Path: utzoo!attcan!utgpu!jarvis.csri.toronto.edu!mailrus!ncar!asuvax!hrc!valley!pfluegerm
From: pfluegerm@valley.UUCP (Mike Pflueger)
Newsgroups: sci.electronics
Subject: Re: Undesired radio reception: mechanism?
Summary: rectification
Message-ID: <46d5b4ea.15840@valley.UUCP>
Date: 14 Nov 89 17:26:16 GMT
References: <19422@pasteur.Berkeley.EDU>
Distribution: usa
Organization: AG Communication Systems
Lines: 80

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
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