Why does a radio still work inside a metal box?

[Paul Uszak]

Have you tried the magnet experiment?

The magnet experiment, and some other actions suggested by contributors:-

The tin IS magnetic, attracting a magnet well
The walls are 0.25mm thick (including paint)
The inside of the tin is unpainted

This is the tin and radio (should have posted a photo earlier )

http://argos.scene7.com/is/image/Argos/5003820_R_Z002A_UC1051961?$TMB$&wid=312&hei=312

I grounded the tin to the Earth wire from a UK mains electrical socket, and repeated the reception test. I filed off the paint where the wire touched the tin. Results:-
FM: Total static. No discernible change with /without Earth wire. No Gaga.
AM: Tinny but recognisable radio station. No, repeat no discernible change in pitch or volume with /without Earth wire. Surprised.

 

[Paul Uszak]

FM detector noise is certainly not random

This vexes me. Some contributors have suggested that the hiss is thermal noise in the early stages of the receiver. I take this to be stochastic Nyquist noise. If so, surely its amplification should also be random...

 

[the_emi_guy]

Doing the math:
0.25mm is 33 skin depths in steel which is 285dB attenuation. It's not getting in that way.

Seems like it must be a seal issue around the lid or bottom

When two smooth metal surfaces come into contact it is not uncommon for the actual contact to be limited to widely separated discrete points. Even if it "feels" tight you may only have a few point of actual contact. This is why we use fingerstock for electrical bonding, and why motors use brushes for contact with armature.

 

[meBigGuy]

The answer is to use an audio spectrum analyzer which can be done with a PC sound card's line input.

I never worked with one but it has 14 day eval and is inexpensive. http://www.zelscope.com/
Maybe someone else has a preferred application. For example, store the data and analyze it in matlab, if you want.

 

[nsaspook]

http://www.chomerics.com/techinfo/EMItheory/index.html

http://www.compliance-club.com/archive/old_archive/990810.htm

 

[Baluncore]

Some contributors have suggested that the hiss is thermal noise in the early stages of the receiver. I take this to be stochastic Nyquist noise. If so, surely its amplification should also be random...

The amplitude of thermal noise (AM) is pink but the phase noise of the 1'st LO (FM) is not.
Also, any AC coupled noise will not be truly random as a big high or a string of highs will automatically adjust the bias to produce more lows to compensate. You will see the time constant of the AC coupling in your assumed-random data.

 

[Jeff Rosenbury]

There's a difference between random noise and white noise and uncorrelated noise. If you are looking to do encryption, you "need" the third. (Technically white noise is uncorrelated, but that is by definition. Proving any supposed white noise is indeed uncorrelated would be a job.)

As I understand the encryption problem, even a tiny bit of correlation effectively cuts your key length in half. One solution would be to double the key length.

OTOH, if you are trying for a one time pad (IMO the way to go), you can't have any correlation.

I have serious doubts about all commercial encryption systems. Anyone who cracks such systems has every motivation to keep that fact a secret.

 

[Baluncore]

I have serious doubts about all commercial encryption systems. Anyone who cracks such systems has every motivation to keep that fact a secret.

Autocorrelation is not important when the PRBS covers all states and is longer than a human lifetime. The weakness of cryptosystems has been in the human component ever since the advent of multiple parallel PRBSs that are XORed together, all with long and different repeat times.

 

[Jeff Rosenbury]

Autocorrelation is not important when the PRBS covers all states and is longer than a human lifetime. The weakness of cryptosystems has been in the human component ever since the advent of multiple parallel PRBSs that are XORed together, all with long and different repeat times.

What you say is completely true -- unless someone smarter than you comes along.

 

[nsaspook]

What you say is completely true -- unless someone smarter than you comes along.

