Jun 282016
 
An example of a signal strength reading from a $25 SDR receiver and using free software on your PC.  Essential for testing your Faraday cage.

An example of a signal strength reading from a $25 SDR receiver and using free software on your PC. Essential for testing your Faraday cage.

So you’ve built your Faraday cage, and are ready to fill it with the precious electronic spares you want (need!) to protect against an EMP.  Well done.

But there’s one thing you really should do before closing it up and moving on to your next project.  Even if you’ve built a simple small and apparently 100% compliant cage, don’t you think you should check and test its effectiveness?  You’re placing a huge amount of reliance on this cage and its ability to protect its contents from an EMP, so isn’t it worth spending a bit of time and maybe a few dollars to check that it works as expected.

The interesting thing is that a Faraday cage is not an ‘all or nothing’ thing.  It isn’t like putting heavy blackout curtains across your windows and blocking out all the light from outside.  It is more like pulling a medium weight curtain across, and then having a searchlight shine on your window from outside.  Some light will get through, the key issue is how much.

The same with a Faraday cage.  It will reduce the strength of the EMP (the technical term is ‘attenuate’) but it won’t block it out entirely.  So how much attenuation will it provide, how much do we need, and how do we measure it?

How Much Attenuation is Desirable

First, how much attenuation do you need?  More is always better than less, but after a point, you reach a realm of vanishing returns and unnecessary extra protection.  We suggest you should look for 40 dB (dB = decibel) of attenuation, and ideally over 50 dB.

What do these numbers mean?  If you reduce something by 30 dB, you have reduced its power 1,000 times.  If you reduce it by 40 dB, you’ve reduced its power 10,000 times.  If you think to yourself ‘every ten dB adds another zero to the number of times the signal is reduced’ then you’d be correct.

So, after the lesson, the test.  How many times would the strength of an EMP be reduced if it had a 50 dB attenuation?  Please tell me that you answered 100,000 times.  Clearly that’s a lot, isn’t it, and probably it is enough.  Let’s see why we say that.

A strong EMP effect will induce voltages in the order of 50,000 volts/meter (it is difficult for EMPs to exceed this – anything higher than that and the sky sort of ‘short circuits itself’, although there are rumors of some ‘super EMP’ weapons that have found a way to create higher voltages).  If we reduce 50,000 by 40 dB, it is down to a trivial 5 volts/meter, and if we take it down by 50 dB, it is down to a hard to measure 0.5 V/m (ie 500 mV/m).

To put this into context, a strong radio signal ranges between 1 – 100 mV/m.  So after about 56 dB of attenuation, the strongest probable EMP is no more harmful than a strong radio signal – ie, totally utterly harmless.  And we’ve at least 10 dB of overload above that and probably 20 dB before there needs to be significant concern about EMP damage to electronic circuits that are switched off.

Measuring Attenuation – Decibels and Frequencies

There is an interesting complicating factor.  An EMP has a mix of different radio frequencies, and Faraday cages block different frequencies by varying degrees.  In general terms, the higher the EMP’s frequency, the less the attenuation.

So that begs the question – exactly what frequencies are likely to be found in an EMP?  The short answer is ‘all of them’, which isn’t very helpful, is it!  A better answer is ‘most EMPs are expected to concentrate most of their energy in frequencies below 100 MHz’.  To put 100 MHz into a meaningful context, it is right in the middle of the FM dial.

That is actually a bit of welcome good news.  The lower the frequency, the better most Faraday cages work at blocking it.  So if we test the cage at 100 MHz, we know that its blocking will be better at lower frequencies and that the 100 MHz result is getting close to a ‘worst case’ scenario.

There are a number of ways you can do this test.

Test 1 – The Phone Test

This is the easiest of all the tests, and is also the least valuable, but as a quick rule of thumb test, it can tell you if your Faraday cage might fail, although it probably can not tell you if the cage for sure will succeed (testing for failure and testing for success are surprisingly different things).

Take your cage and your cell phone to somewhere with good cell phone coverage.  Ideally, go somewhere where the cell phone is receiving on the approx 850-900 MHz range of frequencies, rather than on the 1900 MHz set of frequencies.  You can do this by using the excellent antennasearch.com website – put in your address, and then download all the different transmitters it finds close to you (chances are you’ll be surprised at how many there are!).  Use the tower map to find a tower belonging to the wireless service you use and which has only 900 MHz not 1900 MHz transmitters on it, then go to it to test.

