Topband: FT-8 question
clive at gm3poi.com
Wed Dec 6 19:12:06 EST 2017
My plan is to add four active verticals each feeding back to my house. They
will be spaced over the entire 7 acres I have here. My hope is I can pick
the vertical that carries the highest noise content and then cancel that
noise from my various RX antennas. If I find I have an existing RX antenna
that is aiming at a noise source then I may use that as well. The days of
trying to deal with noise sources is near over.
73 Clive GM3POI
From: Topband [mailto:topband-bounces at contesting.com] On Behalf Of Grant
Sent: 03 December 2017 04:14
To: Jim Thomson; topband at contesting.com
Subject: Re: Topband: FT-8 question
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).
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
> 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
> 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
> 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
> Jim VE7RF
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