So a whole lot of wailing and gnashing of teeth has occurred lately over in prepper land about EMP. I attribute this mainly due to survivalist authors looking for some easy national/global catastrophe scenario to hook into without really having much if any understanding of it. I won’t criticize specific books as some are more accurate than others, but the net result is there is a TON of disinformation out there on EMP IMO.

I am not an expert, but I am literate and do have a few letters after my name and I am an amateur radio operator with a basic understanding of physics and electronics. There are likely people on this very forum that know a shitload more about this than I do, but since no one has stepped forward to do this I’ll take a stab at it, and if those knowledgeable individuals want to correct anything I post along the way please do so. My main source of information is the following document prepared at ORNL which I will reference, since they are the experts, not me.

Big long detailed report by actual experts

So to start with:

We are talking about a very specific phenomenon known as E1 HEMP which is generated in the first few nano-seconds of a nuclear blast. This will generate a large electromagnetic field over a wide frequency range, but the worst from about 1mhz or less to about 1ghz where it really starts to drop off (Page 2-7). It is also very similar to a what happens during a lightning strike (which is actually more powerful) so a lot of measures that help with lightning strikes can also help with E1 HEMP.



One thing to clear up E1 HEMP is mostly yield independent. You don’t need a huge nuclear device to generate it, the examples in the book are 500kt (2-11).

What determines the coverage of an EMP is its height above ground, obviously the higher it is the more it affects. However the higher it is the weaker it is. The “optimal” height above ground is typically less than 100km, in their example 75km is optimal. Total coverage at that height is about 800-1000km (2-14). So it’s a fairly large are, but not the whole US by any stretch of the imagination. Also, what isn’t covered by any of the models used in the study are local terrain effects, like shadowing or reflection. These will be important locally however. So if you are in a valley and “can’t” see the nuke at 75km high, you will have some protection.

The next thing to get an idea of is how “strong” the E1 HEMP is. As with any sort of electromagnetic phenomenon it follows the inverse square law. So that means it gets weaker the further you are away from it very quickly. For comparison they use an EM field generated by a FM radio station for comparison. At 1 mile from 10KW transmitter, the peak field is .68V/m, a peak field at ground zero (directly below the nuke) will be 50,000 kV/m (its a lot), however since this falls off very rapidly with distance, average exposure will be 6.21 Kv/m (still a lot, but its not 50,000 Kv). The example states that a 10cm conductor at ground zero (50,000kV) will have an induced current of 5kV. So using the 6.21 kV average field that same conductor will have an induced charge of 621 Volts (2-35).  
You can see this effect on their charts 2-29. If you have the nuke over Omaha, it F’s up Omaha, but the effects near Chicago, Denver, or Dallas are fairly minimal only .5% of the energy in the 10-100mhz range makes it out that far.



I mentioned frequencies earlier and that is both significant and important. The highest energy range is 1-10mhz. So as HAMS you should understand that means it covers the 160-30m bands the most and then drops off as it approaches UHF frequencies. It’s the wavelength that’s the concern. So most of the energy in the E1 HEMP will want to couple to things of a similar wavelength just like a radio antenna.  So its typically things like cabling attached to devices (antennas, power cables etc.) that is the major concern (2-35) So unplugging your radio from the antenna is a very good idea if you are worried about this (2-38). If the device is powered chances of damage are higher (2-36), there are also other post nuclear blast effects, TREE effects (radiation) that can also damage electronics but these aren’t E1 HEMP.
When it comes to shielding against EMP, there are a lot of things discussed, but how E1 HEMP couples to things is very important and more importantly difficult to predict (2-37).

So things that EMP will couple very well to are going to be metallic things that are 30 meters long (10mhz) and longer. So power lines, telephone lines, railroad tracks and the like. Most of the report is focused on what exactly is going to happen to the electrical grid and telecommunications, because that’s where the largest vulnerability is (2-45). And ironically enough high voltage power lines aren’t actually super problematic since they are designed to withstand lightning strikes, and EMP coupling doesn’t really scale past 1km or so.  

Peak current on a power line.




