Its fairly easy to hear discrete signals below the noise level of the
receiver. Ears are sometimes marvelous narrow band filters that can
ignore noise and concentrate on signals. Its easier when the noise is
more constant than atmospheric static, such as shot noise from circuits.
At VHF its common to work signals somewhere from a few dB to as much as
30 dB (the golden eared operators) below the noise level. An EME signal
that's consistently equal to the noise gets a 599 signal report!
Remember that noise is a time varying process with much variation in
strength no matter what the time frame of the measurement. Which means
that sometimes though the noise power is consistent measured over the
receiver bandwidth its value is less than at other times. And the
receiver is a linear system so that noise and signal can coexist, noise
does not exclude signal. Noise power competes with the signal, and the
fact that the signal level is more consistent than the noise level helps
in the discrimination process. Also the fact that the spectrum of the
signal is narrow while that of the noise is broad gives another foot up
in the discrimination process. Which can be by ear or by computer these
days.
73, Jerry, K0CQ
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Entire content copyright Dr. Gerald N. Johnson. Reproduction by
permission only.
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