[Elecraft] Sensitivity - Was K4 Observations

[Edward R Cole]

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
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