[OKDXA] Radios and noise

[Roger Simpson]

Gentlemen:

Thanks for your replies regarding S Meter Readings on CW vs. SSB.  I 
never thought of this before but I guess you are saying that assuming 
the signal level for every 10Hz slot from 7,100,000 Hz  up to 7,101,000 
Hz was exactly -60 Dbm then the way receivers are designed then if your 
total bandwidth is say 2.4Khz then you are going to hear more noise out 
of the speaker than you would if you have a 500Hz bandwidth -- given 
that the background noise on the band is the "same" in any given 10Hz 
slot.

I guess I was a little slow to the draw but I did not realize that the S 
meter was reporting on the aggregate signal that is in the bandwidth -- 
not just the strongest signal.  I guess another way to say this is that 
the S-meter reading relates to how loud the noise is that is heard on 
the speaker -- assuming that the audio level "volume control" is not 
changed as you change modes / filters.

I guess I've been living in a cave over here. I just did an experiment 
on CW where everything stays exactly the same except I switch from a 
250Hz filter, to a 500Hz filter, and then a 1200Hz filter. Lo and behold 
the S-meter changes. So the S-meter doesn't really have a DIRECT 
correlation to the noise level going on in the band. So I can't just say 
I've got a high noise level [or a low noise level] on 40m right now 
because my S meter says some given reading.

Implicit in this is that if W5XYZ says "the noise level on 40m CW right 
now is S9" and W5ABC says "over here at my QTH the noise level on 40CW 
is S6" it could be that they are both seeing the same thing. We don't 
know unless we know what the bandwidth is that they are using. Also, of 
course, who is to say that rig manufacturer X and rig manufacturer Y 
have comparable S meter readings even if the mode and bandwidth are the 
same. Also various stations have different antennas. An antenna with a 
40meter 3 element yagi at 100 feet might given a different "noise level 
S meter reading" than a 40m dipole at 40 feet.

I guess it is the margin between the signal to noise in terms of DBs 
that is important -- not some particular noise level S meter reading.

73  Roger  K5RKS



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