This is more difficult than it sounds, and probably you’ll compromise by simply testing at home.  That is okay, but to get a helpful result, make sure that your cell phone has four or five bars of signal strength before putting it in the cage.

Close up your cage, and call your phone.  Maybe you’ll be able to hear the phone ringing inside.  If the phone rings, you know your cage is probably not working well.

If the cage material insulates the sound, simply check to see if the phone reports a missed call when you open it up again.  If it shows a missed call, that suggests your cage isn’t fully optimized.

Your cell phone can probably receive signals down to a signal strength of about -115 dBm.  You can probably get your phone to tell you the strength of the signal it is receiving if it is an Android or iPhone – here’s a good article on how to do so, and if you’re interested, this article tells you more.

So if your phone was showing a signal strength of -85 dBm before it went into the cage, and it didn’t ring, you know that the cage is probably providing at least 30 dB or more of attenuation (ie dropping it from -85 to -115 or more) but you don’t really know how much more than the 30 dB it is giving you.  And with only 30 dB of attenuation, that is close to a ‘failure’ so not hearing the phone ring, in this case, isn’t as positive as you’d think.

And if your phone is showing -55 dBm before going into the cage and still rings, you know the cage is providing less than about 60 dB of attenuation, but you don’t know how much less.  At 60 dB of attenuation, having the phone ring can still be considered a ‘success’, although you mightn’t think so by hearing the phone ring

So you need to interpret your phone results with care and caution.  You may be getting either false positive or false negative results; it helps if you know the signal strength the phone was receiving before going into the cage, and what the minimum signal strength is your phone will work on.

Plus, you are ‘torture testing’ your cage by checking its effectiveness at 900 MHz or 1900 MHz.  There is unlikely to be significant energy in an EMP at those sorts of frequencies, and the cage will probably offer better protection at lower frequencies.

So let’s now look at a slightly more helpful test.

Test 2 – Two Walkie Talkies

If you can walk inside your cage, go in with a walkie-talkie and have someone outside with a second one.  Close the cage up and see if you can communicate between the two of you.  If you can, that is suggesting a ‘fail’; if you can’t, that is suggesting a ‘pass’ for your cage.

The walkie talkies ideally should be on the MURS frequencies (about 150 MHz).  If you don’t have MURS capable radios, then FRS or GMRS (about 450 MHz) will work in a pinch, as well.  CB radios would not give as meaningful a result.

If you can’t walk inside your cage, that’s okay too.  In slightly simplistic terms, a cage not only blocks external signals from traveling into the cage, but also blocks internal signals from traveling out.  So all you need to do is use a rubber band to hold down the transmit key on a walkie-talkie, put it into your cage and close it up, and see if you are receiving any signal on the other walkie-talkie outside.

Note that walkie talkies sometimes have a ‘timer’ that stops their transmitting after a period of time.  Check that, when you reopen the cage, the walkie-talkie inside is still transmitting.

This is a good test with fewer ambiguities than the cell phone test.  You can be reasonably certain that if you can’t get a walkie-talkie signal through your cage, then it will block an EMP too.

But if the radios can still communicate with each other, is that necessarily a fail?  It depends on how much the signal strength has dropped.  If one of the radios has an accurate S-meter on it, this is helpful.  In theory, each ‘S unit’ on an S-meter represents a signal strength change of 6 dB, so you would want to see a 7 S unit movement or more between the two signals.  However, particularly, with inexpensive walkie talkies, they either have no S meter at all, or the S meter they do have is woefully inaccurate.

It is not prudent to rely on S meter values, even on quite expensive receivers.

Test 3 – Walkie Talkie and SDR

Good news.  You can actually do a very accurate test of the efficiency of your cage with very inexpensive items.  You want to have a walkie-talkie to place inside your cage, and use a SDR to measure the signal strength, accurately, on the outside.

For the walkie-talkie; if you don’t have a bunch of them already, get a Baofeng UV-5R.  While it is some years since we reviewed these lovely units, and while there have been later models released, the standard UV-5R, at a cost of around $30 each on Amazon, still remains an unbeatable value and excellent performer.  Tune it to a frequency around 145 – 155 MHz, it doesn’t really matter what.  You could tune it down to a lower frequency (they work down to 136 MHz) but best to keep it around the radio’s ‘sweet spot’.

