Hi Hugh,

Thanks for the question - it's an excuse not to settle down to the marking 
again ;-)

>  there is a lower 
> level of QRP which cannot be received by ANY receiver, no matter how well 
> endowed with antennas and low noise amplifiers - simply because it is less 
> than 1 photon.
> I wonder how small it is ?

The energy of a photon is E = 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 = E / T (T the period of the cycle), i.e. P = 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 
efficient, 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