[time-nuts] Difference in antennas

[jimlux]

On 11/22/19 2:29 PM, Hal Murray wrote:
> 
> jra at febo.com said:
>> I like the idea of inserting attenuation until the SNR or Cn values start to
>> go down.  That may be the most practical solution.
> 
> Inserting attenuation is a good trick for the tool box.  It is also used to
> measure error rates on fiber links.
> 
> With a reasonable fiber setup, the error rate is so low that it is hard to
> measure. 

I don't know that this is the case all the time.  On radio links, the 
kind of coding affects the BER vs Eb/No curve - and there's a variety of 
impairments that have the top part of the curve looking ok, and the low 
BER part looking funny.

Figure 6 in this report (figure attached) shows this problem - "BER Flaring"
https://ntrs.nasa.gov/search.jsp?R=20140008859

Granted, that example is because there was a bug in the implementation 
of the receiver software, however it can happen on other systems, due to 
various non-ideal implementation aspects.


  At low error rates, there is a simple relation between signal/noise
> and error rate.  So insert enough attenuation until you can easily measure the
> error rate, then compute what it would be without the attenuation.
> 

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