1 - Akademi Sains Malaysia

Submarines and Space:
Creating an Antenna in the Sky
by In Dr. AHMAD FAIZAL
MOHD ZAIN F.A.S.c.
SEVERAL questions can be asked: How
can an antenna exist in space Why do
we need an antenna in the sky above
What has a submarine got to do with
space
Is "space, the final frontier" Trekkies or
fans of the Star Trek TV series that debuted
in 1966, would be familiar with this cliche.
Space has intrigued humanity since the
creation of man on earth. Man has looked
to the skies searching for answers as to his
origins.
With naked eyes, man is able to see two
of the brightest objects; the sun and the
moon.
Ancient Greeks and Arabs had studied
the night sky looking for celestial objects.
Aided by the telescope, invented by Lipperhey
and made famous by Galileo in
the early 17th century, man is able to look
deeper into space.
With modem technologies, space is just
an instrument away.
When people talk about space, they almost
always think about outer space. However,
closer to home, we have what is termed
as near space.
This starts about 20 km above the earth
in the stratosphere, to the edge of the thermosphere
for about 100 km.
This region contains small concentrations
of neutral and ionised particles, and
an abundance of electrons.
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Electrons, as being amongst the lightest
atomic particles, would be found at even
higher levels, up to more than 600 km.
These particles are generated by solar radiation,
in the form of x-rays, gamma rays,
ultraviolet, and infrared- rays, that reach the
earth.
These radiations knock the electrons out
of the neutral molecules. Being lighter, they
rise until a point where gravity takes over,
and form, a cloud of electrons.
This part of the earth's atmosphere is
called the ionosphere because it consists of
ions and electrons.
It is a well known fact that the depth of
penetration of an electromagnetic wave depends
on its frequency and the conductivity
of the media.
This is known as the skin depth effect. The
lower the frequency of an electromagnetic
wave, the deeper it penetrates seawater.
That is why things deep or in the ocean
look red because it is of a lower frequency
than blue light.
A submersible vehicle like a submarine,
it has to surface to communicate with the
outside world, either via satellite or high
frequency waves (HF).
How can it be submerged and stay
stealthy, if it wants to communicate
The obvious solution is to use a very low
frequency radio carrier. The downside is
that it needs a very big antenna.
For the submarine to communicate at a
depth of about 7 to 10 meters in seawater,
the frequency would have to be about I
kHz.
To radiate efficiently, a simple half-wave
dipole antenna would have to be 150 km
long, at a minimum height of 75 kin above
the earth!
The answer to this problem can be found
in the abundance of electrons in the ionosphere.
This layer of electrons can be periodically
agitated by using very high power radio
transmitters operating at around 3 to 6
MHz.
Vertical radiation is obtained by using directional
antenna. By pulsing the transmitters
on and off, the electrons will be heated
and cooled in tandem with the frequency
of pulsing. This action will in turn radiate
electromagnetic waves at the pulsing frequency.
If the pulsing is done at a much lower frequency
of about 1 to 10 kHz or in the ELF/
VLF (extremely low frequency or very low
frequency) band, an electromagnetic wave