[HN Gopher] A scientific curiosity that happened to me
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A scientific curiosity that happened to me
Author : thunderbong
Score : 56 points
Date : 2023-09-29 16:38 UTC (6 hours ago)
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| nuancebydefault wrote:
| Are local radio stations sometimes using A.M.?
| jmholla wrote:
| Why wouldn't they? AM stations still exist.
| nuancebydefault wrote:
| I did not know. Where I live, all are/have to be in the
| 100...108 FM bands.
| jmholla wrote:
| Oh. Interesting. In the US, FM goes down to the high 80s.
| And while I've definitely been in AM dead zones, there's
| usually barely any FM stations in those places too because
| there is no audience.
| renewiltord wrote:
| Interesting. Reminds me of how if you had a Nokia phone of the
| time next to computer speakers you could tell when you were about
| to get a call because the speakers would make a bup-bupbup bup-
| bupbup sound before the phone itself woke up and rang.
|
| But OP has to be AM, no? Somewhat curious.
| geocrasher wrote:
| Indeed, in some cases all you need is an antenna, a semiconductor
| of almost any kind (which can be improvised) and an audio amp. In
| some cases, it's all-in-one. My family used to talk about a brass
| headboard that you could hear Morse code from, coming from the
| local military base.
| 1letterunixname wrote:
| Calling bullshit. Semiconductors cannot be "improvised". PN
| junctions aren't found anywhere accidentally.
|
| And a brass headboard isn't a crystal radio and isn't a
| circuit. A crystal radio needs a diode, a capacitor, a coil, a
| ground, an antenna, and a quartz headphone stuck in your ear.
| These things don't configure themselves by accident. The reason
| analog headphones with a long wire can do this is they have
| almost all of the attributes necessary to pickup AM radio. FM
| radio requires a vastly more complicated circuit.
| dekhn wrote:
| See https://www.americanradiohistory.com/Archive-Electrical-
| Expe... page 325 and 360. Couple of sewing needles across a
| carbon block. He noticed that when he removed the dry
| battery, he got a better signal.
|
| These are things that would have been available to just about
| anybody at the time he did his experiments.
| fortran77 wrote:
| This is extremely misleading. See:
|
| https://en.wikipedia.org/wiki/Crystal_detector
|
| https://en.wikipedia.org/wiki/Foxhole_radio
|
| Flagged.
| floxy wrote:
| You don't need PN junctions for an AM radio. Accidental
| point-contact diodes might be enough nonlinearity (for
| mixing) and rectification.
|
| Here's a fun jumping off point for a deeper dive:
|
| http://www.sparkbangbuzz.com/
| kelseyfrog wrote:
| The stories about hearing radio through bad tooth fillings
| are all untrue?
| slfnflctd wrote:
| I was wondering that too, since I heard/read about it
| several times as a kid.
|
| This thread is actually pretty decent, although it's still
| mostly speculation:
|
| https://www.reddit.com/r/rfelectronics/comments/oh4zpy/how_
| d...
| geocrasher wrote:
| Calling bullshit. Semiconductors cannot be "improvised". PN
| junctions aren't found anywhere accidentally.
|
| https://en.wikipedia.org/wiki/Foxhole_radio
| One common type was made from an oxidized razor blade (rusty
| or flamed) with a pencil lead pressed against the blade with
| a safety pin. The oxide layer on the blade and the point
| contact of the pencil lead form a semiconductor Schottky
| diode and only allow current to pass in one direction.
| 1letterunixname wrote:
| Yep but that doesn't happen accidentally. And good luck
| getting a quartz crystal in a form that can be heard.
| geocrasher wrote:
| Where did anybody say accidentally? I said it could be
| improvised. Are you arguing just to argue?
|
| Edit: I realize that the headboard story implies an
| "accidental" semiconductor. And yes that's totally
| possible.
| fortran77 wrote:
| It could absolutely happen accidentally. Any bad contact
| between two conductors can cause a non-linear junction
| that can detect an AM radio signal.
|
| TFA refers to, presumably an FM signal (because of the
| 3.3 meter band and what's usually broadcast there), so I
| doubt this story a bit.
| geocrasher wrote:
| Yep I mentioned that in a comment below, an accidental
| semiconductor IS possible, but that wasn't my point
| really. I get the arguers point though, a headboard
| implies accidental, I wasn't thinking of it that way. But
| anyway, as we've established, both improvised and
| accidental are possible and documented.
| [deleted]
| atoav wrote:
| You are aware that there are even such things as natural
| nuclear reactors that happen "accidentally"?
|
| So ruling out a simple PN junction is a weird hill to die
| on.
| bisby wrote:
| Certainly a brass headboard doesn't have a semiconductor in
| it. But that doesn't mean the story is bullshit. Just that
| it's not working in the way they think it is.
|
| Heres an example of a thistle being used to convert an AM
| signal into audio. Granted, they are touching the plant
| directly to the broadcast tower, and it's outputting enough
| power that it starts burning and even burns the hand of the
| person holding the plant.
|
| https://www.youtube.com/watch?v=b9UO9tn4MpI
| geocrasher wrote:
| I admit, it's apocryphal. But, if there is a gap anywhere
| in joint in the headboard, and it oxidized, that's likely
| enough. Still, I believe my family. Maybe I have details
| wrong (likely) but it was a headboard and they heard radio
| from it.
