Topband: FT8 on 160m
grants2 at pacbell.net
Mon Nov 27 19:14:46 EST 2017
My WSTJ waterfall shows about 50Hz bandwidth for ft8 signals. In "band
plan" theory the ft8 segment is 1840 to 1842.5. However, many rigs
won't pass audio much below 300Hz and may not above 2.5KHz. I run "wide
open" DSP on my Pro3 so see to 3Khz. Above 2.6Khz there are few ft8
signals to 1843KHz but some other very slow even weaker signal modes per
the band plan. An ft8 signal only needs that 50Hz but the decoder shows
all transmissions better than the limiting S/N within the 0 to 3Khz range.
In CQWW I heard several CW ops at the low end of the ft8 plan segment,
in 0 to 600 Hz. I could decode ft8 fine thru them which surprised me a
bit, the cw speeds ranged from teens to 30+ wpm. Of course "reserved"
is not true, but there is a band plan and it seemed to be generally
observed by the contest ops.
While my dial shows 1840KHz, with AFSK USB the "carrier" really isn't there.
At least that is how I think it works !!
On 11/27/2017 12:43 PM, Roger Parsons via Topband wrote:
> As we all know, CQWW produces huge activity on all HF bands. This year I was hearing stations up to well above 1850kHz. I have found that trying to run Europe is often more productive in the 1840-1850 segment, presumably because it is less crowded than lower in frequency.
> Late in the contest I started to CQ on about 1842kHz, and after a while, and several QSOs, was told (on CW): "Please QSY. 1840 is reserved for FT8. Good luck in the contest." Ignoring the fact that nobody has a reserved frequency on any band, I thought OK, don't want to upset anybody, and moved to 1842.9kHz. Shortly, the same message arrived.
> My (probably incorrect) understanding is that FT8 stations are using USB with a carrier frequency of 1840kHz. So a CW station on 1842.9 should be out of their passband. I did listen carefully and could not hear any FT8 transmissions above 1842.2. (I have multiple receive antennas in a very quiet location.)
> I believe that FT8 transmissions in principle use a microscopic bandwidth, but it appears they actually need a wide clear channel.
> Yes? No?
> 73 Roger
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