Peak currents on a 5M long vertical antenna (below)



EMP and HAMs
What about the stuff you as HAM care about, a radio for example… First of all many devices, and especially radios are shielded against EMI already (2-44), which isn’t exactly the same as E1 HEMP, but it can help, there are often RF shielded modules in most radios.  As I mentioned earlier, the best thing to do is not have the radio connected to the antenna when not in use, and I would argue have the antenna feedline grounded. In fact there is a huge amount of similarity between E1 HEMP and lightning strikes, so all that stuff the AARL tells you to do when constructing a shack is pretty useful when it comes to E1 HEMP.
There is a lot written about faraday cages Wiki, but I think this must be placed in the context of the frequency range you are talking about (look, they have big ass holes sometimes). The principle of waveguide cutoffs is a good one to consider (2-42), your faraday cage, and lets use the example of a metal trashcan, is probably a good and cheap EMP defense, first off its much smaller than 30m, so that big pulse isn’t going to couple to it too well, but some of the energy will be at the 1m wavelength, but that’s pretty far down the slope of the energy curve so there will be some coupling. Well what about holes or small gaps in the trashcan, yes it helps not to have them, but looking at the frequencies that will get through a 1mm gap is a 300GHZ, well above the energy generated by E1 HEMP which mostly drops to zero by 1GHZ or 300mm or about 11 inches. Also the trashcan is made of relatively (thick) metal which it needs to not be permeable i.e. skin depth (2-40) and why I would seriously doubt that ESD bags marketed for the purpose would actually be effective (no skin depth).

Myths
Finally there is a “myths” section in the very last appendix that will likely be helpful.
Cars dying: small fraction is possible (turns out your car will act as faraday cage if its made of metal) and cars are EMI shielded
Wristwatch dying: Too small to be affected by E1 HEMP.
Electronic devices that are turned off will likely fare better (true)
Conductors. Shorter is better since it will couple less with the EMP.

Conclusions
So, is EMP a viable existential threat? And the answer I think, is that its really hard to say. I think the idea of a 1 bomb EMP taking out the US is pretty non-sensical. However a 1 bomb scenario could still do significant damage to localized areas i.e. a large metro area, or for example the northeast united states. And as is mentioned in the report, EMP is largely yield independent, so even a fairly reasonable 500kt weapon could do a lot of damage. A bomb detonated above say NYC would likely take out Boston, NJ, DC etc.  and the associated infrastructure. That being said, a coordinated strike involving several warheads detonated roughly around the same time could in fact take out most of the US power grid.  As mentioned in the report the primary vulnerability is the power grid and telecom systems, since of course antennas will be connected to radios when the bombs go off.  Smaller electronic devices are primarily vulnerable if they are plugged in at the time since the power cords can couple to the EMP and produce high voltage. Protection against EMP is a faraday cage, and probably the cheapest and best option IMO would be the famous metal trashcan example. Alternately if you need something smaller an Ammo Can would probably work just as well.

Comments/Questions?

Excellent post.

Bravo

Do you have links to where we can get the best trash cans . . . the ones with built in grounding lugs?


JUST KIDDING!!!!

Excellent post, even for those of us with only  few letters behind our names.

// John, CC&BW

Interesting writeup.  I will assume everything is factual, particularly since the base document you linked comes from (or was prepared for) Oak Ridge National Labs.  I think they probably do know a little about EMP.

I am curious about how the rail system will be impacted considering most of the engines are diesel/electric and sit directly on those long conductors you mentioned.

I would think the rail roads would be grounded pretty good.

It's mentioned on page 2-19. And implies large currents can flow through them.

What we need in this thread are EEs and a EMP physicist. I'm neither unfortunately. But I did take a day to read and do my best to understand the article. That and I have stayed at the holiday inn...

 I think in this forum the Holiday Inn stay only counts if wire antennas were deployed from the window.

Thanks for the write up. I've never worried about the emp issue. If I can prepare for more likely scenarios I should be somewhat covered for a long term power outage and the associated issues surrounding it.

