Paging physicists: looking for answers about a Faraday cage

[Bunsen]

There's more to it than just compensating for the de-tuned antenna. His arm is acting like the center conductor of a transmission line, and is introducing a lot more RF into the bottom of the can than would be there without it. If you've got a big metal can around, you can try it yourself using a radio that doesn't need its antenna extended. It should even work on AM.

Married to a physicist and majored in it myself, so you can actually give me a good answer or link me to one (aka no need for the obligatory car analogy )...

Why is the arm acting like a transmission line? I'm probably missing something obvious, but I don't see why arm would do a better job than air.

This would be better with diagrams, but mspaint sucks compared to a chalkboard. Try looking at the difference between a waveguide and coax. A waveguide has only one conductive surface (i.e. it's just a hollow tube), and any signal with a wavelength longer than about twice the diameter can't propagate in it. Below the cutoff frequency, any field that's present at an open end of a waveguide is exponentially suppressed as you look deeper into the tube. Coax has two separated conductors, and has no cutoff frequency. It can have fields all along its length right down to DC, where it just looks like a capacitor.

The empty can is like a short section of waveguide. Low-frequency EM waves are suppressed as they try to travel down the inside, because the walls are effectively a short circuit at low frequencies, so the radio hears a rather weak signal. Shortening and detuning its antenna makes it weak enough to get scratchy.

The can with they guy's arm stuck into it now looks like a section of coax, with the can acting as the shield and the arm as the center conductor. Low-frequency signals can propagate freely, because there are now two separate conductors (i.e. there's no low-frequency self-shorting effect). The coupling from the arm to the radio is capacitive, but there's more to it than pulling the circuit back into resonance. The same behavior should still be observed if you use a compact radio without an extendable antenna.

[williaty]

Thanks.

The explanation makes sense but I have to say I'm pretty surprised that the arm is a good enough conductor to have such an effect. In other words, I wouldn't expect that explanation to work if we stuck a pool noodle into the can and, intuitively, arm seems closer to pool noodle than metal rod. Intuition often fails at these things though.

[KnightoftheRoc]

Thanks.

The explanation makes sense but I have to say I'm pretty surprised that the arm is a good enough conductor to have such an effect. In other words, I wouldn't expect that explanation to work if we stuck a pool noodle into the can and, intuitively, arm seems closer to pool noodle than metal rod. Intuition often fails at these things though.

The pool noodle isn't mostly water, tho, like an arm is. The arm is more electrically conductive- had he used a length of pipe to 'poke' at the antennae, the effect would be even greater. A length of fiberglass rod, the effect would be lessened.

[Bunsen]

Thanks.

The explanation makes sense but I have to say I'm pretty surprised that the arm is a good enough conductor to have such an effect. In other words, I wouldn't expect that explanation to work if we stuck a pool noodle into the can and, intuitively, arm seems closer to pool noodle than metal rod. Intuition often fails at these things though.

Below the skin, we're salty enough to be moderately conductive. Also, water has a dielectric constant of something like 50, which makes it act more conductive at RF than at DC. Make it a pool noodle full of salt water and you'd be closer to reality. It's a craptastic lossy conductor, certainly, but it's good enough make a difference.

[44Dave]

That's why you can grab hold of the rabbit ears and move your arms around to strange positions to get decent TV reception.
The house I used to live in was made of enameled steel. RF reception inside was very poor despite the large poorly insulated windows. I had a FM radio in one room where I had to place the FM antenna just so to couple with my body to achieve decent reception. If I moved out of my customary spot all I got was static.

[MacAttack]

Does RF travel in straight lines are does it go around corners to reach an antenna?

[williaty]

Does RF travel in straight lines are does it go around corners to reach an antenna?

Yes.





The easiest answer is that it mostly goes in straight lines and, to a limited extent, reflects off things. Think of it like a ray of light. Which, actually, RF is exactly the same thing as light, just a different color. The more detailed answer is that RF is a wave and therefore will diffract around corners and refract through thing with different refractive indices. However, light does these things to! It's just that light, and radio waves, do it to such a small amount that we can easily ignore diffraction around corners for common uses. It's the kind of thing that only matters to a physicist.