But what is an SDR?  The acronym stands for ‘Software Defined Radio’.  Instead of a traditional radio with knobs and dials and everything, a SDR is a computer device – often in the form of a USB stick that connects into your computer, and which is then controlled by a computer program rather than by old-fashioned ‘analog’ controls.

These are amazing gadgets that can be had for as little as $25 (actually, you can get ones for even less than $25, but this specific $25 SDR is the best performing/best value unit until you start going way over $100).  Download the free SDR+ (pronounced ‘SDR Sharp’) software from here, plug the unit into your computer, and run the SDR+ software.  (If you have a Mac, you could try this free software.)

SDR radios have many uses for preppers, particularly as broad-band multi-mode scanners.  We’ll write more about them in future articles.

Tune to the same frequency as your HT (walkie talkie), make sure that you’ve zeroed out the RF gain and turned off the AGC, and you can get information on exactly the dBm signal strength that is being received by putting the cursor on the top frequency display.  If the signal is too strong – ie, you don’t get a nice single peak on the SDR’s display, but a whole series of peaks and a general lifting of all the frequencies being monitored, take the antenna off the SDR and maybe replace it with just a very short piece of wire.

Then stick the HT into the cage, close it up, and you’ll see exactly the drop in signal strength.  Maybe you go from +5 dBm to -40 dBm, or something else.  In such a case, clearly the cage has provided you with 45 dB of attenuation.  How easy (and accurate) is that!

sdrc

In this screen shot (a larger view of the image at the top of the article), you can see that the display is monitoring a signal at 145.330 MHz, with a signal strength of +2.5 dBFS (forget about the FS, it means ‘relative to full scale’ – a concept which is not relevant to our needs).  That is a very powerful signal, and as you can see, it ‘stands up’ a long way above the random background ‘noise’ (the other data tells us the background noise is 56.2 dB below full-scale, and so in total, there is a separation of 58.7 dB between the background noise and the measured signal) and some other weak signals that you see small peaks for.

After getting your ‘out of cage’ reading, which this is, you then simply put the radio in the cage and get a second ‘in the cage’ reading.  The difference between the two dBFS readings is the amount of attenuation, and hopefully it is reporting more than 40 dB.

Summary

You shouldn’t assume your Faraday cage is giving you the protection you need and are relying on.  Fortunately, there are some relatively easy ways to test its functionality.

Ideally, you should be getting more than 40dB of attenuation from your cage.

Jun 142016
 
A garage sale can seem to be full of tempting bargains. But are they really as great as they seem?

A garage sale can seem to be full of tempting bargains. But are they really as great as they seem?

This is the first part of a two-part article about the wisdom (or lack thereof!) of buying used gear rather than new gear.  Please also see the second part of our article that specifically looks at considerations to do with buying used electronics.

So there you are, browsing through eBay or the local Craigslist; maybe you’re walking through a second-hand store or at a garage sale, but somehow, you find yourself looking at a tempting bargain.  Should you buy it?

Or maybe, instead, you’ve simply decided that the best way to make your dollars go further is to buy as much used gear as possible.  You already know that a used car that has dropped in price down to one-quarter of new can still have many years and tens of thousands of miles of good life in it – surely the same is true of electronics, too?

Well, yes and no.  There are several things to consider when looking at buying used electronics such as radio gear, computers, and pretty much all other ‘gadgets’.

Pricing

The first surprising point is that while some electronic items drop in price very quickly, others do not.  A 5 – 10 year old computer – well, that’s probably going to be available at pennies on the dollar.  But a 5 – 10 year old radio transceiver?  Not so much.

Indeed, something as old as a 15 or 20 year old radio might still be selling for a high percentage not only of its original price but of what you’d pay for comparable gear, new, today.

There’s an interesting implication of this, and the answer is perhaps not what you’d expect.  If a radio still costs 50% of more of its new price when it is 15 – 20 years old, does that mean that it still has half its life to go?  Does this suggest that radio gear has a 30 – 40 year life?

Another consideration that is increasingly becoming relevant – with the growing availability of low-priced Chinese gear, you sometimes find yourself with a choice between a ‘brand name’ product (ie primarily the big three Japanese brands – Icom, Yaesu and Kenwood) that sells new for perhaps $750, which sells if 15 years old for, say, $500, or a brand new Chinese product with similar capabilities, for $250.  How does it make sense to consider the $500 item when the brand new name brand item, several models newer and ‘better’ is not very much more, and a similar and possibly better new Chinese product is half the price?

