed.
As we repeat the motion again and again, more of these field bubbles are produced.
A magnetic field oscillates at right angles to the electric.
We’ve created an electromagnetic wave travelling away from the source at the speed of light.
Since magically oscillating charges don’t exist in the real world, in order to transmit
radio signals we use a conducting rod with an alternating current source in the middle.
This pushes the electrons to one end, making it negative, and the other end positive.
In order to receive these signals all we need is another conducting rod with a voltmeter
in the middle.
This can record as we move the electrons up or down because of the incoming wave.
To transmit information all we need to do is encode a signal in either amplitude or
frequency modulation.
This is what's known as AM and FM radio.
Although satellites could use either of these types, it's more common that frequency modulation
is used because it’s able to store more data and is less prone to interference.
Now that we understand how radio waves work, it makes more sense that car antennas are
just straight rods.
However, as someone who’s been driving through the Australian outback for the last few days
can attest; radio doesn’t stretch all that far.
You can go for hundreds of kilometres [or miles] with nothing but static or, after the
first day, some pre-downloaded episodes of Hello Internet.
That’s because a simple rod is a form of low gain antenna; it transmits and receives
radio signals from a full 360 degree arc.
This means that energy disperses quickly.
When you’re driving through the town you can pick up a signal no matter where you are,
or which direction your car is facing, but outside a 40 kilometre [25 mile] range the
signal is too weak to detect.
A satellite which can’t talk to the Earth is pretty useless, which is where the weird
and wonderful world of dish design comes in.
We’ve now arrived at the Carnarvon Sugar Scoop.
This 12.8 m [42 ft] wide, folded horn antenna is the last of its kind in the world.
It was first used in 1966 when, after Intelsat-2A failed to reach the correct orbit, engineers
had a brief window through which they could communicate from here to the satellite to
the UK, allowing new immigrants in Carnarvon to talk with their families back home.
This was Australia’s first live broadcast outside the country.
In order to reach from Carnarvon to Intelsat drifting inexorably Westwards above the Indian
ocean, radio signals needed to be carefully directed to prevent them from spreading out
and losing strength.
We start out with a straight rod, but now rather than allowing the waves to spread out
everywhere instead we can direct them using the sides of the horn.
The sides themselves are made of aluminium which has much the same reflective properties
as a mirror does, but for radio waves rather than visible light.
When receiving the downlink, this process works in reverse; scooping up 12.8 metres
[42 ft] of wave fronts and slowly combining them together.
This increases the amplitude high enough to move the electrons in the receiving rod and
register a signal.
This is what's known as a high gain antenna.
From lightning strikes and pulsars to TV stations and power lines, Earth and space are full
of different sources of radio waves.
These can interfere with our signal, especially if it's really faint.
Another benefit of the horn antenna is that the sides provide some form of shielding from
these extra sources of radio waves.
Unfortunately, the sugar scoop antenna isn’t particularly efficient and couldn’t communicate
with things much beyond Low Earth Orbit.
Because of this, in 1969 it was replaced with this 30 metre [100 ft] dish.
Now, engineers in Carnarvon could relay messages to and from the Apollo missions on their way
to the Moon.
Parabolas are special because of their high directionality, they take a spreading wave
front and direct all of it in the same direction.
You’ll find mini parabolas inside of torches and car headlights.
In fact some of these parabolas can be so efficient that rather making the bulb brighter
or dimmer instead we move it towards or away from the focus point in order to change the
amount of light being emitted.
As with all things radio, the same principle also works in reverse.
This carefully designed parabolic shape means that no matter where incoming radio photons
hit, they’re all going to be directed toward the central focus point and then down into
this mini feed horn.
Eventually our condensed radio signals make their way through the end of the horn.
At this point we’d be placing two small antennas; one of them vertical and the other
one horizontal.
These would then be able to receive our magnified radio signals.
Because of the high directionality of these signals, it becomes necessary to point our
satellite dish precisely at our spacecraft.
In order to do that we use this set of gears to do the up and down motion and this set
to rotate.
Despite being more than 50 years old these gears still work today.
To walk on the dish safely we needed to orient it vertically, but now that I’m off we’re
lowering it to face the town of Carnarvon.
This 30 metre [100 ft] dish takes our messages to the edge of Earth’s influence, all the
way to the moon.
To go further we use almost the exact same technology, with a few slight modifications,
and make the dish quite a bit larger.
With some receivers 70 metres [230 ft] in diameter, NASA’s deep space network is able
to communicate with Voyager One, 23 billion kilometres [14.3 billion miles] away.
This network also allows us to stay in contact with all of the other rovers and space probes
exploring the solar system.
If Voyager does eventually make it to the other side of the galaxy, then to communicate
we would need a dish some 500 kilometres [310 miles] across.
The crushing weight of Earth’s gravity makes building and then pointing such as a gargantuan
receiver a nearly insurmountable engineering challenge.
But, who said that we needed to build it here on Earth?
We could construct this telescope at the L2 lagrange point where the gravity of Earth
and Sun cancel out.
With a few asteroids worth of material and some microgravity manufacturing, there's almost
no limit to how big we could build and how far we could reach.
Although your phone bill at the end of the month is going to be pretty astronomical.
As we explore the outer limits of our solar system, and eventually the universe, radio
receivers such as these will allow far-flung travellers, alone on alien worlds, to call
home.
This has been James Dingey from the Atomic Frontier.
Keep Looking Up.