[TowerTalk] RF Below S=0

[Jim Reid]

Ok,  you guys forced me to get out my step attenuator
and do some experimenting!

All the following on 14047.5,  W1AW CW signal between
2100 and 2130 UTC,  July 20th,  as rcvd on the Island of
Kauai.  W1AW seems to have gone QRT just after 2130,
at least I can no longer hear them,  thought there are
other CW signals on the band near by.

W1AW this morning is managing only an S=0 signal
out here this morning.  Now what does that mean?
Only that the AGC circuit in my FT-1000D does not
detect sufficient signal to produce a negative feed
back voltage,  so the voltmeter(S meter) reads zero.

But,  with Tom,  W8JI's,  designed MFJ-762 step
attenuator I can insert 25 to 30 dB of loss in the line
feeding the rig -- and I can still hear and easily copy
the W1AW signal!  With the S meter still reading zero,
I can occasionally just hear the CW signal with 35
dB of added loss!  Now this is using the 500 Hz stock
Yaesu CW filter.  If I open the IF band to the stock
2.4 kHz filter,  the S meter voltage still reads zero.
These with the FT-1000D.

Now I live on a relatively small island out here near
mid-Pacific,  so,  yes,  background noise is always
quite low.

With W1AW attenuated to just readabiltiy,  but still down
some 25 dB below zero AGC voltage (S meter = 0),  I
tried experimenting with audible deltas a 1, 2, and 3 dB
signal change made;  that is,  clicked in these added dB
amounts to push the signal down closer to 30 below S0.
Incidently,  I also tried comparing the percieved audio
noise level when the meter was reading 0 to the same
audio sound when the antenna input to the rig was
grounded.  Required about 30 db of attenuation in the
step attenuator to reduce to the same level as grounding
the inpur antenna jack on the rig -- that is when you hear
the rig generated noise.  So here on 20 meters,  my rig
noise is about 30 db down from S = 0;  another interesting
point.

I could not hear real amplitude changes much less that
an added 5 dB;  a 5 dB delta when already 25 dB down
below S0 I could hear,  but tough for smaller increments
to hear the difference.  Signal would usually become
inaudible below 30 db down,  but some times I could
still read it even 35 dB below S0!

Interesting stuff,  not sure how meaningful.  Except if
the noise level is quite low,  you can work CW stations
down at least 25 dB below the level where your rig
stops developing an AGC negative feedback voltage.

73,  Jim,  KH7M


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