by Sean Taeschner
Radiohax
Spark gap transmitters are the oldest type of radio transmitter made
by man. They were first used around 1888 and remained legal until the
1920s when their use became greatly restricted. World War II delayed
their complete ban outside of emergency communications for a few years.
Now the only way to use them legally is inside a
faraday cage. They operate as jammers for the same reason they were banned, they take up a lot of the radio spectrum.
A spark gap transmitter is fairly simple. Send a high voltage current
through an air gap, when the resistance of the air breaks down a spark
will cross the gap. When this happens electromagnetic radiation is
emitted. You can test this in your house fairly easily. Turn on some
speakers so they are powered but nothing is coming out of them. Computer
speakers that are on will work as will a stereo set to CD or tape with
no CD or tape playing. Flip your room lights on and off in rapid
succession, you should hear a clicking from the speakers. The clicking
is RF energy that is being picked up from the light switch.
Effective Range
The range of this type of jammer is based on a few properties.
- Antenna resonance
- Field strength
- Modulation
- Terrain
The antenna of this particular device will be more resonant on one
frequency than others. As a result there will be more RF energy on that
frequency, and harmonics or multiples of that frequency. The range for
this frequency will be further than others. Using multiple antennas will
cause the transmissions to be greater on more frequencies.
The field strength, or amount of RF energy given off, is directly
related to the difference in voltage between the two states. With the
type of transmitter we are making one state will be totally off or 0
volts while the other will be on or the full voltage. It is not required
to switch between an off and on state you may switch between a lower
and higher voltage as well. The higher the input voltage the stronger
the RF, the more range you will have. You have normal
free space losses that
all radio transmissions have which means that in order to double the
effective range you will have to apply 4 times more power.
Another consideration is the modulation that is being utilized.
Modulation also
includes some of the data that is carried over that mode such as
digital data. FM signals for example will lock onto the strongest
relative signal from the receivers point of view, a weaker but closer
signal will appear stronger than a further but more powerful signal due
to free space losses. FM can flip back and forth quickly between two
stations that have a nearly equal signal strength at the receivers
antenna and make it appear like it is mixing the two stations, the
reality is that it is not. SSB will mix the two signals, while you may
hear noise in with the desired audio you can still hear the desired
audio. Spread spectrum will tolerate the most noise.
Digital systems can be jammed even if the underlying modulation is
not by introducing noise the decoding circuitry in the radio may not be
able to decipher the bits correctly and reconstruct the message. This
can result in heavy packet loss on that system rendering it unusable.
Many digital systems are more sensitive to noise than their less
sophisticated counterparts. Many digital systems are either FM or spread
spectrum. FM is more often used for longer range applications.
The specific type of system that you are trying to jam is important
to know as it will tell you how much power you have to have and at what
distance. The best way to determine what is suitable is to get a similar
system and see how far it will work from the spark gap transmitter at a
given power.
Terrain includes buildings, trees and other objects that the radio
signals will interact with. If you stash the spark gap transmitter in a
metal box the signal will be attenuated compared to that same
transmitter being placed in an open field. Elevation can also influence
the distance the signal may go.
Construction
The construction of this type of spark gap generator is fairly
simple. You should be able to get everything for under $50 new, used
items may be even less expensive. You will need the following items:
- high voltage source
- batteries
- small bit of wood to mount things on
- small motor
- a few nails or screws
- wire
The high voltage source could be an automotive ignition coil. This
takes the battery or alternator output and makes the voltage much higher
so that it will jump across the electrodes of a spark plug igniting the
fuel. A spark plug is a spark gap, although it is not suitable in its
current form as a radio jammer, it could be made into one.
The battery should be suitable to run your high voltage source and
the small motor. A motorcycle battery is smaller but will go dead faster
than a larger car battery.
The wood should be large enough to let you mount the ignition coil,
motor and screws on. This can be any scrap wood or similar
non-conductive material that you can find. It should be rigid enough
that the components will not just fall apart although it does not have
to be that strong. It’s purpose is just to hold everything together. A
block of cheese or dried mud would work if that is all you have
available.
The small motor can be anything from a vibrator out of an old pager
or mobile phone, an electric fan or one you may find at an electronics
or hobby store. It just has to have a shaft that allows mounting of a
cross member onto.
The first step will be to assemble the motor so that it will function
the way you want. When mounting the cross member you should make sure
that it is insulated from the rest of the motor either by a plastic
shaft or a plastic mounting block. The rod can be a nail although you
will want it equidistant from the center.
You can see how the metal rod goes all the way through the plastic
insulating block. You may install either one rod all the way through or
two rods forming an X. If you install two rods you will have twice the
sparks and thus the transmission cycle will be twice as great.
Once the insulating block is mounted on the motor you should affix
two other nails or screws on either side so that they almost touch the
cross members you installed in the previous step. You do not want them
touching but you do want them as close as possible. The closer the
better. You should connect the motor to your battery and ensure that it
can spin correctly with no problems and that it does not vibrate itself
off of the wood.
Now prepare your antenna. This is a length of wire cut to
1/4 wavelength of
the desired target frequency. If you do not know the target frequency
you can just make some guesses. You can also connect multiple antennas
together to cover various frequencies. Remember you will have RF emitted
on all frequencies it will just be more powerful on the frequencies
that the antenna is resonant for. Mobile phones are about 0.5 inch
antennas and older police radios are 2.86 inches. For a good spectrum
coverage if you make antennas of 0.5, 1.0, 1.5, 2.0, 2.5 and 3.0 inches
most of the usable spectrum will be covered.
Connect the antenna(s) that you made to one of the stationary side
rods. To the other end you will need to connect a ground. This can be to
a metal water pipe in your house or to a rod driven into the ground. If
you do not have a ground the radio will still transmit although it will
be at a slightly lower power.
Now connect the output of the ignition coil to the stationary rods on
either side of the motor. Connect the negative terminal to the ground
side and the positive terminal to the side with the antenna(s). The idea
here is that when the motor spins current will pass from one rod to the
other and out the antenna.
The motor will switch the output of the coil
on and off in rapid succession.
The last and final step is to connect the battery to the motor and
the coil. You may optionally install a switch to turn the device on or
off or you may just disconnect the battery.
Remember building this is not illegal (in most places) but using it
is. You will be causing harmful interference and probably upsetting
people.
http://radiohax.wikispaces.com/Spark+gap+transmitter
http://www.fromthetrenchesworldreport.com/how-to-build-a-radiodrone-jammer/34089/