Faraday cages

My brother once built a satellite dish using wire mesh. I was wondering how radio waves reflected off wire mesh so I was looking into how a Faraday cage works and realized I didn't know as much about it as I thought I did. I realized I don't know how wires reflect radio waves and how close or far apart the wires need to be. I didn't know chicken wire would work for RFs and it does. I don't know why my rooftop antenna absorbs radio waves while my satellite dish reflects them.

Can anybody help me understand how a Faraday cage works and how an antenna works? Are there frequency limitations with a Faraday cage? Gamma rays have very short wavelengths. What about ELF waves?

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@metal-brain said
My brother once built a satellite dish using wire mesh. I was wondering how radio waves reflected off wire mesh so I was looking into how a Faraday cage works and realized I didn't know as much about it as I thought I did. I realized I don't know how wires reflect radio waves and how close or far apart the wires need to be. I didn't know chicken wire would work for RFs an ...[text shortened]... ys have very short wavelengths. What about ELF waves?

https://www.youtube.com/watch?v=aWGm8kmoK6A

The mesh width needed depends on the wavelength. RF is anywhere between 1mm and a few kilometres. Gamma rays have a wavelength of the order of a billion times shorter than EHF radio-waves so they just go through without interacting.

You don't seem to have a very clear question. I need to remind myself but can go through the basic theory, Sonhouse probably has the most practical knowledge.

@metal-brain said
My brother once built a satellite dish using wire mesh. I was wondering how radio waves reflected off wire mesh so I was looking into how a Faraday cage works and realized I didn't know as much about it as I thought I did. I realized I don't know how wires reflect radio waves and how close or far apart the wires need to be. I didn't know chicken wire would work for RFs an ...[text shortened]... ys have very short wavelengths. What about ELF waves?

https://www.youtube.com/watch?v=aWGm8kmoK6A

Faraday cages are like ANTI antennae. The idea is to totally block RF seeping into a system so you can get measurements of sensitivity and bandwidth of components without external signals screwing up measurements. So there will be a box with copper sheet covering the entire guts of the box and metallic seals on the doors so the only thing inside the box would be a low power RF transmitter for the frequency in question and some kind of antenna to measure how well the antenna performs. And other uses as well.
But like DT said, the chicken wire used as a dish reflector surface will work only up to a maximum frequency because if you are trying to reflect say 20 Gigahertz, which has a wavelength of only a few millimeters, the chicken wire opening is too wide so will not reflect that frequency or only very poorly.

The typical TV antenna has several elements tuned to the wavelength of interest so the low TV channels have a wavelength of a few feet or so and if there is say, an element of the antenna just hanging in space, impinged upon by the right wavelength (frequency) it does absorb some of that signal but also re-emits that signal also. So if you have several of these elements like you see in TV antennae, the elements send a reinforced signal level then there is the actual receive element, usually about 5% longer and the other elements ends up doing two things: one is more gain, sometimes a gain of a hundred or more, (20 DB gain) and decreases signals coming from the sides so signals from say 40 degrees off the main aiming area, that interference is limited so more of the desired signal gets to the receiver.

Then signals coming from 180 degrees from the front also gets limited by a large amount, called 'Front to back ratio' and that can also be a hundred to one or better for some specialty antennae.
In my ham band use, for instance, 2 meter band, uses antenna elements of half wavelength so the elements are only one meter long, about 40 inches. Then the more of those elements feeding to the actual receive element, the more gain, the more back to front ration and so forth.

In fact there is a subset of amateur radio called 'moonbounce' where they build antennae where not only is there one antennae like I have used, but whole banks of them, one rig I heard about used 64 such antennae properly phased to give a signal boost of a thousand to one and they aim that assembly at the moon and use the maximum power available and the signal goes to the moon and some of it bounces off to go back to Earth where a similar rig can listen and they can talk with a signal path of 500,000 miles so there is a few seconds of delay between "Hi there' and 'How are you doing'. Pretty expensive deal though😉

@deepthought said
The mesh width needed depends on the wavelength. RF is anywhere between 1mm and a few kilometres. Gamma rays have a wavelength of the order of a billion times shorter than EHF radio-waves so they just go through without interacting.

You don't seem to have a very clear question. I need to remind myself but can go through the basic theory, Sonhouse probably has the most practical knowledge.

What about ELF waves? Even though the wavelengths are long it can go through the earth so a Faraday cage would not stop it either I would think. Is that correct?