Some very smart people have been using LSFR keystream generator systems for a very long time.
https://www.nsa.gov/about/_files/cryptologic_heritage/publications/misc/tsec_kw26.pdf

 

[Baluncore]

We are coming up to the centenary of the Vernam patent of 1919. http://en.wikipedia.org/wiki/Gilbert_Vernam

 

[Jeff Rosenbury]

I once broke the RSA system. (I found a way to trade memory for execution speed in a brute force attack. It only worked for short key lengths.) I approached the company; they weren't interested. I approached the NSA; they weren't interested. I couldn't find any ethical way to use this hack. So I dropped it.

My point is that no one wants to fix a broken encryption system. The people using it don't want to spend the money and effort to find a better system. The company which sells the broken system prefers no one know it is broken. And anyone who breaks it wants to keep the break a secret (I'm looking at you NSA). So I expect most of the systems sold have been broken (more than once), not because they are bad systems, but because no one wants to deal with breaks.

I do agree that most breaks are human breaks or partial breaks (like mine), but so what? If no one is motivated to fix the systems, they are still broken.

 

[Tom_K]

The magnet experiment, and some other actions suggested by contributors:-

The tin IS magnetic, attracting a magnet well
The walls are 0.25mm thick (including paint)
The inside of the tin is unpainted

This is the tin and radio (should have posted a photo earlier )

View attachment 84698
http://argos.scene7.com/is/image/Argos/5003820_R_Z002A_UC1051961?$TMB$&wid=312&hei=312

I grounded the tin to the Earth wire from a UK mains electrical socket, and repeated the reception test. I filed off the paint where the wire touched the tin. Results:-
FM: Total static. No discernible change with /without Earth wire. No Gaga.
AM: Tinny but recognisable radio station. No, repeat no discernible change in pitch or volume with /without Earth wire. Surprised.

To eliminate a possible entry around the lid you can try putting some copper tape around the lid seam.

Another possibility is the AM signal is not going through the tin, but is being re-radiated inside the box, with the tin box acting as a passive repeater. Nothing much you can do about that, I think.

Have you tried removing the AM antenna from the radio?

 

[nsaspook]

To eliminate a possible entry around the lid you can try putting some copper tape around the lid seam.

Another possibility is the AM signal is not going through the tin, but is being re-radiated inside the box, with the tin box acting as a passive repeater. Nothing much you can do about that, I think.

Have you tried removing the AM antenna from the radio?

It's not so much it's being re-radiated. Even if the box had perfect E field shielding the H field would still not be totally excluded by absorption or reflection from skin-effect due to the low relative permeability and low cross-section of the metal at 1mhz. The actual loss might be much less than a typical skin-effect loss calculation. Usually the internal ferrite loop antenna (that's sensitive to the magnetic field component of the AM signal) is part of the frequency tuning circuit in a small AM radio so it can't be removed.

 

[Paul Uszak]

Audacity (http://audacity.sourceforge.net/) is available for Linux & Windows. It's free. It can perform frequency analysis, autocorrelation and plot a spectrogram.

 

[Paul Uszak]

I don't really have any copper tape to hand for this unfortunately. And, the AM aerial is built into the radio so can't be removed.

I think that we have to conclude that I'll find it difficult to effectively screen an AM radio. I'll have to restrict my entropy source experiments to FM frequencies only. That should be all right. I can procure 1mm sheet copper so I can solder up a better box.

 

[Baluncore]

I think that we have to conclude that I'll find it difficult to effectively screen an AM radio. I'll have to restrict my entropy source experiments to FM frequencies only.

How are you going to get the power into the box and the audio electrical noise out of the box without letting RF in ?

 

[Paul Uszak]

How are you going to get the power into the box and the audio electrical noise out of the box without letting RF in ?

To be honest, I don't know exactly but have some ideas and a blind faith that this is a common surmountable scenario in electronics .

I also don't know whether this should be broken off into another PF question (I'm a Newbie).

