Topband: FT-8 question

[Grant Saviers]

Hi Jim,

Well, yes on the 5 acres, but still suburban.  But, I do notice that FT8 
on the vertical 160 T often decodes as well as my DXE 4 sq receive which 
has directivity and thus less noise.  Same comment from another op here 
locally.  I don't see any advantage to subsegment filtering, and in fact 
I think a wide bandwidth DSP/IF will have less phase distortion.  That's 
what the FT8 "manual" says also.  I run "wide open" 3 KHz on my Pro3 and 
don't see any problems other than AGC pumping and desense from locals. 
(its hard to turn on/off AGC completely).  I checked out 160 tonight 
with the cw contest on, FT8 decoded most of the FT8 signals even with CW 
all over the FT8 "segment".  (let's not revisit THAT!!)  Listening on 
your 80m dipole will probably help, it did for me at another QTH. Then 
there is all the advice from N4IS.   It is also fascinating to watch the 
FT8 decoder pretty much ignore the woodpecker on 80m when I work 80m AM 
greyline into Asia from here (Seattle).

Grant KZ1W

On 12/2/2017 9:07 AM, Jim Thomson wrote:
> Folks on FT-8 use a 2.5 khz  ssb wide RX  filter.    FT-8 is supposed to be good for 20 db  below the noise floor.
> I assume that means 20 db below the noise floor of the 2.5 khz RX filter ?
>
> But a 250 hz cw filter would drop the noise floor by 10 db... vs the 2.5 khz wider filter... if CW mode used.
>
> So  if Im reading this correctly,  FT-8 mode, using a 2.5 khz filter....is really only 10 db better than a 250 hz cw filter, using cw mode ?
>
> If that is the case,  being able to copy signals 10 db  weaker than the noise floor of a  250 hz filter is still nothing to sneeze at.
>
> The issue I see with FT-8.. on any band is the requirement for a  2.5 khz filter.... and possibly being prone to qrm.
>
> Right now, my issue is extreme high noise levels on 160m... on a 100 by 130 city lot.    Point a semi directional RX ant in the desired direction.... and its
> also pointed at a noise source.  Seems like I am surrounded by noise on 160m.   Im going to drag out by noise canceller and try some more rx experiments,
> b4 I throw in the towel.    FT-8 might just be an option for folks like myself that are plagued with high levels of noise.   Another possible option might be
> the use of real time  remote RX.   Another possible option might be using my 80m rotary dipole for 160m RX.   As is, its infuriating listening to high noise
> levels on 160m.   If I cant hear on 160m...except for the usual louder stations,  Im not going to even try TX.
>
> I have tried using a pair of  500 hz filters, and also a pair of 250 hz filters, and also a 125 + 250 combo, in both my MK-V..and also 1000-D.
> The MK-V also has a 240-120-60hz  dsp filter.  The 1000-D has a tunable audio cw filter.   The problem with the narrow xtal filters is...
> with noise levels so high, the noise...  rings out the filters.   The signals coming out of  each filter... get stretched a bit in time duration.
> What Im left with is this mess whereby the desired signals +  noise end up all mashed together.  Typ noise on 160m, using a 2.4 filter
> is S9 to S9 + 10 db.   Right now, Im trying to evaluate if 160m is even worth the effort required.   Are the  rest of you on 5 acres out in the woods ?
>
> Jim   VE7RF
>
>
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