Topband: FT8 qrm
don.field at gmail.com
Wed Nov 29 11:38:18 EST 2017
I've been watching this thread with interest, having recently taken the
plunge and experimented with FT8 (but got bored very quickly!). Three
comments on aspects that I don't think have been covered:
1. FT8 is very new (albeit it seems to be taking the world by storm) and I
suspect many operators are unaware of where it exists on the bands, unless
they are data modes operators themselves (which, typically, many long-term
topband operators are not)
2. Many FT8 "users" are actually computers - I know of a number of folk who
have automated FT8 to the extent that they go to bed at night and in the
morning look to see what their PC has "worked". As far as I know, their PC
doesn't specifically check for a clear channel although it will normally be
trying to find a slot between other FT8 signals within the audio bandwidth.
3. Apropos of which, most FT8 QSO are split frequency - it makes no sense
to call a station co-channel. So even if a frequency appears to be clear,
it may be that there is a station already in QSO with an FT8 QSO partner
1kHz or more away (the WSJT software handles this "split" operation
automatically). So a quick "QRL?" will not be heard.
As is usually the case in life, nothing is simple!
73 Don G3XTT
On 29 November 2017 at 15:49, Roger Parsons via Topband <
topband at contesting.com> wrote:
> 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
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