[MacAttack]

So basically 'no' it does not bend around corners to reach an antenna. But it can be bent by gravity like light.

It needs to reflect off of a surface to reach around a corner with any significance.

[williaty]

So basically 'no' it does not bend around corners to reach an antenna. But it can be bent by gravity like light.

It needs to reflect off of a surface to reach around a corner with any significance.

Correct, radio waves, light, x-rays and more are all the same thing: electromagnetic radiation. The only difference is the frequency (or wavelength, whichever way you like to look at it). So, the same thing that makes us see red as a different color than blue (they're different wavelengths of electromagnetic radiation) is the same thing that makes us call some of them radio waves and some of them visible light. All waves diffract, refract, and reflect. The refraction and diffraction of radio waves and light is just so small that it's not on a human scale and we conveniently ignore it.

So, you're absolutely right, for normal-people purposes, radio waves don't bend around corners. Reflection can be a bigger deal. If you read up on "multipath interference", you'll find that multipath is often caused by radio waves bouncing around between buildings in a city (reflecting off of them). This is one of the reasons sometimes you can move your car like half a car length in stop-and-go traffic and go from strong FM radio to static.

[NYKh]

I know this is an older thread, but new to me.....

Thanks for the cut and dry info......

[S100]

I was planning on putting together my own cage,

Using the SS garbage can, tossing in some fine copper mesh on the interior, with plastic sandwiched between the steel and copper, and then cardboard with tinfoil on the inside of that, yay or nay on that being effective enough to stop most of the original posters concerns?

[williaty]

I was planning on putting together my own cage,

Using the SS garbage can, tossing in some fine copper mesh on the interior, with plastic sandwiched between the steel and copper, and then cardboard with tinfoil on the inside of that, yay or nay on that being effective enough to stop most of the original posters concerns?

Not really. You'd be better off taking a single metal can and using RF weatherstripping to make sure the lid is electrically bonded to the can without any gaps at all. The multiple layers of stuff lining the can gain you nothing.

[SRO1911]

NOT a physicist!

One of the earlier posts/questions was talking about grounding in an apt. Several people mentioned just using the ground lug on any old wall-plate - not a great idea.
When any device on that circuit faults to ground there is a potential (slim) that it will energize the ground circuit a whole.

Your best bet in an apartment - and what I personally have on one item in my apartment is to tie into one of your COLD water risers i.e. any copper/steel fitting on your cold water system such as - cut-off for sinks or toilets. You can buy grounding clamps for practically any size pipe.

[KJ4VOV]

NOT a physicist!

One of the earlier posts/questions was talking about grounding in an apt. Several people mentioned just using the ground lug on any old wall-plate - not a great idea.
When any device on that circuit faults to ground there is a potential (slim) that it will energize the ground circuit a whole.

Your best bet in an apartment - and what I personally have on one item in my apartment is to tie into one of your COLD water risers i.e. any copper/steel fitting on your cold water system such as - cut-off for sinks or toilets. You can buy grounding clamps for practically any size pipe.

Cold water lines used to be acceptable as grounds, back in the days of only metal pipes, but since the advent of PVC, CPVC, etc. can no longer be relied upon as any kind of ground. Also, if you've read through this entire thread you should realize that a ground is not needed for this purpose. Your sole purpose here is to RF shield the gear and you need a ground for that about as much as you need a drain line connected to your raincoat to protect you from getting wet.

[KnightoftheRoc]

NOT a physicist!

One of the earlier posts/questions was talking about grounding in an apt. Several people mentioned just using the ground lug on any old wall-plate - not a great idea.
When any device on that circuit faults to ground there is a potential (slim) that it will energize the ground circuit a whole.

Your best bet in an apartment - and what I personally have on one item in my apartment is to tie into one of your COLD water risers i.e. any copper/steel fitting on your cold water system such as - cut-off for sinks or toilets. You can buy grounding clamps for practically any size pipe.

Cold water lines used to be acceptable as grounds, back in the days of only metal pipes, but since the advent of PVC, CPVC, etc. can no longer be relied upon as any kind of ground. Also, if you've read through this entire thread you should realize that a ground is not needed for this purpose. Your sole purpose here is to RF shield the gear and you need a ground for that about as much as you need a drain line connected to your raincoat to protect you from getting wet.

