Re: How many joules --> e.m. waves?

[inet@intellisys.net (brian whatcott)]

At 09:53 PM 4/23/97 -0400, Bob Sciamanda wrote:
> LUDWIK KOWALSKI wrote:
> > ...
> > ... Does anybody know what a
> > typical noise level of a radio set is, in term of microwatts?
> >                                                          Ludwik Kowalski
>
> I don't think you're asking the right question.  We are looking at a
> signal source, not a power source.  
> The pertinent parameter is the E and/or H field 
> at the receiver. (Like a high impedance scope measuring V without
> drawing power.) 
> Reminds me: as a kid on a farm in the boonies, I used to get minuscule
> signals  from very distant radio stations to palpably activate the
> headphones of my crystal set (no batteries!).
> -- 
> Bob Sciamanda	

Several interesting points here:
Ludwik is asking about the noise power at a radio receiver.

This is the very same question that radio astronomers ask, though
they often  prefer to express it in terms of noise temperature.
 At low frequencies man-made noise dominates, but at VHF,
thermal noise becomes important, as human noise sources
become quieter.

While it is usual to express the electromagnetic field in terms of
the electric component, the unspoken basis is the unvarying
impedance of free space, close to that of the atmosphere.
 Hence it not strictly correct for Bob to dismiss the electrical power 
consumed by a scope or a radio.

 If we were to measure the electric component of a field traversing
a block of praffin wax, we would not be surprised to see a reduction.
It is noteworthy that dielectric lensing of UHF fields was demonstrated
 very early in  radio research.

Regards
brian whatcott <inet@intellisys.net>
Altus OK