Tuesday, August 31, 2010

Why I'm Not (Very) Afraid of EMP


When I started this blog six months ago, one of the things I pondered was what I could contribute to the world that other bloggers don't.  As I've spread out, reading more and interacting more, I think I can add some useful information about topics I study on my own, work with, or do as a hobby. 

Over the course of the next few posts, I'm going to look at EMP from a specialist's perspective.  EMP is a term that brings a lot of fear with it.  While I don't specifically work on protection of electronics systems from EMP, I do work with lightning protection and EMI (electromagnetic interference) protection, both of which are very similar.  I have the tools and background to do the math, and think I can improve the signal to noise ratio of the conversation.  I'm going to start with the optimistic side: why I'm not afraid of EMP, and then talk about the concerns I do have about it. So let's start at the beginning. 

Electromagnetic Pulse.  It's widely talked about whenever people talk about potential catastrophes.  There is at least one "open source" book ("Lights Out", currently offline as the author shops it for sale to movie studios, but you can probably find a copy around) and a commercial book ("One Second After") about the aftermath of an EMP, typically depicting the destruction of society - The End Of The World As We Know It. (note: again - I don't make anything off links to Amazon.  It's here simply as a convenience)

To begin with, what is an Electromagnetic Pulse?  Most people who are familiar with the term know it as a side effect of a nuclear detonation in the ionosphere, and that's a good way to generate one.  The ionosphere is a layer of the atmosphere, from about 50 miles to 600 miles or so up, where gas atoms are turned into ions by the UV rays from the sun.  The Federation of American Scientists offers this explanation (from Wikipedia).
A high-altitude nuclear detonation produces an immediate flux of gamma rays from the nuclear reactions within the device. These photons in turn produce high energy free electrons by Compton scattering at altitudes between (roughly) 20 and 40 km. These electrons are then trapped in the Earth's magnetic field, giving rise to an oscillating electric current. This current is asymmetric in general and gives rise to a rapidly rising radiated electromagnetic field called an electromagnetic pulse (EMP). Because the electrons are trapped essentially simultaneously, a very large electromagnetic source radiates coherently.

The pulse can easily span continent-sized areas, and this radiation can affect systems on land, sea, and air. The first recorded EMP incident accompanied a high-altitude nuclear test over the South Pacific and resulted in power system failures as far away as Hawaii. A large device detonated at 400-500 km (250 to 312 miles) over Kansas would affect all of the continental U.S. The signal from such an event extends to the visual horizon as seen from the burst point.

The effect seen at ground level is a very brief, very high voltage pulse.  The very short duration spreads the energy out in frequency, (the universe is just put together that way).  The electric field (E-field) produced is felt across the entire portion of the continent "seen" by the explosion, although the intensity does fall off with distance, as with any source of E-fields.  That is, the areas farthest from the epicenter will get a lower voltage peak than the people directly under the blast. 

How much voltage are we talking about?  Standard models talk about a voltage pulse that reaches 50,000 volts in five nanoseconds (five billionths of a second).  For comparison, lightning may reach similar voltages, but takes much longer.  A lightning strike's rise time is often more than 1,000 times longer, over a microsecond.  While lightning strikes are not all the same, a lightning strike that is less than 100 nanoseconds is quite unusual.  Here's a mathematical simulation of what the pulse looks like.  The time axis is in nanosecond s, so "1" is 1 nsec.  Note that the time display is in log scale, so the pulse is really short; it's down to 1% of its peak value in just over 100 nanoseconds.  If any damage is going to happen, it's going to happen in the first 50 nanoseconds.


One of the main things the EMP stories focus on is how everything electronic is blown out.  Airplanes fall out of the sky.  No communications at all.  They say to store your radios wrapped in foil in a metal ammo can. 

I don't believe that. 

SurvivalBlog has recently run reports of several tests where many vehicles were exposed to a simulated EMP (generated in other ways, so they affect much less area).  Almost all of the cars kept working or simply needed to be restarted.  These were modern cars, not old cars without electronic ignition systems.  Amateur radio magazines have carried articles where tests were done, and results calculated for typical amateur radios.  Most typical installations were not damaged.  Radios did not need to be in a sealed metal box.  I am familiar with the ways commercial aviation electronics is tested, and I don't believe that failures would be widespread.  Airplanes will not come crashing down.   

