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

[Bob Sciamanda <Sciamanda@worldnet.att.net>]

LUDWIK KOWALSKI wrote:
> On 4/21/97 Bob Sciamanda wrote:
> > . . .
> >         E(r) = {Io*A*w^2*sin(theta)}/4*Pi*epsilon*c^3*r
> > . . .

> Are you sure it is not cos(theta) which would mean maximum E along the
> axis? . . . 

Sin(theta) is the correct variation; the field has a null along the
normal 
to the plane of the coil. (On this normal the vector potential A(r)
cancels 
to zero by symmetry; then E=-dA/dt.)
> > Don't under-estimate the sensitivity of (even simple) radio receivers.
> > The specifications of my (cheap Radio Shack) stereo claim a sensitivity
> > of 900 microvolts/meter for the AM tuner.
>
> Do you mean that E<0.9 mV/m would be below the noise level? No matter how
> sensitive a detector may be it is very hard to recognize a signal which is
> below the noise level. By the way, my cheap AM radio was tuned on a station
> near 550 KHz while the natural f (for L=0.0028 mH and 3000 pF) is 1.74 MHz.
> Tunning the receiver on a station closer to 1.74 MHz would be more logical
> but I did not think about this.
>                                                       Ludwik Kowalski

This specification is usually taken to mean that a noise free signal of 
amplitude .9mV/m (at the receiver's built in antenna) will give a signal
to 
noise ratio of the order of 20 db. Thus this is a characterization of
both 
gain and internal receiver noise. If the signal is swamped in external
noise 
(or a competing signal) of comparable strength then all bets are off!

Your "rig" is putting out a broad band signal; I don't expect much of a 
tuning effect over a broad range of the receiver "dial"!

Yes, I specified that the E(r) which I gave you was the 1/r "radiation
field" 
the only term which "propagates to infinity".  The point is that if the
numbers 
show this field to be detectable by your radio, then "a fortiori" the
total 
field should be detectable at near points (avoiding interference nulls).

P.S.
	You said that you were "holding" the loop. If there is significant 
capacitive coupling between you and the loop, you become part of the
antenna!
Notice how an AM radio often receives weak signals better when a person
places 
a hand near its loop antenna.
-- 
Bob Sciamanda		sciamanda@edinboro.edu
Dept of Physics		sciamanda@worldnet.att.net
Edinboro Univ of PA	http://www.edinboro.edu/~sciamanda/home.html
Edinboro, PA		(814)838-7185