Also, as the electrical ground connection in an outlet stands the CHANCE of becoming energized, so does the plumbing used as a part of the grounding process- the plumbing is usually tied into the electrical panel's ground line so the water main (if copper) can be used as a ground rod. The advent of plastic supply piping for water supplies and even for the heating systems, means that grounding to a metal fixture piece gives you a ground ONLY IF THE WATER IN THE LINE REMAINS UNINTERRUPTED to conduct the electricity. Given the high, and usually harmless, chance of an air bubble interrupting the water stream in any line, depending on today's plumbing systems for electrical conductivity is NOT a good plan.

If having a grounded cage gives you a warm, fuzzy feeling, fine- buy a length of cable, the connectors, and an actual ground rod designed for the purpose. Don't depend on the plumbing system.

[ctbarb]

This has been a very informative thread! I was not a physics major in college, just radiography, so I know more about x-ray radiation and general radiobiology than the rest. I have been very curious as to how to make an effective Farady cage for our small electronics. We live in a 2nd floor condo apt so are limited in what we can do. We really have no space for a
SS trash can, but do have a dedicated closet for prepping. I was thinking of wrapping the radios, etc in layers of newspaper and foil and keeping them in the closet or under the beds or wherever I can. Any thought as to if this will work? I do not have room in the prep closet for the SS trash can as it is full of food and other essentials!

[williaty]

The newspaper isn't going to do anything for you. The foil I wouldn't trust because it'd be nearly impossible to ensure it wasn't "leaky". Just get a continuous metal box or can, get some RF weatherstripping, and properly bond the lid to the container and you'll be good to go.

[Zimmy]

Sorry about being a noob at all this.....but (you knew there was going to be a but ),

What kind of stuff should be in a faraday cage? I'm not entirely sure what is threatened by the hazards mentioned earlier. I understand radios, computers, ect would possibly fry. If solar flared or EMP'd I imagine my faraday protected phone would reach almost exactly nobody because there wouldn't be cell towers ect working.

My main question is regarding my old vehicles. Do I just need to put spark plugs and wires, alternators, voltage regulators, distributers, points, ect in there? Or will the whole wiring harness be fried? I could rewire my old '48 Ford 8N fairly easily, and the '57 6 ton Chevy wrecker with more difficulty, but if the '64 Chevy would need reharnessing that would almost be time and cost prohibitive because of increasing complexity. Just getting the wires run enough to get a fire lit in her would be doable, I suppose.

I don't want to even think about the Mercedes UniMog. Germans build stuff in a very simply complicated way.

Also, my wind turbines, batteries, dehydrators, LED lights, and small solar panels. Safe outside or no?

Thanks for any info

[williaty]

Sorry about being a noob at all this.....but (you knew there was going to be a but ),

What kind of stuff should be in a faraday cage? I'm not entirely sure what is threatened by the hazards mentioned earlier. I understand radios, computers, ect would possibly fry. If solar flared or EMP'd I imagine my faraday protected phone would reach almost exactly nobody because there wouldn't be cell towers ect working.

My main question is regarding my old vehicles. Do I just need to put spark plugs and wires, alternators, voltage regulators, distributers, points, ect in there? Or will the whole wiring harness be fried? I could rewire my old '48 Ford 8N fairly easily, and the '57 6 ton Chevy wrecker with more difficulty, but if the '64 Chevy would need reharnessing that would almost be time and cost prohibitive because of increasing complexity. Just getting the wires run enough to get a fire lit in her would be doable, I suppose.

I don't want to even think about the Mercedes UniMog. Germans build stuff in a very simply complicated way.

Also, my wind turbines, batteries, dehydrators, LED lights, and small solar panels. Safe outside or no?

Thanks for any info

My stance on your cars, based on understanding of the physics involved, testing by the government, and testing by the major auto manufacturers is that you don't need to worry about your car, new or old. Simply put, today's cars are astonishingly resistant to assault from electromagnetic radiation. Because they rely entirely on electronics to function, the engineers design them to be quite robust. The key is that much of the work required to make cars resistant to interference from electromagnetic noise also happens to make them resistant to an EMP as an unintended benefit. Manufacturers test their cars up to field strengths approaching 100kV/m because they don't want the cars to stop functioning when struck by lightning. Getting zapped in testing often makes the cars do strange things such as switch radio channel, turn on a light, or something similar. However, it doesn't render the car useless.