Plus, whereas used cars have a number of different services that publish valuations to help you understand if you’re getting a good value or not, there’s nothing comparable for used electronics.  This of course works both ways – maybe you’re getting a tremendous value, but maybe you’re being offered something shamefully overpriced.

Age vs State of the Art vs Fashion vs Value

Some things have technological obsolescence long before they actually wear out.  Computers and cell phones are good examples of this, although both product lines seem to be ‘maxing out’ and we’re all buying computers and phones less regularly than we used to.  But, do you really want to buy a ten-year old computer at any price?  Do you really want it with an old-fashioned CRT VGA monitor, some sort of Pentium processor, a mere 1 GB or so of disk, and so on?

We suggest that this is false economy and not a good choice.  Remember, after TEOTWAWKI, there isn’t going to be a repair store to go to, there aren’t going to be online help forums, and there won’t be spare parts.

We might buy a ten-year old refrigerator or vehicle, but no way would we buy a ten-year old computer.  We wouldn’t even accept one, for free.  With many electronic items, the ‘state of the art’ has changed so much as to make the older product truly obsolete, and useless at any price.  It isn’t even useful for spare parts.  What use is incompatible memory; an old and power-hungry screen with such low resolution as to be useless, a hard drive with an out-of-date interface, etc?

The trap in that scenario is buying something that is very inexpensive, but also very useless.

Sometimes the latest ‘state-of-the-art’ features truly are valuable and worth paying extra for.  Before you settle for something ten or more years out of date, make sure you know what you’re missing out on.  And even seemingly ‘old fashioned’ technologies like radio transmitters and receivers are changing (quite drastically due to digitization) and with much/most electronic gear, the newer model with newer features can truly be worth paying extra for.

Another factor that encourages faster replacement than is indicated by simple measurement of things wearing out is fashion.  Mercifully this afflicts women more than men (such as me!), but marketeers even try to encourage us to change our clothing long before it is worn out.  Wide lapels or short.  Bell-bottom flared trousers or straight/narrow/skinny.  And so on.  ‘This season’s colors’ – gack!  Cars used to be sold on an annual model refresh cycle, that has slowed down a bit too, but generally we all buy clothing – and probably cars too – long before the economic and effective life of the item we are replacing has expired.

The opportunity in that scenario is buying something that still has a lot of good working life left, and which has been valued lower than it is worth simply because it isn’t fashionable.

Opportunistic Buying – Yes or No?

By nature, many of us preppers are acquisitive and tend to eagerly accept anything we can get, particularly if it is free.  Anything we have space to store and which might possibly be of some value in the future seems like a no-brainer to accept – no downside to taking it, and who knows what upside, right?

We don’t entirely disagree with that concept, and if you saw the cartons and closets full of junk we have, clearly we’re as bad as anyone else!  We laugh at fashion – we just dig far enough back in our closet to find clothing that matches the ‘new’ fashion but from the previous time it was in fashion.

But there is a danger, if/when you buy opportunistically, that you start confusing irrelevant actions with important results.  Which is better :  To have a double garage you can no longer drive either car into because it is full of old junk that you’ll never actually use, even in an extreme Level 3 situation?  Or to have just a couple of cartons of essential items that you will use and need, for sure?  To buy $1000 worth of junk that maybe is worth much more if you ever have a need for it, but then to lack the money to buy a $1000 item that you will definitely for sure need?

The garage full of junk obscures the fact you might be missing some essential items.  And your ability to repurpose the junk in your garage will also be reduced after TSHTF, because you can’t just go to the local hardware store or wherever/whatever to get an extra piece of two of stuff to modify/repair/adapt the junk item to a practical purpose.

You’ll also have very much less spare time; you’ll need to focus your time on productive essential tasks, and the same will be true of your friends and neighbors (and, excuse us for saying this, but who knows how many of them will survive through the stressful times and still be available as resources for you to turn to).

A useless thing is a useless thing, no matter how little you pay for it.

Why is it Being Sold?

If you like hearing lies, ask any seller of anything ‘Why are you selling this?’.

Now sometimes you don’t need to ask the question, because the answer is sadly obvious.  The item is little better than junk (at least in the seller’s mind); maybe it doesn’t work, maybe the seller doesn’t even know what it is, or maybe it is no longer needed (eg a baby’s crib).  You see a lot of that sort of stuff at garage sales.

But when you’re looking at higher value items that apparently still have value and life left in them, it is a question to ask, even if the answer is meaningless.