I am an EE (and a whole lot more ). That by itself isn't enough. The best data on this is certainly all classified. However there are two significant data points that one can use as a rough guide to how little a problem EMP really is in this modern age of electronics:

1. There is open source data that shows that automotive systems are generally unaffected by even fairly substantial EMP fields. This includes relatively modern engine control units. Sometimes the automotive systems needed to be reset, rarely was anything actually broken.

2. Modern military electronic systems are not built with any special anti EMP characteristics anymore. Not for a long time, in fact. And there is extensive use of COTS (commercial off the shelf) products in the military, even (especially) on ships and aircraft.

So go buy your garbage cans and be happy!

Yeah, thats mostly what this report has said on point 1. And personally I think an unplugged radio (i.e. no power or antenna to it) would likely survive unshielded.

I have seen some old data when they were testing PRC-77's and with the antenna unplugged (IIRC) they were GTG (declassified canuckistan data). I also own radios that I know were specifically EMP hardened and tested so I figure I'm ok on the radio - EMP front.

Can you comment on solar panels and EMP? I've been wondering about that, and I mean the cells themselves not the associated transmission lines or downstream circuits.

A-effing-men!

I stopped commenting on EMP threads on the survival forum because hitting my head against a brick wall
is more fun than dealing with armchair experts that have one or two books that say "it's bad,, mmmay?"

Another big clue is that white sands rents their EMP simulator out to the car companies and puts out
press releases about it. If it killed cars easily, the car companies wouldn't discuss it, and if it were so
secret, white sands wouldn't be boasting about renting the time.

I used to worry about EMPs and HEMPS, but after reading for myself and listening to people who actually know what they are talking about I agree with what you

have stated. Safe manufacturers started putting out "EMP proof locks" and locks with digital and mechanical mechanisms to "safeguard from an EMP", which I find

to be a marketing gimmick. I see a certain hysteria about EMP's and HEMP's on survival forums and from people I know and I think it's due to all the movies and books

that are out there. I do agree that even though it wouldn't be as bad as the movies and books paint it to be, losing power to a major city or area ie east coast would wreck havoc

with all those people around that can't take care of themselves. Thanks for the write up, it was a good read.

Nope, don't have a clue about solar panels and EMP, except to say that if they are small ones I wouldn't be worried.

I'd love to get one of those rooftop arrays here, but every ham I know that has one or is near one has said that they are horrible RFI generators. I suspect that is because they are almost all now based on distributed microinverter technology.

I was about to post aww not this shit again  


thanks for a great post - now to find time to go through the source doc and read for myself.

Basically, you don't need to put your radios in a Faraday cage, or metal trash can, or ammo can.  Just unplug
the antenna and power cords when not in use... which I do anyway for lightning protection.

I too use to think about stuff like this then came to the realization that it doesn't matter. If I'm  truly close enough to be impacted I'm  probably dead anyways.

Practice good habits like deenergizing devices when there is a risk and that is about as good as you can do.

Pray for the best, prep for the worst but don't let the worst run your life.

Well, it sort of depends. EMP won't kill you outright... But yeah its pretty much at the bottom of my list of things of things to worry about. If anyone asks I tell them to go buy the trashcan (plus it doubles as an AK storage case ).

 
It will charge your entire battery bank in like 10 milliseconds...

Is that the EMP or the extra sun in the sky?

E1 HEMP hates Omaha.

I need to kick my ak's out of the trash can and get a backup radio and antenna.

Well now this is a good point...since batteries will protect your radio from lightning I am sure adding one on the antenna feed will also protect it from EMP.

