On Mon, 15 Nov 2010 17:56:16 -0600, "amdx" wrote:

I can only go by what I have read on the crystal radio groups and they
say 1pw
is audable with very sensitive headphones..

Hi Mike,

You should consider their being very good at this then. 1 pW is the
lowest limit of hearing at 1KHz. No speaker/transducer is 100%
efficient. So, automatically, this claim you have read is suspect in
the highest degree.

Let's work these numbers further. The transducer elements you have
been describing may be sensitive, but that is not the same thing as
efficient. Given that they are ancient magnet and diaphram
constructions, they would be pushing the limits at 10%.

Further, your chosen power level of 1pW would not be heard but in a
very special anechoic chamber, and even then your heart and your
breath would be in competition as QRN. Speach as perceived to be at
normal quiet talking levels would be 40dB more powerful (and, again,
in a quiet surrounding like a library). This would be a normal
expectation of program content sound level.

Being generous (good efficiency and whispers at 1 meter in the
library), I would suspect that your receive power level would be
closer to 1nW and this would be straining things for a real listening
experience.

For sensitivity the starting point has got to be the tank circuit, you
want to build
an inductor with very high Q and then mate that to a good quality
capacitor. A Q of 1000 is possible over much of the AM BCB.
Can we agree on that?

Have you considered what Q=1000 does to the bandwidth of the received
signal?

Ya, it would limit bandwidth. But to much Q is easy to solve and hard to
get.
That is only unloaded Q of the tank, adding the antenna brings that down to
Q=500.
Then comes the load from the detector and then the audio transducer
assembly.

In the middle of the band, it would give you poor telephone audio
quality. However, many hams find it suitable for DX work.


Now you need to couple in energy from an antenna. If this is adjusted for
maximum power transfer, we have reduced the Q by 1/2 or Q=500.
Assuming a 240uh inductor and frequency of 1 Mhz the XL is 1507 ohms,
multiply that by the antenna loaded tank Q of 500 and we have an Rp =
753,500 ohms.
Does that work for you?

Good enough to pitch back and forth, much as your power level.

73's
Richard Clark, KB7QHC