Here's why radios won't be burned out:


This is a plot of the electrical field created by the 50,000 Volt EM Pulse pictured above, derived by taking the FFT of that pulse. E-fields are usually referred to in units of Volts per meter.  This plot adds the bandwidth of the measurement, so Volts per meter per Hertz.  The fields are small: less than .002 Volts/meter.  For comparison, commercial aircraft electronics is typically tested to withstand fields of 20 V/m, often 200 V/m.  Military systems are tested to higher levels. 

And it gets better.  If you have a narrow band antenna - and most are - the power received is just not that great.  For example, pretend you have an amateur transceiver for the 20 meter band, where your group has a meeting nightly.  You have a dipole antenna for that band.  The power you gather is less than 1/1000 watt (-2 dBm for those curious).  There is no way this power level would be dangerous to your radio.  To your "2 meter" radio (146 MHz), the signal is quite a bit weaker (-16 dBm - less than 1/10 the power of the lower frequency).  In both cases, your radio might well be exposed to signals of this strength without EMP.   

The local broadcasters: TV, AM, and FM, all use narrowband antennas.  The air traffic control systems and navigation systems at the airport use narrowband antennas.  In the case of the broadcasters, their transmitters are putting out thousands of watts or tens of thousands of watts - or more.  It would be an amazing feat to even find the incoming signal from an EMP in the presence of that sort of transmitter power.  Transmitters won't be damaged.

Now if you have a broadband antenna and broadband receiver, the levels will get worse because of the broadband energy the pulse causes.  That's where the bandwidth really becomes apparent; the broadband E-field voltages increase by 10 times the log of the bandwidth.  A 14-30 MHz log periodic dipole array antenna will make the fields stronger by 72 dB, or voltages around 4000 times higher.  But the average E-field values in this frequency band are around .0006 V so multiplying by 4000 still only gives you 2.4 V/m/Hz. 

Calculating how strong the levels would be in the radio is a bit more complicated, because the architecture of the radio matters.  The very first circuits the EMP hits will see higher powers, but they are designed to limit the bandwidth (reflecting most of that pulse back out of the radio) and you should still not have a problem.  The signal on channel is still the same; the power seen at the input filter is higher.  If you integrate up all the power the antenna will bring into the radio, it gets up to a full Watt.  Again, the protection in the front of the radio will keep it from being damaged.   

I don't want to play climate scientist here, and just believe my computer model, but it's agreeing with other things I've seen published.  Always compare computer models to experiment!  If that model pulse is too weak, it still lets us estimate performance.  If the voltage pulse is 100 kV instead of 50, we double the voltages we derived. 

I don't believe that EMP is terribly hard to handle in a radio.  If your radio is VHF, for example, a simple low frequency blocking capacitor might be all you need.  A UHF or microwave radio will be completely unaffected - there's no EMP energy out there.  It's a mistake to think that you use the steady state ratings (that is, your capacitor is rated at 100 VDC) when handling a surge like this.  It's better to look at total energy in the pulse, and since it's so short, there's less energy than in a lightning pulse.  

In short, I think in the aftermath of an EMP, your radio is going to work, the broadcast AM/FM/TV stations are going to work, air traffic control is going to work, the radios on the planes are going to work, your car is going to work, and most electronics will work.  That doesn't mean nothing bad will happen.  

In my next post I want to talk about the things about EMP that do bother me. 

(This work was derived from some industry applications notes, largely from Ansoft, a leading manufacturer of EM analysis software). 

3 comments:

BS Footprint said...

Lovely! Thanks for that. (You might want to add links to the other parts, if you have posted them.)

Anonymous said...

What about powerline transmitted EMP?

Graybeard said...

Anon -9/1/11 - In the second part of this series, I talk about this. The short answer is that the biggest risk I see is powerline and phone line conducted EMP. Some areas of the power grid will undoubtedly be damaged, and because these giant components are not just in a warehouse somewhere, large portions of the effected area will be without power for a long time.

If you haven't found the rest, read the second part.