Topband: Re: Minimum discernible signal ?

[Tom Rauch]

I wrote:

> > >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.

Mario replies:
 
 > 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.

I know you keep saying that Mario, but the numbers indicate 
otherwise.

My noise floor at night through a low-gain preamplifier that just 
offsets feedline loss is about -119 dB with a SSB filter bandwidth. 
That is 0.25 uV!

An FT1000D with preamplifier ON barely makes that sensitivity.  
With a CW filter, even more sensitivity is required.

That is why **virtually everyone** with Beverages uses a 
preamplifier. There is a very clear need for them!

The poor close-spaced blocking and third-order intercept of 
receivers also is a problem, and the HF bands are often very 
crowded with strong signals next to weak signals. Everyone who 
works 160-40 meters knows what a problem it can be to hear noise 
floor signals next to strong local signals.

Despite what you claim, few receivers are good enough for the 
anything but the most casual local 160 operations.

73, Tom W8JI
w8ji@contesting.com


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