As Joe-W4TV nicely explained, digital modes excel due to occupying
less bandwidth which also reduces noise bandwidth. There is some
"high-tech" coding that adds to the overall sensitivity of the
modes. CW eme operators are said to be able to reduce bandwidth "in
their heads" to 50-Hz. When I ran CW eme, I found setting my radio
to 100 to 200 Hz worked best for me. 50-Hz DSP filter caused too
much ringing for me to discern the CW note.
Radio sensitivity requirements are mostly set by band noise whose
minimum is established by "celestial" (or sky noise). Such noise is
commonly characterized as applicable sky noise temperature (in
Kevin). Tsky (144-MHz) is thought to be about 250K. At 432 that
lowers to 70K and above 1000 MHz approx 10K.
Receiver sensitivity is tied to noise figure (which also can be
thought of as a temperature (Trx).
Overall receiving sensitivity Te = Tsky + Trx + Tant
The last factor, Tant mostly refers to how much noise the antenna
sees. Earth at 70F is 290K. So if your antenna sidelobes see the
earth, that adds to minimum sensitivity one can achieve. A typical
144-MHz eme receiving system noise temp: Te = 250K + 70K + 29K =
349K. Trx=70 is roughly a noise figure of 0.5 dB.
As one goes higher in frequency, sky noise is less so one wants the
receiver to be less, to improve overall sensitivity.
But as one goes lower in frequency sky noise rises a lot. Tsky
(50-MHz) is roughly 2000K and Tsky (28-MHz) is 5000K (or more).
Making a HF receiver super low noise (low noise figure and thus more
sensitive) is severely limited by Tsky (which is in 10,000K to 100,000K).
And note that I did not add any factor for human generated noise
sources. Te = Tsky + Trx + Tant + Tman-made
Sensitivity is measured in signal power which is related to system
noise temperature b the formula: Pn = KTB.
K is Botlzmanns constant. T is Te derived above. And B is detection
bandwidth in Hz.
If noise power, Pn is in terms of dBm, then Pn = -198.6 + 10Log(Te) + 10Log(B)
SNR = Ps - Pn, where Ps is signal power in dBm. SNR=0 is at the
noise level (where Ps = Pn).
K3 (with PR6) is spec at Pn = -143 dBm at B=500,000 Hz which is very
sensitive. That level would only make a difference on 10m or 6m due
to lower sky noise.
73, Ed - KL7UW
http://www.kl7uw.com
Dubus-NA Business mail:
[email protected]
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