dls@euler.Berkeley.EDU (David L. Steere) (11/11/89)
I read an intriguing newspaper article yesterday, and thought that it might make an interesting topic for this newsgroup to discuss. It concerned the undesired reception by people in the Los Angeles area of a radio station's transmissions in the nearby area. Apparently, there is a strong local station with a narrow transmission pattern, and as a result, people are apparently finding that they are picking up the broadcast quite clearly on a variety of items around the house, including telephones, stereo systems, and even the house water pipe systems (!). Now, maybe the newspaper report was exaggerating, but it seems reasonable that radio broadcasts might indeed be picked up by poorly shielded electronic devices, whose internal circuitry might provide the detection/demodulation functions of a simple crystal radio, resulting in audible evidence of the radio broadcast in a variety of unwanted areas. Apparently, the local phone company has been inundated with complaints, and their response has been that the radio station's transmission adheres to FCC regulations, but that the various (cheap) telephones are not properly shielded. Can knowledgeable readers comment on the exact mechanism of this pickup, and offer examples from their own experience? And, how on earth would a water pipe system pick up and render audible such radio transmissions? The article explicitly mentioned someone who claimed that they were hearing the radio station while seated on their toilet. Since the station was broadcasting "rap" music, one can easily "c" what the end result of this would be...
wb8foz@mthvax.cs.miami.edu (David Lesher) (11/11/89)
#
# I read an intriguing newspaper article yesterday,
[]
# concerned the undesired reception by people in the Los Angeles area of
# a radio station's transmissions in the nearby area.
Ah, the old WLW syndrome..
{warning, third hand story follows. Those sensitive to such may
wish to remove young childern from the area ;-]}
Back before there was an FCC to set the rules, WLW in
Cincinnati used to run a bit more power..... about 500 kw.
Locals will tell you that people with tin roofs heard them. Farmers
out working on barbed wire fences heard them. (Those who grabbed long
wire fences often felt the burns too!) Cars talked. Some dentists found
out those patients hearing voices in their heads were NOT crazy. Then
alas, Frank's Cookie Company came along and made them use only 50 kw.
Lots of problems disappeared.
But the fact of the matter is, all it takes to detect AM is a diode.
ANY junction of metal, with a little corrosion will do. If the
junction is the bridgework in a mouth, with a little acid salivia, even
better! Manufactured diodes are best. The ones in your phone network
work real well, they have several miles of antenna.
Oh, BTW, rumor has it WLW still has the old XMTR, just waiting
for the rules to change again.....
--
A host is a host & from coast to coast...wb8foz@mthvax.cs.miami.edu
no one will talk to a host that's close..............(305) 255-RTFM
Unless the host (that isn't close)......................pob 570-335
is busy, hung or dead....................................33257-0335greg@sj.ate.slb.com (Greg Wageman) (11/14/89)
Opinions expressed are the responsibility of the author. In article <1044@umigw.MIAMI.EDU> wb8foz@Mthvax.Miami.Edu (David Lesher) writes: ># I read an intriguing newspaper article yesterday, >[] ># concerned the undesired reception by people in the Los Angeles area of ># a radio station's transmissions in the nearby area. > >Locals will tell you that people with tin roofs heard them. Farmers >out working on barbed wire fences heard them. (Those who grabbed long >wire fences often felt the burns too!) Cars talked. Some dentists found >out those patients hearing voices in their heads were NOT crazy. > >But the fact of the matter is, all it takes to detect AM is a diode. Yeah, but that doesn't answer the original question, which was: how do the electrical signals get transduced into audible vibrations? Why does that pulsing DC in the tin roof make it vibrate? I am familiar with the piezoelectric effect, but how many common materials actually exhibit it? Or is it the earth's magnetic field interacting with the induced field from the pulsing DC? Or is this another urban (rural?) myth? Copyright 1989 Greg Wageman DOMAIN: greg@sj.ate.slb.com Schlumberger Technologies UUCP: {uunet,decwrl,amdahl}!sjsca4!greg San Jose, CA 95110-1397 BIX: gwage CIS: 74016,352 GEnie: G.WAGEMAN Permission granted for not-for-profit reproduction only.
