Re: LF: RE: Re: Testing now a new RX antenna on MF

["Roelof Bakker" <roelof@ndb.demon.nl>]

Hello Clemens,

> E.g., if your FFT is set to a bin width of say 5Hz and a carrier lies  
> 20dB over
> the DANL (displayed average noise level) it will have a SNR of zero dB  
> in a
> 500Hz "IF"-bandwidth and therefore will be barely audible.
The brain of a skilled listener can simulate a bandpass filter with a  
width of 50 Hz.
Hence your FFT bin width of say 5Hz and a carrier of 20dB over DANL will  
be clearly audible.
Unfortunately the human brain needs white noise to work against, so a band  
full of interfering signals will exhaust the listener before long. On a  
quiet band it works great.
This problem is solved by using a filter bandwidth of 10 - 20 Hz for aural  
copy of Morse Code.
I believe that the "human brain filter bandwidth" is frequency dependent.
When I started chasing NDB's, a beat note of 500 Hz was used.
Over time this has changed to 300 Hz!
Such a low beat note can be a problem with an analogue receiver due to  
less than perfect filter skirts. With SDR's the problem does not exist.
73,
Roelof, PAoRDT