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From: dww@stl.UUCP (David Wright)
Newsgroups: net.physics
Subject: Re: freq. spectrum
Message-ID: <199@stl.UUCP>
Date: Fri, 26-Jul-85 19:04:37 EDT
Article-I.D.: stl.199
Posted: Fri Jul 26 19:04:37 1985
Date-Received: Tue, 30-Jul-85 06:32:56 EDT
References: <99@biomed.UUCP> <232@unccvax.UUCP>
Reply-To: dww@stl.UUCP (David Wright)
Organization: STL,Harlow,UK.
Lines: 22

<...eater...>

Re the discussion on how low can an AC signal go and still be radiated - no
20kHz is not the lowest ... here is an experiment:

Take a hi-fi amplifier with a sensitive input.  Connect a piece of wire to 
said input - a metre or two will do.  Measure frequency of hum at output.
Result = 60Hz	==> you are probably in some reasonably advanced country (e.g.
                            the USA) outside Europe and are picking up RADIATION
			    at 60Hz from house and/or local grid wiring
Result = 50Hz   ==> Ditto but in Europe
Result = 440Hz  ==> Are you in an aircraft or what??
No result          ==> You must be in the middle of the Sahara or Antartica or
                            somewhere ....

This is why audio wiring for low level signals uses screened cable.  It is true
that this is also needed to prevent induced mains hum, and that hum can also
come from ground loops, but there's plenty of low-level radiation at 50/60 Hz!

[I thought everybody knew this, but as the discussion's been going on about
 whether 10 or 20 or 50kHz was too low I've added my bit ...  and yes there
 is NO frequency so low that you cannot radiate it given sufficient power]