Re: Topband: FT8 qrm

[Roger Parsons via Topband <topband@contesting.com>]

Perhaps I shouldn't have started this thread! 


The whole point of my original posting was that I was definitely transmitting 
more than 500Hz HF of the FT8 tones, so from an 'analogue' perspective there 
should have been no problem. As others have mentioned, FT8 is received though 
an SSB bandwidth filter. Where wideband noise is the limiting factor on 
reception (eg VHF) it is a valid technique to put the ultimate selectivity at 
the end of the receive chain. It is not valid where there are likely to be 
strong signals from some other mode within the receiver IF passband. Therefore, 
my opinion FT8 (and many other digital modes) are not suitable in situations 
where there is intense activity on nearby frequencies. I think it is 
unreasonable to suggest that there should be an unused 'guard band' just to 
overcome receiver and system deficiencies. 


(And of course I am aware that these limitations could be overcome using SDR 
architecture, but that is another story.)


73 Roger
VE3ZI
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