[Elecraft] [K3] dB Changes, Loudness, and Signal to Noise Ratio

[Bill W4ZV]

Kok Chen wrote:
> 
> 
> On Dec 15, 2009, at 9:52 AM, Jim Brown wrote:
> 
>> ...we are working on the signal to noise ratio, NOT LOUDNESS. It's apples
>> and 
>> oranges!  The only thing in common is that we're using a log ratio to 
>> describe it. 
> 
> I couldn't agree more with Jim on this one.  In our "business," we are
> trying to get information across, instead of determining if one signal is
> louder than another.  This is demonstrated especially well with digital
> modes, where we can measure with objective detectors instead of using the
> subjective gray matter between our ears.
> 

There was an interesting series of low power beacon receiving tests
conducted in December 2004 through January 2005 by N2XE.  John sent a beacon
ID message including a unique code word at various power levels on 80m over
a several day period.  You had to correctly copy each code word correctly to
verify reception.  There's a lot of documentation on the Topband and QRP-L
lists over that period, but here's a brief summary of the results:

80 uW - 8 stations correctly copied.
40 uW - 1 station 
27 uW - 1 station
20 uW - 0 stations.

The power difference in dB at each succeeding lower level was (top to
bottom):
 
-3 dB (i.e. the 80 uW to 40 uW test)
-1.7 dB
-1.3 dB

I copied the beacon at both the 40 uW and 27 uW levels but could not
decipher the code word at 20 uW even though I could detect presence of the
signal.  Unfortunately N2XE ran 20 uW only briefly (because at that point
nobody else was hearing it).  I believe I could have copied it if he had run
it as long as he ran the other tests.

The point of this is that small changes in dB do indeed make a difference in
the ear/brain's ability to **copy** weak signals in noise, which is really
what the discussion is about.  Tom W8JI added the following interesting
observation (which I also noticed and agree with):

"Now for the curious effect I observed.

While I didn't spend a large amount of time listening to 80
and I never listened to 40 at all, I did notice one thing
that I found interesting. Listening to the very low power
transmissions on 80 meters I observed a very pronounced
signal peak just before sunrise. The effect was very much
like the effect called "search light" or "spot light"
propagation. While the signal was largely in the noise and
unreadable from 0900 Z  until 1120 Z, I observed a very
clear "strong" peak between 1125 and 1135 Z where the signal
was the equivalent of  about a "559" or better DX report. By
1138 copy was back to nil with an increase that provided a
short 2 minute long readable signal starting at 1143 Z .
After 1145 Z that was it for the signal. I never really
noticed this effect over the same path when power levels
were higher and signal levels stronger, although I'm sure
the path loss went through similar variations. This is why
even one dB sounds like a large change when signals are in
the noise, and is meaningless with "579" signals."

http://lists.contesting.com/archives//html/Topband/2005-01/msg00023.html

73,  Bill



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