GBPPR Tech Bulletin #9 - Nuclear Electromagnetic Pulse (NEMP) Survival

This was originally going to be a big long article on some myths/facts of nuclear electromagnetic pulse (NEMP) effects on modern communication gear.  Well, it's just going to be some ramdom quotes and comments for awhile.

Excerpt from Gary, KE4ZV

>On the contrary, an EMP will not really affect people, but it will do
>a good job of destroying most unprotected transistor circuitry. Old
>tube technology is pretty much immune.

That's the myth, but it has been pretty well discredited.  For a normal low air burst or ground burst detonation, the EMP effects reach no further than the flash burn and blast effects, IE roughly 30 miles for the typical "city buster" bomb.  In other words, if it cooks your radio, it'll cook you too.

For a very high altitude detonation, EMP effects can spread over a wide area outside the range of flash burn and blast effects.  But to be damaged, electronic equipment needs to be connected to fairly long unprotected exposed conductors in order for enough voltage to be induced to cause breakdown.  The ARRL published tests on a number of amateur radios, tested in a military EMP simulator, to see what would typically happen in such a case.  Battery powered VHF/UHF equipment using a "rubber ducky" antenna was immune to damage.  Equipment connected to unprotected mains power suffered power supply damage.  And HF equipment connected to unprotected outside antennas suffered receiver front end damage.  For solid state equipment, this damage was to the first RF stage transistor, for tube equipment, it was damage to the first stage grid resistor.  No transmitters suffered damage.

And there is effective EMP protection available.  EMP can be considered merely as a fast risetime form of nearby lightning strike.  In both cases, an induced surge enters the equipment via a long exposed conductor.  If you are using good lightning surge protection with fast risetime protectors, such as the EMP rated units sold by Polyphaser (and others), and have practiced the kind of station layout I've preached about here many times, there should be no more cause for concern about EMP damage than from lightning surge damage.

Thanks to inverse square effects, an EMP detonation at a 200 mile altitude, the ideal height to maximize EMP effects, has a field strength at your antenna about equivalent to a lightning strike at 7 miles.  But the effect on very long exposed conductors, like the power grid or the telephone grid, is as if there were simultaneous lightning strikes 7 miles from *every* point of the grid.  This causes a huge voltage to develop on these extended grids, and can cause severe damage to them.  Your radio and antennas don't represent such a widespread grid, so they are only subjected to the equivalent of a single lightning surge.

In any case, if you were to suffer EMP damage, the receiver and mains power supplies would be the most likely candidates for damage.  CW transmitters offer you no advantage in either of those cases since you still need a working receiver and power source to establish communications regardless of operating mode.

And frankly, for some time (perhaps years) after a massive exchange of nuclear weapons, HF would be useless thanks to disruption of the ionosphere.  Only VLF would work for long range communications, which is why the military operates VLF stations for nuclear command and control.  VHF+ would continue to work for short range communications.  Since neither typically uses Morse, Morse knowledge would be virtually useless after a nuclear exchange.

Excerpt from Phil, KA9Q

No, not HF CW.  Try LF packet radio.  Have you heard of the Ground Wave Emergency Network (GWEN)?  It is specifically designed to work after the Big One is dropped.

You have to admit that for this application, packet has some nice advantages over CW.  For example, it'll keep working after all the humans on the planet (yes, this includes all of the CW operators) have been killed off by radiation.  (Remember "On the Beach"?)

I may not think much of the doomsday thinking that led to the construction of GWEN.  But I must admit to being glad to have it around because I can use it to annoy the heck out of the "CW, vacuum tubes, blood and guts forever" types who keep insisting that they'll have a complete monopoly on communications after a nuclear war has toasted all of the more modern electronics... :-)

Excerpt from Gary, KE4ZV

According to the ARRL test data in the Navy's NEMP simulator, that's not the case.  The real hazard is to radios connected to long exposed unprotected conductors, IE mains power or HF antennas.  In the tests, tube type HF rigs sustained damage to the front end coil assemblies.  Battery powered VHF solid state radios sustained no damage.  HF radios, solid state or tube, protected by suitable NEMP suppression (properly installed) suffered no damage.  And no disconnected radio suffered any damage.

The high impedance of tube equipment tends to make it more susceptable to flashovers in the input circuitry.  Since that's harder to change than a tube, the advantage of tubes is moot.  NEMP protection is available today, and with that properly installed, there is no reason to fear NEMP.

Excerpt from QST August 1986, "EMP and the Radio Amateur"

... condensed from NCS TIB 85-10 "EMP threat testing of protection devices for amateur / military affiliate radio systems equipment".  I quote "The electric field strength remains fairly constant in the 10 kHz to 1 MHz band; it decreases by as factor of 100 in the 1 to 100 MHz band and continue to decrease at a faster rate for frequencies greater than 100 MHz" So, it appears that EMP field strength decreases by at least an order of magnitude for each decade of frequency above 1 MHz.  So.  Your quarter inch vent will resonate and allow radio wave to pass above about the 10 GHz range.  10 GHz is 4 decades above 1 MHz so:

The electric field strength transmitted through a quarter inch hole will be less than one ten thousandth the electric field strength in open air.

