"Jeff Liebermann" wrote in message
...

On Thu, 31 Jul 2014 21:29:11 -0400, rickman wrote:

I would suggest you check again. Receivers aren't that sensitive. Most
unlicensed transmitters are in the 100-500 mw range, and have a coverage
of maybe 100 feet. And picowatts aren't even worth discussing.

I'm unclear, is mW microwatts or milliwatts as you wrote it? The reason
I ask is that a 500 milliwatt transmitter would certainly have a
receivable distance much greater than 100 feet, no?

He's right. The receivers aren't that sensitive. For example, I just
excavated the Silicon Labs Si470-31 data sheet, which shows an FM
sensitivity of 1.1 uV into 50 ohms for a S+N/N ratio of 26dB. The
reason is the wide (200KHz for FM and 500KHz HD Radio) occupied
bandwidths. The bigger the bandwidth, the more noise gets in, and the
lower the sensitivity. Crudely, double the bandwidth and lose -3dB
(i.e. half) in sensitivity.

Much depends on the antennas and the field strength sensitivity. Let's
see what the Friis Equations produces. If I assume a best case of a
2dB gain dipole at the transmitter, but a crude earphone cord antenna
at the receiver (-3dB at best). Then what I get is:
TX 27dBm (500 mw)
TX ant 2dB
path loss ????
RX ant -3dB
RX sens -107dBm
Path Loss = 27 + 2 -3 -107 = 81 dB
Plugging into:
http://www.proxim.com/products/knowledge-center/calculations/calculations-free-space-loss
I get 1.7 miles at 100 MHz. Looks like it should work.

Things get messy when I run the numbers again with the typical
receiver sensitivity found in analog receivers. These have typical
sensitivity of 9 dBf. That's dB(Femtowatts) or 10^-15 watts
reference. Converting to a milliwatts reference, that's:
9*10^-15 watts / 1*10^-3 watts = 9*10^-12
converting to dB, or 10 log of the ratio:
dB = 10*log(9*10^-12) = 10 * (-11) = -110 dBm
which is allegedly 3dB more sensitive than the all digital chip. I
don't believe it. So, with 3dB less sensitivity, you should get about
half the range or 0.85 miles.
================================================== =====

When I was in the Navy, I was detached from my ship for a short school. I
bought a Radio Shack FM broadcaster that used a 9v battery. A couple of
guys and I fashioned a folded dipole for the FM band as the transmit antenna
and used a BCB radio for the audio input. We pushed the folded dipole a few
feet out a second-story window on a broom handle and proceeded across the
base to see how far we could hear our signal on a portable FM radio.

We went about a 1000 feet before the signal became useless, although there
were some dropouts closer than that. I do not know the output of the device
in milliWatts, but I suspect it is much less than the nominal 100 milliWatts
that limits the no-license operation.

"Sal"