From: Chris Trayner <C.Trayner@leeds.ac.uk>
To: "rsgb_lf_group@blacksheep.org" <rsgb_lf_group@blacksheep.org>
Date: Tue, 1 Jun 2010 16:05:33 +0100
Thread-Topic: LF: QRO / QRP
Thread-Index: AcsBm+LKRm60nwtrS6+q1LmhxTMgdg==
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Hi Hugh,

Thanks for the question - it's an excuse not to settle down to the marking=
 again ;-)

>  there is a lower=20
> level of QRP which cannot be received by ANY receiver, no matter how wel=
l=20
> endowed with antennas and low noise amplifiers - simply because it is le=
ss=20
> than 1 photon.
> I wonder how small it is ?

The energy of a photon is E =3D h f
where h is Planck's constant, 6.626 . 10^-34 Js
and f is frequency, Hz.
(Physicists call frequency nu, but that is harder to typeset than f.)

Thus the energy of a photon at 136kHz is about 9 . 10^-29 Joules.

To get a power in watts you have to decide how often you are going to emit=
 those photons - you could emit one every few days and get an arbitrarily=
 low power.
To get some figure, suppose we emit one every cycle, i.e. 136k of them per=
 second.
Then the power is P =3D E / T (T the period of the cycle), i.e. P =3D E f
For 136kHz this gives a power of 1.23 . 10^-23 W, i.e. 12.3 . 10^-24, i.e=
 12.3 pico-pico-watts.

If your transmitter was only one million-million-millionth of a percent ef=
ficient, you could power it off the dynamic microphone you spoke into.


Ho, hum, back to the real world of marking.
73, Chris G4OKW

-----------------------
Dr Chris Trayner
School of Electronic & Electrical Engineering,
The University of Leeds,
Leeds LS2 9JT, United Kingdom
Tel: +44 113 34 32053
Fax: +44 113 34 32032