Nine times out of ten, the answer will be a lie, and the tenth time, it will probably be an obscured truth.  For example, if the seller says ‘I got the newer model’, then the obscured truth might be ‘This one failed and I had to replace it’.

If you think about yourself, two things are probably usually true.  You only replace things when you uncover limitations or problems with them, and you generally keep things that are working well, even if you buy additional or replacement units.  You’re not alone in this approach – many other people do exactly the same, and only sell items when they absolutely for sure have no remaining value, or when something bad has happened to them.

So, know this :  There’s almost always a ‘bad’ reason why anyone is selling anything.  You may or may not uncover that reason, but expect there to be one.

There are additional lies that specifically relate to electronic gear being sold.  For example, ‘it has a nearly new battery’ and ‘it hasn’t been used much’.  Unless you see a new battery still sealed in its original packing and with a recent manufacturing date stamped on it, you probably should plan on replacing the battery (or at least buying a new one as spare).  The same goes for ‘I’ve just replaced all the tubes’ – unless you can test the tubes, consider them all as near the end of their life – and even if the tubes were recently replaced, you don’t know how much remaining life there is in the new tubes.

Some people might also tell you it has recently been ‘re-capped’ – that all the electrolytic capacitors have been replaced.  Ask to look inside the unit and see for yourself – do they look new or old?  Is the soldering fresh and bright, or older and duller, like everything else?  Has every electrolytic been replaced, or just the ‘easy to get at’ ones?

You might also been told ‘it has just been serviced by an authorized dealer’ – only accept that claim if you see the invoice and perhaps, if it is a high value item, you’ll even want to call the dealer and confirm that the work order was to ‘check/overhaul everything and make the unit in perfect like-new order’ and see if the dealer has any notes about issues they found and weren’t authorized to repair.  Just because you see a $200 invoice that says ‘repair item’ doesn’t mean that every fault with the item was repaired, or that the repair used new replacement parts, etc.

One more lie that some people can tell with a straight face – ‘I haven’t used it for a while, but last time I did it worked perfectly’.  If it can’t be fully operated and demonstrated to you prior to you buying it, you should prudently expect the worst.

Bartering and Negotiating

It should go without saying that you should avoid paying the initial asking price on anything that is being offered for sale.  Experts at negotiating deals consistently tell us two things – the first is that the first person to name their price loses the negotiation, and the second is that the magic phrase to use is, and say this slowly and thoughtfully, in an uncertain but helpful tone, ‘What is the best price you’d accept for this?’.

If you think about it, the two pieces of advice are two sides of the same coin, aren’t they.  By asking the guy to name his best price, he is the first person to put a number out there.  You might be able to talk the guy down further, but for sure, you know there’s no way you’ll have to pay extra above that revised asking price!

Once you’ve done most of the dickering over price, see if you can then switch to another line of bargaining.  ‘Could you throw in the —- as well?’ – see if you can have him include something else as well.

Maybe try to negotiate a deal for two items, but then, when you’ve beaten the guy down as low as you can for a ‘quantity discount’ for the two or more items, then look disappointed and say ‘Thanks for trying to help me with this.  Unfortunately, the price is over what I could afford for all the items we’re talking about.  But, I tell you what.  I’ll take the xxxx off your hands for $—-.’  That way, you’ve managed to get a quantity discount for only buying one thing!

Another thing.  Sometimes you might be able to trade something you have and are willing to dispose of as part or full exchange for the item the other guy is selling.  This can be the best deal of all.  If you have something you don’t need but the other guy wants, and he has something he doesn’t need but you want, then it ends up with you both giving away something unimportant and getting something of value in return.  A ‘win-win’ deal like that is the best of all.

If you don’t really need something, but would be willing to buy it at a bargain price, a useful strategy is to say to the seller ‘I’d be interested in helping you out by taking your xxxxx off your hands, but the thing is, I didn’t come here today looking to buy one, and I don’t really need it.  So I could only justify it to my wife it I got it at a heck of a deal.  What say you try selling it to anyone else for the best price you can for the rest of the day, and I’ll come back at closing time, and if you still have it, then I’ll give you $— for it?’.

This makes best use of the pressure of time in the deal.  If you’re going to a one day sale event somewhere, at the start of the day, there’s a rush of buyers all wanting to get the best bargains, and the sellers are optimistic that the rush will continue and they’ll get their asking price for everything they have.  But that first rush doesn’t last long at all, and half way in to the day, it is over, and sellers are starting to gloomily think to themselves ‘no-one has even shown any interest in my xxxxx at all’ and they’re starting to think they’ll need to pack up unsold items and take them back, instead of the cash they might have sold them for.

