Topband: Re: Minimum discernible signal ?

[i4jmy@iol.it]

W8JI:
 >My 160 meter noise floor, after pre-
 >amplifiers, on a quiet winter night when the band is open to Europe
 >is about -117 dBm measured at 3.5 kHz bandwidth. With a 350 Hz
 >bandwidth, the "off-the-air noise floor" of my system is -127 dBm.
 >With a 35 Hz filter it is -137 dBm.

S56A: 
> I stated long ago that the claims of nV sensitivity for 160m RX is 
excessive.
> We can now all see that 20 dB attenuation is optimal on any 160m SSB 
>RX.

Hi Mario (S56A),

unluckily we can't agree just that 20db of extra attenuation is optimal.
Average receiver sensitivity is probably unnecessary and S/N remains 
good even attenuating 20 dB if the BW and the antenna used are 
equivalent to those of the example. 
When the antennas are quite marginal in terms of efficiency (like small 
wire loops resistively terminated), the receiver ground floor is quite 
comparable with signals (and BW can't be reduced), any extra 
attenuation have an impact on the S/N. 
On the other hand it's true that in most cases very low noise stages 
have a reduced dynamic range and this fact should always suggest not to 
use antennas with fractional efficiency and thus requiring 
extremely "silent" preamplifiers.

73,
Mauri I4JMY


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