After listening to people on the forum talk about EMP destroying this EMP destroying that, to the point that It makes me want to hurl.
I have decided to draw up the plans for a home made EMP bound wave test cell that you can make with readily available parts. So you can see for
yourself if item X will be destroyed by an EMP.
It will be made with aluminum foil that you can buy in a grocery store.
There is a simple version using one spark gap, and an alternate two stage spark gap. You can terminate the test cell in three ways. Each will produce
a different response.
General rule of thumb is a NEMP will generate a pulse with a rise time of 2 to 10 ns. A air spark gap will reliably produce a rise time on the order
On a two stage spark gap, most research people use a SF6 gap for the second gap. That allows them to get sub uS rise times. That allows them to
generate an “ultra wideband EMP” We don’t need sub uS times. Normal air will work for that application. It will get us close to, or beyond the
2us rise time of a normal EMP in the peaking gap.
You can build the unit with one spark gap. A two stage spark gap. And/or A capacitor driven gap.
You can terminate it with a an open termination, a shorted termination or a non reflective termination.
Length of test cell could be as long as you want it (a whole roll of aluminum foil for that mater), longer the better, but it should be three or four
times as large as the largest dimension of the item you are testing.
It should be built on a metal ground plane. If you don’t have one (wood floor) then take two or three sheets of Al foil and lay them side by side
with the edges overlapping an inch to make up the ground plane. The total length should be just a bit longer than the length of the top part. If you
are using 12 inch AL foil for the top part, then you could use a single strip of 18inch wide foil for the ground plane.
The length of the transmission line section that drives the test cell could be as short as you want if you are using a single spark gap. You could
have the gap right on the end of the tapered section. If you are using a two stage gap, you will want a couple feet between the primary gap and the
AL foil should be the heaviest that is normally available at the local store.
To avoid impedance bumps you should keep the height/with ratio the same between sections with a tapered section between them. 12 inch high/12 inch
wide driven by 1 inch wide/1 inch high. If you use 18 inch wide AL foil for the top of the test cell, then use put it 18 inches high.
Use a piece of cardboard for the AL foil backing to make it ridged. Suspend the top with fishing line or use PVC pipe for supports.
With the open termination, The entire unit forms the capacitor that stores the charge. The entire top piece is charged to threat level then the EMP is
created by it being shorted to ground through the spark gap. That forms almost a square wave style EMP. Something that is farm more damaging than a
With a capacitor driven unit the top piece is at ground voltage and the capacitor formed by the glass plate that is charged to threat level and
discharged into the test cell through the spark gap. The glass plate driven gap produces a peak with a constant decay that is more typical of a
The reason why the two state gap increases rise time. When a gap breaks down, the volt per meter gradient will affect how fast it breaks down. When
you slowly increase the voltage to the point that the gap just breaks down, it will produce around a 10us rise time. If you quickly increase the
voltage to a point that is far higher than breakdown, then the gap will ionize far quicker, which will produce a far quicker rise time. On a two stage
gap, the first gap is spaced where it will break down at 12kv. The second gap is spaced where it will break down at a low lower voltage (smaller gap).
All the voltage in the system is across gap 1. When gap 1 breaks down, it will create a pulse that has a rise time of 10ns. That pulse travels down
the line until it hits the break in the line (formed by gap 2). The traveling wave reflects off the dead end causing the voltage across the gap to go
to almost 2X the system voltage in only 10uS. considering that the potential across the gap is probably 10X breakdown, then the second gap quickly
avalanches producing a 1 to 2 uS pulse. Which travels to the test cell. The air core inductor across the second gap keeps the pre fire voltage to
zero and it act like an open circuit to the high frequency component when the unit fires, thus not affecting the firing of SG2.
With an open termination, the wave will bounce back from the end of the cell and will form a constructive wave with the one heading from the spark gap
end. Total peak voltage will be double the threat voltage.
A closed termination will reflect a wave that is destructive to the primary wave.
A non reflective termination won’t reflect anything (duh……..)
But trying to build a termination for a strip line is more than I want to get into here, so I won’t.
The simplest way is a one stage gap with an open termination.
The next simplest is a two stage gap with open termination.
Then a capacitor driven single stage gap with open or closed termination.
Then a capacitor driven two stage gap with open or closed termination.
The hardest is a capacitor driven gap with a non reflective termination.
The two stage gap with open termination is the most destructive. The pulse bounces back and forth along the test cell until it is dissipated. That
means the item under test is hit with a EMP several times, each time the unit fires.
The capacitor driven gap with non reflective termination is the least destructive.
If your unit survives a hit in a two stage open termination test cell, then a NEMP won’t touch it.
It almost doesn’t need any current to charge the unit. Just enough to charge a low value HV capacitor. Power supply can be one of those little low
current HV supplies (flyback ecec). Or it can be a Vandergraph generator if you used a good enough insulating material, And it is the safest.
Heck, if your test unit is small enough, you should be able to get it to firing voltage by driving it with an ignition coil.
Your charge voltage depends on how tall your test cell is. If it is 12 inches tall, set the spark gap to go at 12 KV. If it is 18 inches, set it to go
at 18kv. That should get you close to 50kvm.
If you really want to get destructive. Use a neon sign transformer that has an output of 15kv and hook it up to the unit via a capacitor and inductor.
The unit will fire 120 times a second (equivalent to 120 high altitude nuclear detonations a second). If you can pass the item in question through the
test cell and not damage it, then the unit isn’t going to be damaged, short of a direct nuclear strike.
The best size for a glass plate capacitor could be calculated for the strip line in question, but I won’t waste the time right now.
If you want to build a taller cell, then you have to build a higher voltage power supply that will deliver a lot more energy.
Basics of the simplest design. The test cell acts as the energy storage capacitor. The top conductor is one side of the capacitor. The bottom
conductor(ground plane) is the other side. You charge it up to threat voltage. Gradient of 50kv meter. When the gap fires, it shorts the end to
ground. That causes a 50kv inverse spike to travel down the test cell.
And a word of caution, the voltages we are talking about can kill depending on the supply you use, so be careful.
And another word of caution. If you build the un-terminated cell, be careful of where the open end is pointed. It will act like a horn antenna and
possibly damage something down rage of the test cell.
Basic test cell layout
SG1 layout with a capacitor drive(made up of a glass plate).
SG 2 peaking gap
edit on 1-1-2011 by Mr Tranny because: (no reason