By giving the seller a fair chance to sell the item for more, and by making the point that at the end of the day, no-one else is likely to be buying it, you might be able to negotiate a very low price such as to make it sensible to buy the thing you don’t really need or want.

Part Two

Please keep reading for the second part of this article, which talks about the special considerations to do with buying used/old(er) electronics.

Jun 062016
 
An artist's impression of an EMP explosion.

An artist’s impression of an EMP explosion.

One of the things that gives us the most troubled sleep of all is the risk of, and outcomes from, an EMP attack on the US.  In case you’re not fully up to speed on this draconian danger, we discuss EMP attacks – what they are, how fearsome their impacts would be, and how easy they are to stage – in several articles here.

Our sense is that the danger of an EMP event is steadily increasing.  To be blunt, the world is becoming an increasingly unfriendly place, and with growing sophistication of both nuclear weapons and their associated delivery systems (ie missiles) by both North Korea and Iran (as well as other countries that aren’t being quite so public about their actions) and some threats that translate quite clearly to ‘if we need to, we’ll use an EMP device to bring your country to its knees’, the thought of an EMP attack is far from impossible to countenance.  At the same time, our lives continue to become more and more dependent on electronics for everything we do.

We are increasingly of the opinion that it is prudent to maintain a spare set of essential electronic items in a protective Faraday cage so that if/when an EMP occurs, you have a backup set of equipment to turn to.

One consideration when planning for this.  There is no point in keeping backup equipment that relies on other equipment or services owned/operated by other people/organizations, unless you are certain that these other parties will also be able to continue to provide services after an EMP.  For example, what is the point of having a backup cell phone if all the cell towers and network infrastructure by the wireless companies is fried by EMP, and also if the cell phones owned by most of your contacts are also fried!

So, with that as introduction, let’s continue…..

As you may already know, and as our other linked articles explain, an EMP attack destroys electronics by creating high voltage surges in them.  These high voltage surges are induced by electromagnetic radiation – a fancy way of saying ‘radio waves and similar type things’.  Because the voltage is induced by electromagnetic radiation, there is no need for electronic objects to be connected to anything – they wirelessly ‘receive’ these voltage surges, whether they want to or not, the same way that radios receive radio waves, televisions receive broadcast tv signals and cell phones receive phone calls.

Worst of all, perhaps, switching off your devices doesn’t protect them from these voltage surges.  You can unplug your devices and take the batteries out, but they are still at risk of being ‘fried’ by the electromagnetic radiation caused by an EMP device.

Note that while EMP effects are a problem to your at-home electronics, solar flares and storms are not a problem, assuming your electronics are not plugged into utility power (or possibly internet connections).  Unplugged, and battery operated, devices will not be affected by the different type of EMF radiation generated in a solar storm.

Back to EMP risks and counter-measures.  There are several ways to protect your electronics.  Some are impractical, at least for us, because they involve a redesign and ‘hardening’ of the electronic items when they are designed and built.

Others are impractical for other reasons, such as burying our electronics at least 10 ft underground.

The easiest approach for most of us would be to select items we wanted to protect and save for use after an EMP, and place those items inside a special type of container, known as a Faraday Cage.  This device is named after the English scientist Michael Faraday, the discoverer of electromagnetic fields, and deemed the inventor of these protective containers, back in 1836, although in truth it was Benjamin Franklin who first observed the properties of such containers, in 1755.

If you think about it, there’s something slightly strange about using a device first observed 260 years ago, to protect against a modern type of risk only developed 60 years ago.  But progress is a funny thing, right?

What is a Faraday Cage

The chances are you may have already experienced the protective effects of a Faraday cage, and without even realizing what was going on.  If you’ve ever been on a plane that was struck by lightning, the reason you lived to tell the tale – and the reason the plane wasn’t destroyed – is because the entire plane acts as a super Faraday cage, protecting itself and its contents.

A Faraday cage is simply an electrically conductive metal container that completely surrounds its contents.  When electromagnetic radiation reaches the container/cage, it has two choices for what it does next – it can either travel through the container, and thereby exposing the contents inside to its damaging effects, or it can travel around the outside of the container on its conductive exterior.

It is ‘easier’ for the radiation to travel on the conductive exterior, and indeed, the conductive exterior works so as to in effect compel the radiation to take this route rather than to go inside, through, and outside the container again.