In the case that moderators judge this thread continue... I believe that there is an opening to frequency ratio that governs the maximum allowable aperture through an EMI shield. I intend to exit the audio with a shielded cable and metal jack plug. 5VDC power will come from a PC but it's case will have been compromised with a large (300mm diameter) hole in one side; it's a modded PC case. I can shield that cable too. I'm assuming that it's only frequencies around 100MHz that the radio can decode in FM mode, thus other spectra are largely irrelevant.

My knowledge peters out at this point. I need advice as to bonding the copper box, PC case, 0VDC rail, audio cable and power cable shields. There's a lot of literature that I've not yet assimilated, or indeed understood. The fact that drives me is that the static audio need not be white (or any other shade) noise, and some interference is acceptable. The absolute and defining requirement is that the noise waveform in its entirety be unpredictable over a continuous sample period such as 1 second. It mustn't be all the same output. The waveform can be any shape. It would actually be fine if it was a majority of 50Hz mains hum, as long as there was an unpredictable component superimposed on top that can be identified with digitisation (16 bit resolution).

I'd like to explore my box's construction further, perhaps as another question..?

 

[Baluncore]

I do not like your chances of success with a radio in a box.

meBigGuy gave a circuit in post #28 that would do the job. The great advantage is that it has no antenna.
It will need a comparator to generate a sequence of binary data bits.

In my opinion, with hardware you are better going for a long pseudo random binary sequence which does not need an AC coupled comparator.

A pseudo random binary sequence can also be generated by software. Most language packages have an excellent random number generator, usually far better than any electronic noise source. Those code generators are available free on the web, you only need to select an initial key to get a random sequence.

 

[nsaspook]

To be honest, I don't know exactly but have some ideas and a blind faith that this is a common surmountable scenario in electronics .

Use a coaxial capacitor feed-through for the power and audio signals on the shield.
http://eis.apitech.com/docs/resin-sealed-bolt-in-filters.pdf

A simple noise diode circuit might be easier.
http://www.maximintegrated.com/en/app-notes/index.mvp/id/3469

 

[meBigGuy]

I'll say it again, loudly --- A RADIO IS A TERRIBLE SOURCE FOR RANDOM NOISE (in general, and also for the reasons I stated). If you do not look at the noise source's spectrum and autocorrelation and do what ever is needed to make it clean, you are not doing your job. PERIOD! You are totally guessing, and are likely wrong in a big way.

The simple circuits I posted, the single chip digital pseudo random generator, and many more google-findable noise sources are all wiser choices, simpler, cheaper and probably outperform anything you can do with a radio.

A radio is a complex design with unintended feedback, power supply noise, many components with possibly long term correlated effects. The list goes on and on.

I really don't understand why you are so fixated on the radio and blind to the superior results of nearly any credible design.

 

[Tom_K]

I'll say it again, loudly --- A RADIO IS A TERRIBLE SOURCE FOR RANDOM NOISE (in general, and also for the reasons I stated). If you do not look at the noise source's spectrum and autocorrelation and do what ever is needed to make it clean, you are not doing your job. PERIOD! You are totally guessing, and are likely wrong in a big way.

The simple circuits I posted, the single chip digital pseudo random generator, and many more google-findable noise sources are all wiser choices, simpler, cheaper and probably outperform anything you can do with a radio.

A radio is a complex design with unintended feedback, power supply noise, many components with possibly long term correlated effects. The list goes on and on.

I really don't understand why you are so fixated on the radio and blind to the superior results of nearly any credible design.

I agree. I lost track of what the experiment was all about. Initially, I thought it was all about shielding RF, and why an AM radio signal was harder to shield than FM.

If it is only about generating random noise in the audio range, he can probably do no better than what is available here.

And everything you want to know about noise and noise measurements is here.