For things like your solar system, the situation isn't quite as good. Solar systems have two big vulnerabilities. First, they're usually attached to wires to bring the electricity from the panel into the house to the control system. That wire acts like an antenna. The longer the wire, the greater the voltage gradient across it during an EMP. Second, most of the equipment is pretty intolerant of reversed polarity and there's no guarantee the EMP is going to induce a gradient that runs in the happy direction.

For everything plugged in in your house, the real threat isn't the EMP or the CME, it's the power company. Think about how often consumer electronics get friend just being plugged in now. Brown outs, spikes, lightning hits, flickering on and off when a recloser goes wonky. The power grid is at real risk of failure during an EMP or a CME because they've got some bloody long wires! The real danger is the death throes of the power grid doing all kinds of horrid things to shit you've got plugged into the wall.

So, if you ask me "what should I do about EMP/CME?", my answer isn't "build a Faraday cage", it's "prep to survive without grid power for a span of months to years depending on how close you are to something important enough to restore power to.

[bigmattdaddywack]

very interesting

[ctbarb]

My interest in having a Faraday cage is not to keep an entire house electrified, but to keep a few small, needed electronics in working order. For example several walky-talkies, a laptop, camera, radios, etc. In all probability, in the event of an EMP or CME cell towers and landlines would be down. Being able to communicate with family members or others in my group would be very important. As I am a newby at this I am just learning about the different types of handheld walky-talkie thingys and want to protect the stuff I have.

A couple years ago we had some bad storms that caused widespread power outages. We went without power for over 5 days. But it was no problem, because we had our preps, books to read, and plenty of water. If we hadn't had our cell phones it would have been tough to stay in contact with each other sometimes. That's why I have an interest in a non-cell phone communication type device. And in protecting said device from the possibility (however unlikely) of EMP/CME. Our other preps are coming along nicely, we have at least 3-4 months food/supplies so now I am thinking about "other stuff"! I think it is important to keep expanding our basic supplies, but I also want to incorporate some luxuries. A laptop to watch movies, for record keeping, etc. would be nice. And a camera to record events and/or damages would also be helpful in any kind of disaster. That is why I want to know more about Faraday cages and how practical, or not , it is to make one.

[ProstheticWeasel]

That is why I want to know more about Faraday cages and how practical, or not , it is to make one.

If you read through all this and the links provided the general consensus is that no; putting your stuff in a garbage can will not save it. Nor will an ammo can. In an ammo can in a garbage can wrapped in tin foil is not much better. You need mass and the amount of mass needed is inconvenient.

30 feet of earth and some reinforced concrete are a good start. Several feet of lead shielding with no gaps has potential as well.

Old school electronics are a lot hardier. learn to make your own radio with plumbing parts and garden materials. That or buy a decommissioned silo and live like a mole.

[williaty]

That is why I want to know more about Faraday cages and how practical, or not , it is to make one.

If you read through all this and the links provided the general consensus is that no; putting your stuff in a garbage can will not save it. Nor will an ammo can. In an ammo can in a garbage can wrapped in tin foil is not much better. You need mass and the amount of mass needed is inconvenient.

30 feet of earth and some reinforced concrete are a good start. Several feet of lead shielding with no gaps has potential as well.

No, you're completely wrong. You're giving instructions for protecting organic tissue from alpha and beta radiation (and to a limited extent ionizing EM radiation), not for making a Farady cage.

You need an unbroken electrically conductive surface completely surrounding the object you wish to protect. This is why a trash can fails: the lid is not sufficiently well mated to the can to be considered "unbroken". Same thing with a large metal cabinet: there's too many gaps around the doors and in the welds. It's not unbroken. Same thing with aluminum foil: the problem is that you'll end up with "leaky" areas were things overlap and fold. If you were a welding god and could weld aluminum foil, you could weld something into a little sack and it'd be well protected.

Best bet, if you really want to do this is some sort of a sheet metal box with continuously welded seams and a door bonded to the carcase with RF weatherstripping.

[Zimmy]

Thanks for the info