So, think of a nice old-fashioned metal biscuit tin with a close-fitting lid.  Instant Faraday cage!  Any type of metal container, of most reasonable shapes and sizes, will work perfectly well as a Faraday cage.

Tight mesh metal screens have sometimes been used as radio frequency shielding too.  We don’t recommend this approach because the wave-length of the energies released by an EMP tend to be shorter, and therefore might be small enough to ‘fit through’ the holes in the mesh.  Plus, unless you have excellent electrical connections at each node where the screen mesh running in one direction intersects with the screen mesh running at sort of right angles to it, you’ll end up with invisible gaps in your radiation blocking.  Best to play it safe and stick to solid metal.

As implied by the comment on screens and their lesser effectiveness, nothing is an absolute in this world, and a Faraday cage – even made out of thick 100% copper, won’t necessarily eliminate 100% of all the radiation.  A little bit might still ‘leak’ through into the inside.  But a well constructed Faraday cage will reduce the radiation inside itself to perhaps one millionth or less of the level of the radiation outside the container, and that is probably sufficient reduction (the technical term is ‘attenuation’) as to reduce the radiation level from a dangerous level that risks the integrity of your devices to a trivial level that poses no threat to them.

Faraday Cage Construction 1 – The Need for a Total Metal Enclosure

Don’t think that a container that has metal on some of its surrounding surfaces – maybe even as many as five of the six sides of a cube shape – will protect its contents.  A typical example of that would be some sort of metal container with a plastic lid.

Such a shape is not a cage at all – rather it could become a ‘wave guide’ which might funnel radiation through it and maybe even concentrate it in some essentially unpredictable manner.

The container needs to have as close to complete metal coverage as possible.  Small holes are okay, but the electromagnetic radiation can travel through holes and other cuts and slits in the container.  The bigger the hole, the more radiation that can go through it, exactly as you’d think if designing a container to block out light (which is also electromagnetic radiation) or to be waterproof.

So don’t, for example, go to Harbor Freight and buy a ‘stainless steel tool chest’ that actually only has some very thin pieces of stainless steel in the center of some of the tool chest’s sides, and plenty of plastic elsewhere.  It might be a great way to carry tools, but it is not useful as a Faraday Cage.

Faraday Cage Construction 2 – The Need for Good Electrical Joins

So maybe you’ve ended up with an enclosure that is metal on all sides.  That’s good, but there’s more to it than that.  Are all the sides connected to each other electrically?  If they are not, you again have a wave guide or possibly a radiation ‘concentrator’ rather than some sort of blocking cage, and might end up with worse outcomes than without an enclosure at all.

Make sure there is no varnish or other ‘invisible’ insulation on each of the metal surfaces.  And have good connections on all sides, as much as possible, so that the shortest physical distance from any point to any other point on the enclosure is also the same as the shortest electrical distance.  As soon as you start to require the radiation to go ‘the long way around’, you start to tempt it to take ‘a shortcut’ through the container rather than around it.

In simple terms, if you have a cube type shape, that means that each of the six sides of the cube should be physically and electrically connected to other sides of the cube on each of its four edges.  A ‘press fit’ is acceptable only if the two surfaces are each clean and not corroded (both dirt and corrosion are usually insulators rather than conductors).  You’ll of course have welded or soldered joins on most of the sides of the enclosure, with just its ‘lid’ being openable in some form, and you need to be sure that all four edges of the lid provide a positive seal and electrical connection.

An Essential but Usually Overlooked Requirement with Metal Tape

Now for an important consideration.  If you are sealing some sort of container, you probably know or can guess that you should use a metal foil type of tape to seal the container with.  Okay, but there’s a trick that many people don’t think through.  The typical adhesive on generic metal foil tape acts as an insulator.  It insulates the tape from whatever you are taping it onto, and also insulates the tape from itself (if you have tape overlapping itself).  This makes the metal foil tape close to useless.

Be sure to get tape that has conductive adhesive on it.  Needless to say, Amazon is your friend, and offers a wide range of different types of metal foil tape with conductive adhesive on it, as you can see from the link.  Better still, the material isn’t very expensive.

Do You Need to Ground Your Faraday Cage?

This is one of the most widely misunderstood aspects of Faraday Cage design and construction.  Faraday cages do not need to be grounded, indeed, as best we vaguely remember our advanced college physics classes, they should not be grounded.