pfluegerm@valley.UUCP (Mike Pflueger) (11/15/89)
In article <19422@pasteur.Berkeley.EDU>, dls@euler.Berkeley.EDU (David L. Steere) writes: > > I read an intriguing newspaper article yesterday, and thought that it [stuff deleted] > a radio station's transmissions in the nearby area. Apparently, there > is a strong local station with a narrow transmission pattern, and as a > result, people are apparently finding that they are picking up the > broadcast quite clearly on a variety of items around the house, > including telephones, stereo systems, and even the house water pipe > systems (!). Now, maybe the newspaper report was exaggerating, but it [more stuff deleted] > telephones are not properly shielded. Can knowledgeable readers > comment on the exact mechanism of this pickup, and offer examples from > their own experience? And, how on earth would a water pipe system pick > up and render audible such radio transmissions? The article explicitly Yes, this can and does happen. The process is essentially the same as that used for the "original" radio receivers, known as crystal radios. To detect radio frequency (RF) energy, a "non-linear" device is required. Linear devices are resistors, capacitors, and inductors. Non-linear devices are devices like diodes and transistors. In modern radios, a diode is usually used, but a transistor can also work. In the simplest receivers, with a very strong signal that has been rectified, the only other thing that is necessary to hear the signal is something to convert the electrical energy to air compression (sound waves). Tuning circuitry is not necessary (and generally ineffective) in these instances because the RF is so strong. In olden days, "cat whisker" detectors were used since diodes were unknown. These consisted of a fine wire which "tickled" a piece of metal or crystal. The metal or crystal surface would have a layer of oxidation, and with the loose contact, would act as a rectifier. The wire was adjusted to find a point which maximize the rectification. Virtually any metal-metal or metal-semiconductor junction which is in loose contact can rectify RF. This explains the plumbing situation - poorly connected joints rectify the strong RF, and there is enough energy to vibrate the pipes. Poorly shielded electronics (designed for other purposes) can rectify the RF in a transistor or diode and amplify the audio in its own amplifier circuits. In an attempt to make modern electronics cheaper (more "affordable"), most manufacturers do not properly shield consumer products. This is a major headache to hams (like me) because we are often blamed for such problems, although the problem is with the unshielded product. You can see good shielding when you look at electronics, if you can peek inside and see the electronics encased in metal boxes, and if the entire package is inside a metal box. Generally, the more the better. And this is usually a sign of a product with high overall quality. I always pick my stereo equipment this way. You also need to shield the inputs and outputs to such electronic devices. Shielded wiring should always be used, and internally, capacitors can be used to bypass (short out, in parallel circuits) the RF coming in on a wire, and ferrite beads (chokes, in series circuits) can also filter out the RF (I'm an EE also). BTW, strong RF fields will even make flourescent lights light, even if just loose (i.e. not installed). I heard a story (don't know if its true or not, but it could be) about someone who lived near a radio or TV transmitter who had to put all the flourescent lights in a closet at night! IMHO, the FCC or EIA is going to have to establish requirements for proper shielding, because consumers don't understand what to look for and avoid in such cheap electronics, or who to blame when problems occur. And they're occurring more frequently with the boom in RF sources (cable tv, CB (legal or not), cellular phones, portable phones, PC's, garage door openers...) and cheap electronic products. A side effect of the cheap transmitters (portable phones, baby monitors, garage door openers...) is that they also are rich in harmonics (PC's too) which means they generate RF on frequencies they're not supposed to, interfering with devices on those frequencies. Ooops... I'm rambling. -- Mike Pflueger @ AG Communication Systems (formerly GTE Comm. Sys.), Phoenix, AZ UUCP: {...!ames!ncar!noao!asuvax | uunet!zardoz!hrc | att}!gtephx!pfluegerm Work: 602-582-7049 FAX: 602-581-4850 Home: 602-439-1978 Packet: WD8KPZ @ W1FJI
yahoo@unix.cis.pitt.edu (Kenneth L Moore) (11/15/89)
In article <1989Nov13.191239.25082@sj.ate.slb.com> greg@sj.ate.slb.com (Greg Wageman) writes: >Opinions expressed are the responsibility of the author. > >In article <1044@umigw.MIAMI.EDU> wb8foz@Mthvax.Miami.Edu (David Lesher) writes: >Why does that pulsing DC in the tin roof make it vibrate? I am >familiar with the piezoelectric effect, but how many common materials >actually exhibit it? > >Copyright 1989 Greg Wageman DOMAIN: greg@sj.ate.slb.com I am a retired steel worker and have seen a coil of very heavy wire (50-60 lbs) attached to a welding electrode jump every time the welder drew an arc. In this case I think that the explaination is that the coil of wire created an electromagnet which interacted with the current flowing to the electrode. I don't know about flat plates of tin. -- yahoo@unix.cis.pitt.edu (Kenneth L Moore)