Topband: FT8 Observations

[Joe Subich, W4TV]

On 2018-04-23 12:08 PM, Tim Shoppa wrote:
> On being able to hear signals at -12 to -17 dB on FT8, I do broadly 
 > agree. A CW signal at those levels would be easily heard and copied
 > by any decent CW operator.

-12 to -17 dB on FT8 (or any of the other "JT modes") is signal to noise
+ QRM in a 2500 Hz bandwidth.  "0 dB" in 'JT speak' is +22 dB S/N for
CW assuming a 100 Hz bandwidth (using the convention of total voltage
from the receiver vs. noise and QRM voltage).

That makes the -17 dB FT8 signal about +5 dB S/N on CW and the mythical
-24 dB (threshold) FT8 signal about - 2dB or -3 dB *below* the copyable
level for a CW signal.  JT9 has a threshold of -27 dB ... another 3 dB
more sensitive with its one minute cycle instead of the 15 second cycle
of FT8.

Since the 'JT mode' "threshold" represents a decoding probably of 0.50
(50%) and CW operators, particularly on 160 meters, often operate with
decoding probabilities as little as 10%, one can assume the ultimate
sensitivity for FT8 is 5 to 6 dB better than CW and JT closer to 10 dB
better than CW but it will take some time for experience to prove (or
disprove) those assumptions.  The nature of noise, fading and QRM will
also impact the relative sensitivities.

73,

    ... Joe, W4TV


On 2018-04-23 12:08 PM, Tim Shoppa wrote:
> On being able to hear signals at -12 to -17 dB on FT8, I do broadly agree. A CW signal at those levels would be easily heard and copied by any decent CW operator.
> 
> I think a lot of the FT8 “processing gain” claims, assumes a really poor CW operator. A 0dB FT8 signal is not at noise level, it is way way above noise level.
> 
> That said, this morning at my sunrise (noon in Europe) I was printing Italian stations on 40M FT8 and I was being decoded in Europe too, often at the -22 to -24 dB level. (I was barefoot and I’m assuming the italiAns too). Those are levels below what I can hear or copy on CW. I can work Europe midday on 40CW in winter but not so easy in spring or summer.
> 
> Tim N3QE