You’ll see many prepper sites that say you should ground a Faraday Cage, but they either don’t say why, or say ‘to bleed off the charge’, or perhaps work on the semi-stated assumption that you ground other electrical things for safety and so, therefore, you should also ground a Faraday Cage.  This is all wrong.  Faraday Cages do not accumulate charge.  They simply allow a charge to pass from one side of them to the other, without passing ‘through’ the inside of the cage.

That process also points to why you should not ground a Faraday Cage.  If you ground the cage, it is no longer an isolated part of normal space that just so happens to be better conducting than the space inside it.  Now it has become an antenna of sorts, and even can be considered as a ‘magnet’ for the radiation.  Think of the process like a lightning rod – a lightning rod actually works by attracting lightning – it ‘pulls’ the lightning to itself rather than allowing the lightning to semi-randomly choose anywhere else to land.  The last thing you want is to change from a neutral Faraday cage to an active receiver of the radiation!  Instead of the EMF passing around your object, it now goes onto the object and travels along it and down to the ground.

Like all antennas, a grounded Faraday cage ceases to be a conductive material and instead starts to become a resonating material with inductive and capacitive properties, with some parts of its length having high resistance (impedance) and thereby potentially defeating the major concept of the cage – its ability to more effectively transfer EMF around its exterior rather than through its middle.  It will be itself absorbing and re-radiating some of the EMF it is now ‘receiving’ and conducting, and while we’re not exactly sure, we fear some of that transmission may be into itself and whatever is inside.

Think of it this way.  Planes get struck by lightning dozens of times every day, but nothing happens to them because the planes are essentially Faraday cages.  The lightning strikes the plane, travels around/through it, and then keeps on going.  But imagine if the plane had a big long antenna trailing off the end of it, all the way down to where it was dragging along the ground.  Now the plane has changed from being an electrically neutral thing that the lightning doesn’t ‘see’ or expend energy on, and instead becomes a huge big ‘magnet’ that draws lightning from miles around to it.

‘Nuff said?  Don’t ground your cage.  Not only don’t deliberately ground it, but also keep it away from any ‘accidental’ grounds – for example, don’t have it resting on solid earth or attached to any metal beams that might lead to the ground, and – whatever you do – please don’t have any electric wiring feeding into it!

Insulating the Contents from the Cage Walls

Something we often see overlooked in articles about Faraday cages is the need to insulate the contents from the cage walls.  When the cage is actually ‘doing its job’, those walls – both inside and outside the cage – are going to be alive with energy, and that energy will be eager to find anything and everything to flow into and fill.

If you think about what would happen if you had the contents actually touching the walls, there’s a chance you’d end up creating an electrical path through the cage that would be easier or almost as easy for the radiation to travel along as it is for the radiation to go around the outside of the cage.  The last thing you want to do is to encourage the radiation to come into your cage and move around.

We suggest you simply line the inside of your cage with thin foam – perhaps 1/4″ – 1/2″ thick.  That’s all you need to do – have some sort of light simply stand-off that keeps your electronics off the walls of the container.

Testing Your Faraday Cage

There are several ways you can test your Faraday Cage once you’ve constructed it.  Go somewhere with a very strong cell phone signal (such as right next to a nearby cell tower) and confirm you’ve all the bars on your phone showing max strength.

Put your phone in the cage and try calling it.  If you hear it ring, you know you’ve got a problem.

Next, place a call from your phone to someone else, then put the phone in the cage and close it up.  Does that cut off your call?  If it doesn’t, you again know you’ve got problems.

Try this with the phone and cage oriented in different directions (and with the phone either vertical or horizontal inside), in case radiation can get in one way but not another way.

Another test is to check for electrical continuity all the way around the cage.  With an ohm meter, set it on its lowest/most sensitive ohm scale and first calibrate it to zero ohms.  Then stick the probes on random parts of the cage, and ensure that everywhere you place the probes, you’re getting under a 1 ohm resistance reading.  Test a range of combinations from any side to any other side.

Those are the two easiest types of test to do.  Happily, there’s not really much of a trick to building a cage, and so there’s every good chance your cage will pass these tests.

Summary

Build yourself a Faraday cage – perhaps out of plumbing ductwork type thin metal sheeting that is easy to work and not too ridiculously heavy.  Follow the design considerations discussed above – perfect electrical and physical seal, nothing touching inside, outside not grounded, and you should have an effective way of protecting an essential set of spare electronic devices for WTSHTF.