 

[arydberg]

The magnet experiment, and some other actions suggested by contributors:-

The tin IS magnetic, attracting a magnet well
The walls are 0.25mm thick (including paint)
The inside of the tin is unpainted

This is the tin and radio (should have posted a photo earlier )

View attachment 84698
http://argos.scene7.com/is/image/Argos/5003820_R_Z002A_UC1051961?$TMB$&wid=312&hei=312

I grounded the tin to the Earth wire from a UK mains electrical socket, and repeated the reception test. I filed off the paint where the wire touched the tin. Results:-
FM: Total static. No discernible change with /without Earth wire. No Gaga.
AM: Tinny but recognisable radio station. No, repeat no discernible change in pitch or volume with /without Earth wire. Surprised.

Tell us more about the wire that goes into the box.

 

[arydberg]

Tell us more about the wire that goes into the box.

The wire is shown in the picture you posted it looks like flat flex cable of 2 or 3 or 4 wires. Is it used for a speaker or power? it may be acting as an antenna and bringing signals into the box.

 

[Paul Uszak]

Hiss

I lost track of what the experiment was all about.

Sorry about that. Two of my questions were merged together for me at the start and thus unfortunately two threads run throughout this post (Hiss & shielding)

With respect, I'm not blind to alternative sources of entropy, and I'm quite familiar with their performance characteristics. I am committed to hardware generation, not pseudo random. I like the idea of using a radio. After all fish tanks, lava lamps and typing have been used before very successfully. One of the two popular random generation sites uses radios...

An earlier contributor suggested that the hiss was created by thermal noise in the the initial stages of the receiver. Is there any evidence to refute this claim?

 

[Paul Uszak]

Shielding

The cable is a standard PC power lead with the computer connector end cut off. It's plugged into a power socket and I've bared and trapped the Earth conductor under the tin's lid. I filed off the paint on the side of the tin to ensure a good Earth contact. The Earth pin works in the power socket. There's no speaker cable, I can just hear the sound from the radio's in built speaker.

 

[meBigGuy]

An earlier contributor suggested that the hiss was created by thermal noise in the the initial stages of the receiver. Is there any evidence to refute this claim?

There is no evidence to support that claim, especially in your specific receiver. Also, others have said that discriminator output is NOT random. (I can't speak to evidence to support that claim). There could easily be (and probably are) design dependent characteristics of the radio that have correlative effects. You need to measure and analyze the noise. To do otherwise is poor engineering.

 

[Baluncore]

An earlier contributor suggested that the hiss was created by thermal noise in the the initial stages of the receiver. Is there any evidence to refute this claim?

The noise floor is Johnson Noise. https://en.wikipedia.org/wiki/Johnson–Nyquist_noise
But if you have an antenna on the radio it will also have man made noise (QRM) and natural noise (QRN) such as lightning crashes added.

The amplitude of Johnson noise is a function of temperature and bandwidth.
What data rate in bits per second do you need from your random number generator ?

 

[arydberg]

Shielding

The cable is a standard PC power lead with the computer connector end cut off. It's plugged into a power socket and I've bared and trapped the Earth conductor under the tin's lid. I filed off the paint on the side of the tin to ensure a good Earth contact. The Earth pin works in the power socket. There's no speaker cable, I can just hear the sound from the radio's in built speaker.

Do you power the radio with the power lead? If so this is what is carrying the signal into the box. Repeat the experiment with a battery powered radio.

 

[meBigGuy]

There are all sorts of ways for non-thermal noise effects to occur in a radio-in-a-box. The RF amplifiers (least likely), the synthesizer, the mixer, the IF amplifiers, the discriminator (in FM) can all have issues. In addition to that is the likelihood (hi-probability) of low level RF signals leaking in. Maybe the audio amplifier noise gets on the power supply and creates phase jitter in the synthesizer. Especially in modern single IC AM/FM radios. Maybe a missing bypass capacitor that saved the manufacturer a tenth of a penny. A crystal that is sensitive to mechanical vibration. The list goes on and on. If you are building a toy so you can say you did it, then fine. Go for it.