| floxy wrote:
| >Certainly a brass headboard doesn't have a semiconductor
| in it.
|
| Are you sure?
|
| http://www.sparkbangbuzz.com/els/photocell-el.htm
|
| https://en.wikipedia.org/wiki/Metal_rectifier
| version_five wrote:
| To receive AM you just need a non linearity and the x^2 term
| iirc will do your rectification. So any element that
| electrically provides some kind of nonlinear behavior (ie is
| not a pure R, L, or C) should give some sort of modulation.
| NotGMan wrote:
| This is basicaly a "radio that doesn't require any batteries".
|
| It's very easy to build one yourself since only ~5 components are
| needed and a long-ish wire.
|
| A variable resistor knob is then used to switch between radio
| stations.
|
| Though it will only catch AM if I'm not mistaken.
| ogig wrote:
| My dad, whose dad had a radio and tv repair shop, got me a kit
| for making one when I was a little kid. I spend the day coiling
| the wire and following the instructions. When I was done and
| the artifact captured radio waves I was so excited! It's one of
| those vivid memories from childhood, a moment that put me on
| the path for a technology focused career.
| dekhn wrote:
| radio is a gateway to abilities that some consider to be
| unnatural
|
| (my grandpa gave me a shortwave and explained I could receive
| far-away signals due the radio signals bouncing off the
| ionosphere. It was my introduction to numbers stations, but I
| never found radio that interesting, when I had the internet
| available)
| sandworm101 wrote:
| The most basic of these are called trench or foxhole radios.
|
| https://en.wikipedia.org/wiki/Foxhole_radio
| supernova87a wrote:
| What's interesting to me is that one particular station came in
| clearly, given how the difference in broadcast FM allocated
| frequencies translates into millimeters of antenna length. (e.g.
| 98.3 vs 98.7 MHz = 3051mm vs. 3039mm)
|
| Just some flexing / coiling of the cord could mean you would
| favor a different station or get tons of mixing in of other
| stations.
|
| Maybe the key is that the author / friend was very close to a
| broadcasting antenna and that overwhelmed any other station that
| would equally have been received on the headphone cord otherwise?
| 1letterunixname wrote:
| Bose QC 35 V1 headphones when charging and used passively
| connected via the headphone jack to an iPad or iPhone also being
| charged from the same source would pickup audible digital bus
| noise of the iDevice. Moving the charging sources to different
| ground planes would solve the problem. It would also happen if
| devices were being charged by different power adapters connected
| to the same neutral or ground. This led me to conclude it was a
| form of ground loop due to a lack of engineering and testing of
| the headphones.
| adhesive_wombat wrote:
| I used to be able to hear my USB mouse scrolling in my
| headphones. It was so annoying I eventually go an external USB
| soundcard to try to decouple it, which worked.
| rdtsc wrote:
| I lived next to an AM station before and could hear their
| broadcasts it in my headphones sometimes.
| 1letterunixname wrote:
| This happened to an engineer where I worked who had an analog
| phone with unshielded, untwisted cord exactly the length of the
| full-wave of the radio station.
|
| It's also worth noting that lightning in the area can induce
| massive voltage and current impulses in long, unshielded,
| straight conductors.
| kemmishtree wrote:
| Some family members of mine were in bed in their house during a
| thunderstorm and during a close lightning strike they heard a few
| seconds of music that sounded like a radio transmission, somehow
| being amplified throughout throughout the house as the lightning
| flashed. I believe their story, and wonder about the mechanism.
| pizzafeelsright wrote:
| https://www.youtube.com/watch?v=uo9nGzIzSPw
| Johnny555 wrote:
| >I found out that the radio station's wavelength she was
| listening to was about 3.3 meters
|
| That's 90MHz, which is in the FM band (at least in the USA). I
| understand (and have experienced) how this works with AM, but how
| does this work with FM? Does some circuitry in the headphone act
| as an FM decoder?
| roamingryan wrote:
| I was left wondering the same.
|
| My best guess would be that slope detection could be converting
| the FM signal into AM. This happens in circuits where the
| response is strongly dependent on frequency (see [1]). A
| specifically designed filter is typically used, but a resonant
| circuit like the headphone wires could do something similar.
| But then you still need to get from AM to audible, and to do
| that you still need some sort of non-linearity in the system.
| Some possibility non-linearies could be a poor connection
| resulting in a whisker-style rectifier junction, or perhaps a
| magnetic permeability non-linearity in the headphone speakers.
| I'm sure there are others.
|
| Related, but probably not relevant: Stereo FM broadcast signals
| in most parts (all parts?) of the world also contain an AM
| subcarrier. The primary FM carrier has the L+R audio content
| and is used by both monaural and stereo receivers. Stereo
| receivers make use of an additional AM modulated sub-carrier
| (+/- 38 kHz from primary carrier) to obtain L-R audio content.
| I don't believe this AM carrier is directly rectifiable because
| it does not manifest as envelope modulation in the full signal.
|
| [1] https://www.electronics-
| notes.com/articles/radio/modulation/...
| jjoonathan wrote:
| Slope conversion is the hard part, once it's AM just about
| any energy conversion will do. abs(x) and |x|^2 are plenty
| nonlinear.
|
| https://youtu.be/uo9nGzIzSPw?t=19
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(page generated 2023-09-29 23:01 UTC)