Excerpt from the Nuclear Weapons FAQ

5.5 Electromagnetic Effects

The high temperatures and energetic radiation produced by nuclear explosions also produce large amounts of ionized (electrically charged) matter which is present immediately after the explosion.  Under the right conditions, intense currents and electromagnetic fields can be produced, generically called EMP (Electromagnetic Pulse), that are felt at long distances.  Living organisms are impervious to these effects, but electrical and electronic equipment can be temporarily or permanently disabled by them.  Ionized gasses can also block short wavelength radio and radar signals (fireball blackout) for extended periods.

The occurrence of EMP is strongly dependent on the altitude of burst.  It can be significant for surface or low altitude bursts (below 4,000 m); it is very significant for high altitude bursts (above 30,000 m); but it is not significant for altitudes between these extremes.  This is because EMP is generated by the asymmetric absorption of instantaneous gamma rays produced by the explosion.  At intermediate altitudes the air absorbs these rays fairly uniformly and does not generate long range electromagnetic disturbances.

The formation EMP begins with the very intense, but very short burst of gamma rays caused by the nuclear reactions in the bomb.  About 0.3% of the bomb's energy is in this pulse, but it lasts for only 10 nanoseconds or so.  These gamma rays collide with electrons in air molecules, and eject the electrons at high energies through a process called Compton scattering.  These energetic electrons in turn knock other electrons loose, and create a cascade effect that produces some 30,000 electrons for every original gamma ray.

In low altitude explosions the electrons, being very light, move much more quickly than the ionized atoms they are removed from and diffuse away from the region where they are formed.  This creates a very strong electric field which peaks in intensity at 10 nanoseconds.  The gamma rays emitted downward however are absorbed by the ground which prevents charge separation from occurring.  This creates a very strong vertical electric current which generates intense electromagnetic emissions over a wide frequency range (up to 100 MHZ) that emanate mostly horizontally.  At the same time, the earth acts as a conductor allowing the electrons to flow back toward the burst point where the positive ions are concentrated.  This produces a strong magnetic field along the ground.  Although only about 3x10^-10 of the total explosion energy is radiated as EMP in a ground burst (10^6 joules for 1 Mt bomb), it is concentrated in a very short pulse.  The charge separation persists for only a few tens of microseconds, making the emission power some 100 gigawatts.  The field strengths for ground bursts are high only in the immediate vicinity of the explosion.  For smaller bombs they aren't very important because they are strong only where the destruction is intense anyway.  With increasing yields, they reach farther from the zone of intense destruction.  With a 1 Mt bomb, they remain significant out to the 2 psi overpressure zone (5 miles).

High altitude explosions produce EMPs that are dramatically more destructive.  About 3x10^-5 of the bomb's total energy goes into EMP in this case, 10^11 joules for a 1 Mt bomb.  EMP is formed in high altitude explosions when the downwardly directed gamma rays encounter denser layers of air below.  A pancake shaped ionization region is formed below the bomb.  The zone can extend all the way to the horizon, to 2500 km for an explosion at an altitude of 500 km.  The ionization zone is up to 80 km thick at the center.  The Earth's magnetic field causes the electrons in this layer to spiral as they travel, creating a powerful downward directed electromagnetic pulse lasting a few microseconds.  A strong vertical electrical field (20-50 KV/m) is also generated between the Earth's surface and the ionized layer, this field lasts for several minutes until the electrons are recaptured by the air.  Although the peak EMP field strengths from high altitude bursts are only 1-10% as intense as the peak ground burst fields, they are nearly constant over the entire Earth's surface under the ionized region.

The effects of these field on electronics is difficult to predict, but can be profound.  Enormous induced electric currents are generated in wires, antennas, and metal objects (like missiles, airplanes, and building frames).  Commercial electrical grids are immense EMP antennas and would be subjected to voltage surges far exceeding those created by lightning, and over vastly greater areas.  Modern VLSI chips are extremely sensitive to voltage surges, and would be burned out by even small leakage currents.  Military equipment is generally designed to be resistant to EMP, but realistic tests are very difficult to perform and EMP protection rests on attention to detail.  Minor changes in design, incorrect maintenance procedures, poorly fitting parts, loose debris, moisture, and ordinary dirt can all cause elaborate EMP protections to be totally circumvented.  It can be expected that a single high yield, high altitude explosion over an industrialized area would cause massive disruption for an indeterminable period, and would cause huge economic damages (all those damaged chips add up).

A separate effect is the ability of the ionized fireball to block radio and radar signals.  Like EMP, this effect becomes important with high altitude bursts.  Fireball blackout can cause radar to be blocked for tens of seconds to minutes over an area tens of kilometers across.  High frequency radio can be disrupted over hundreds to thousands of kilometers for minutes to hours depending on exact conditions.



  1. Atmospheric NEMP burst tend to be vertically polarized.
  2. The Effects of Nuclear War, Office of Technology Assesment, OTA-NS-89, May 1979.
  3. NEMP has a pulse shape corresponding to the NCS TIB-85-10.
  4. Solid state electronics are more power efficient than legacy tube-type equipment, making for easier portable battery operation.


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