Brilliant!

what happened to that guy??!?!?!?

ar-jedi

good summary.  a couple of notes.



compared to even the 1990's, today's telecom networks are far, far, far more robust against (N)EMP due to one simple reason: except for the last mile, fiber has almost completely supplanted copper.  gone are the days when microwave repeaters were connected via DS3's and channel banks had dozens of DS1's hung off them.  nowadays, a span of optical fiber carries 10 terabits/sec without any issues due to environmental interference.  



https://www.ar15.com/forums/t_1_5/1601048__ARCHIVED_THREAD____Fall_River_Man_Arrested_For_Videotaping_Police_Officer__Video_erased_in_police_possession.html&page=2#i46130941



a vehicle with huge glass areas and door/hood seams and such can in no way be characterized as a faraday cage.  the car only "dying temporarily" is not a result of effective shielding at the "skin" of the vehicle.  it is the result of a myriad of uncontrollable and non-deterministic factors.  example: rotate the car 90 degrees on the test stand, and you may get a different result.

ar-jedi

You should come check out what we've got going on in our COs in Alaska.  Microwave, copper, and satellite (yes, satellite) are the orders of the day.  We've got some fiber, but nothing like the lower 48 has.  We're geographically almost 100% rural.

ok ok ok, i will start out next time with "in modern, civilized areas, ..."  

ps
i was just reading a paper the other day where some arctic/antarctic scientists/physicists were complaining about the scarcity of surplus 1970's era RTG's.  
it turns out that if you need 50W for a long, long time (like dozens of years) at a remote weather/seismic monitoring site, a Sentinel 25F or Sentinel 100F RTG is a really, really good solution.
https://en.wikipedia.org/wiki/Radioisotope_thermoelectric_generator#Terrestrial

ar-jedi

Thanks Jedi,

I guess the main point about the Faraday cages that I took away, is that the bulk of the energy in the E1 HEMP is in the much lower frequency part of the spectrum, so lets say a window is 2 feet tall, only a small amount of the total energy is at frequencies that aren't being shielded by that (unless I'm totally misunderstanding Faraday cages).  And a huge part of that article talks about how difficult it is to predict what dies due to the huge number of variables involved when it comes to how an EMP field will couple to various electrical components and induce currents and voltages. Obviously more modern "plastic paneled" cars are much less shielded than stuff from 90's. But the actual computers are typically EMI shielded anyway. The authors specifically stated cars probably wouldn't be a big problem.

Any comment about solar panels since you know about this stuff?

Do you mean I shouldn't keep my spare car battery and a starter in the expensive grounded model trashcan ? Should I at least ground it? I keep the trash can on the second floor. I was told to keep grounding wire as short as possible. So I figured bringing ground closer was easier than running a wire all the way to the ground.  I placed a 55 gallon drum full of good, rich dirt (ground) next to the trash can and put two 4 ft copper rods into it.

I just laughed so hard here that my coworkers came over to check on me. lol. That's awesome, I'm going to have to reuse that line.

The only issue I see is that you didn't connect the battery in series from the equipment to the ground.

I hate to be that guy, but...

Would wrapping a radio in aluminum foil qualify as a Faraday cage?


Would a gun safe qualify (assuming no attempt to ensure electrical connection between the body of the safe and the door)?

Generally speaking,  yes, as long as there is an insulating layer between the foil and the radio. The foil should not have any gaps.
Most safes have large gaps around the door. An effective Faraday Cage should provide at least 50 db. attenuation at around 400 - 600 Mhz, for man made EMP. Your safe will probably provide around 15 db attenuation unless you tape the gaps around the door with an aluminum duct tape. Also many safes have small hole openings for electrical wires. You'll need to cover those with an aluminum tape as well. Remove any electrical wires passing through the safe walls (for heaters, etc).
Same rule applies for trash cans or any metalic boxes. A tight lid is not enough for proper shielding at higher frequencies. Even a tiny gap will act as an antenna thus passing RF into the enclosure.
BTW, I'm  an EE and have done some research on this subject.

Long ago I posted a bunch of info from LANL's website in one of the numerous EMP threads. I can't find the post or I'd link it.

The field intensity of the high altitude NEMP was roughly the same as an average lightning strike 3/8 mile away. If you're been through a thunderstorm, your radio, car, aimpoints, etc will be just fine.

The problem is that intensity is there over a tremendous area, so that very long conductors can accumulate more energy.

tag

Nothing wrong with that.

If it's stupid, but it works, it's not stupid.

that's ... innovative.

should have used a steel drum though...  

ar-jedi

Wow...