@sonhouse said
Faraday cages are like ANTI antennae. The idea is to totally block RF seeping into a system so you can get measurements of sensitivity and bandwidth of components without external signals screwing up measurements. So there will be a box with copper sheet covering the entire guts of the box and metallic seals on the doors so the only thing inside the box would be a low power ...[text shortened]... s a few seconds of delay between "Hi there' and 'How are you doing'. Pretty expensive deal though😉

So a satellite dish reflects as well as absorbs the RF just like a TV antenna does?
What exactly is happening when a RF wave hits a satellite dish made from wire mesh? Is it only reflecting part of the wave? How does it make a predictable bounce to the receiver in the center? I watched satellite TV from that dish for years when I was young so I know it worked, I just want to know why it worked.

@metal-brain said
What about ELF waves? Even though the wavelengths are long it can go through the earth so a Faraday cage would not stop it either I would think. Is that correct?

The earth is not a tremendously good conductor. My knee jerk response is no, but long wavelength transmissions are used for submarine communication through salt water so I need to look this up. It might depend on how good a conductor it is. A superconductor is impermeable to magnetic fields so if the Faraday cage were made of superconducting material then it would exclude anything with a wavelength longer than the interatomic spacing of the material.

@deepthought said
The earth is not a tremendously good conductor. My knee jerk response is no, but long wavelength transmissions are used for submarine communication through salt water so I need to look this up. It might depend on how good a conductor it is. A superconductor is impermeable to magnetic fields so if the Faraday cage were made of superconducting material then it would exclude anything with a wavelength longer than the interatomic spacing of the material.

What exactly is happening when a RF wave hits a satellite dish made from wire mesh? Is it only reflecting part of the wave? How does it make a predictable bounce to the receiver in the center? I watched satellite TV from that dish for years when I was young so I know it worked, I just want to know why it worked.

@metal-brain said
So a satellite dish reflects as well as absorbs the RF just like a TV antenna does?
What exactly is happening when a RF wave hits a satellite dish made from wire mesh? Is it only reflecting part of the wave? How does it make a predictable bounce to the receiver in the center? I watched satellite TV from that dish for years when I was young so I know it worked, I just want to know why it worked.

Mostly it reflects. It would absorb only if the elements had a higher resistance than the wires of the mesh or dish. Mesh will absorb some of the energy but most of it is reflected. The shape of the dish is the biggest factor, it has to be a parabolic shape or some variation of that shape. The standard sat dish of today uses an offset parabola designed in such a way as the actual receive antenna elements are out of the way of the incoming RF. If you ever looked through a reflector telescope, you see a mirror in front canted off 45 degrees and then more optics to focus the incoming light. That little mirror effects the image for one thing by just reducing some of the light reaching the actual reflector mirror and the supports of the small mirror adds artifacts to the optical path.
It is similar in a TV Sat dish. If we used the old fashioned steerable dish, you remember the big ones, 6 to 8 feet in diameter? The actual pickup array is right in line with the incoming RF so it blocks a small amount of the receive signal. Not a huge deal with an 8 foot dish, the difference of the block Vs no block would not be detectable.
On the tiny Sat dishes, however, the receive elements are around the same size as the old big dishes. So a dish of say one foot diameter with the receive elements in place would block a rather large amount of the incoming signal. So they use an offset parabolic curve so the whole area of the dish sends the RF to the receive elements without those elements interfering with the signal. But it is basically a parabolic curve which focuses the RF or optical energy to a point, limited by diffraction but generally pretty good at focusing RF or light.

So you got the idea that ELF may not be blocked by a mesh but it will be stopped by a solid panel of copper which is what professional level Faraday cages use so NO RF gets through no matter the wavelength. For instance, a 3000 mile long high voltage transmission line loses about 7 percent of its energy in that long trip from source to load. Most of that loss is in the resistance of the wires conducting the energy but some of it is actually transmitted, and a 3000 mile long wire is a fair imitation of an antenna at the frequency we use, 60 Hertz, 60 cycles per second.
At that frequency, the wavelength is about 3100 miles and the wire could be something like that length so it would be an antenna, of some efficiency so the entire US is saturated with some low level of 60 hertz radiation. Also the same from house wiring but much less effectively turned into actual RF but still some of it is transmitted.

That kind of wavelength is what the subs use to get messages to the sub when other methods are dead. If a sub is 1000 feet underwater no RF of normal wavelengths will penetrate that deep but 60 Hertz will. Of course such a low frequency means there is a limit as to how much information can be stuffed down that small hole. It would basically be a Morse code link, a few words per second, maybe 20 WPM or so and that would be the word to every sub in the world so they would have a code for a single sub and they would have to wait their place in line to get their particular message.

Really slow but no weather, clouds, storms, lightning or such will stop that signal getting through.

@sonhouse
BTW, the wavelength tells how much gain the dish gives. If you have a wavelength of one inch, and a dish of one foot square, it would concentrate 144 of those wave packets. Dishes are more or less circular so it would concentrate about 0.8 of that number, say 115 packets to one and 100 to 1 is 10 DB of gain and a much larger front to back ratio. Now the frequency of that pack you can work out like this: a Packet of one meter size is very close to 300 megahertz. One meter is about 40 inches, close enough for government work😉 so 40 times 300 yields a frequency of 12,000 megahertz or 12 Gigahertz. That is a bit higher than the actual sat TV frequency, which might be 6 Ghz. So a wavelength of 2 inches and that one foot dish now would concentrate about 60 to one which works out to roughly 8 DB of gain. It works because the satellite in space is a straight shot, unless you are in a severe storm and they use fair high power at the satellite, couple hundred watts or so. So you can figure the approximate gain of any dish based on the frequency, converted to wavelength.

@sonhouse said
Mostly it reflects. It would absorb only if the elements had a higher resistance than the wires of the mesh or dish. Mesh will absorb some of the energy but most of it is reflected. The shape of the dish is the biggest factor, it has to be a parabolic shape or some variation of that shape. The standard sat dish of today uses an offset parabola designed in such a way as the a ...[text shortened]... Really slow but no weather, clouds, storms, lightning or such will stop that signal getting through.

How does the mesh reflect? I get that the wavelength is wider than the mesh, but how does it reflect the wave? You would think wire mesh would only reflect part of the wave, not all of it. Why doesn't the wave pass through the part in between the wires? Is that what is happening? Does some of the wave always get through unless it is solid metal?

How effective is a Faraday cage at reflecting if it is made of wire and not solid? Is it only partially reflecting?

@metal-brain said
Why doesn't the wave pass through the part in between the wires?

Answered in intuitive layperson terms and here I will try to keep the technical terminology to a minimum, and the following applies to all the electromagnetic spectrum thus not only to radio but also to visible light and also applies whether we are thinking of the electromagnetic radiation in terms of being 'waves' or 'photons' (photons are particles as opposed to waves):
It can be considered to be a general law of physics (albeit not a primary one because it can be deduced from other more fundamental laws of physics) that if the wavelength of the photon/wave is more than double the diameter of some feature of something (such as a gap between some metal wires) then that photon/wave doesn't 'see' that feature which means, to the photon/wave, it is just as if that feature isn't there!
So, to answer your question, the reason why the (radio) wave doesn't pass through the parts between the wires is because those gaps between the wires are to small (in diameter) for the wave to 'see' them and thus, to that wave, there are no gaps for it to pass through!

P.S. I know of at least one loophole in this above law.

@metal-brain said
How does the mesh reflect? I get that the wavelength is wider than the mesh, but how does it reflect the wave? You would think wire mesh would only reflect part of the wave, not all of it. Why doesn't the wave pass through the part in between the wires? Is that what is happening? Does some of the wave always get through unless it is solid metal?

How effective is a Faraday cage at reflecting if it is made of wire and not solid? Is it only partially reflecting?

Think of the mesh as the size of chicken wire but what is hitting it is a bowling ball. The ball will bounce off even though the chicken wire has holes in it. The photon packets are like that somewhat.

@sonhouse said
Think of the mesh as the size of chicken wire but what is hitting it is a bowling ball. The ball will bounce off even though the chicken wire has holes in it. The photon packets are like that somewhat.

and, using your 'bowling ball analogy', the diameter of this analogous bowling ball is half of its wavelength.

@sonhouse said
Think of the mesh as the size of chicken wire but what is hitting it is a bowling ball. The ball will bounce off even though the chicken wire has holes in it. The photon packets are like that somewhat.

But why? Why does the whole wave get reflected instead of part of it? Are you sure it is the whole wave? Doesn't some get through?

@metal-brain said
Why does the whole wave get reflected instead of part of it?

the only way to truly understand why is in terms of maxwell's equations of electromagnetism. Those equations are so difficult to understand in intuitive terms that THERE IS NO layperson way to explain them simply. This is often the case in science that there isn't a simple layperson way of explaining some part of it and the only way to understand it is to spend YEARS intensively studying it and, even at the end of that study, you STILL might not understand it! In general, such things can only be understood by people a LOT smarter than me or you. If you don't believe me, just try and understand those equation here;

https://en.wikipedia.org/wiki/Maxwell%27s_equations#Vacuum_equations,_electromagnetic_waves_and_speed_of_light

Until if or when you can understand such equations, you will just going to have to trust what the experts (like I did at university), who are a lot smarter than me and you and know much about it that we don't, say about it is correct. You have just going to have to accept our word that the whole length of the radio wave does get reflected and accept you cannot possibly understand why so.

Doesn't some get through?

A small amount of the radio radiation might get through due to such effects as quantum tunneling etc BUT that has nothing to do with the gaps between the wires proving those gaps are much less in diameter than half of its wavelength.