minimum signal strength for cw

roger na4rr

Just wondering what the minimum signal strength required to receive a CW signal.  On FT8 I can decode signals on regular basis at -20 to -24 db. Just wondering how strong the same station would need to be if it was sending cw. Assuming 0 noise level. thanks roger

Rick - WC8D

The S meter on a Flex radio typically reads S-9 as -75dbm. The reading for FT8 is actually a signal to noise ratio. Not the same thing. I have been able to copy CW signals just above the noise limit on the Flex which can be as low as -110 dbm.

Please see http://www.arrl.org/forum/topics/view/1957 for an explanation on FT8 S/N ratio.

Rick

roger na4rr

Thanks for the reply.  The reason I ask is I have a friend that does only cw he refuses to do ft8 To him "fake mode" hi.  Well i was wondering how strong the ft8 signals needed to be for him to able to raise someone on cw.  When I am ft8 on 6m must of the time I see no cw stations on my panadapter.  I'm also talking about the db report from wsjt not the flex s- meter reading

KY6LA_Howard

In simplistic terms... FT8 has a bandwidth of 6.25Hz and CW has a bandwidth of 25Hz

This means that under the exact same noise conditions FT8 has a gain advantage of 

10 LOG(25/6.25) = 10 LOG (4) = 6dB or 1 S unit

But if gets better for FT8 as the signal contains error correction bits and is integrated over 15 seconds--- albeit the human brain does some error correction on CW.

so in the real world, FT8 likely has a 10 to 15 dB (2-3S Unit) gain advantage over CW 

Lasse Moell

I can relate to the question beeing a CW op mainly. Using FT8 as a beacon for band openings, the snr numbers reported on FT8 is useful. FT8 bases the snr wrt 3 kHz BW, and I find myself often use 50 Hz filter when digging deep down into the noise. 3000/50=60 or ~18 dB. Usually I need a few dB of SNR to copy CW so I would expect a -15 dB snr on FT8 should enable me a CW QSO given the power and antennas are the same
For me a marginal CW QSO is a lot more rewardig than bagging a FT8.....

KY6LA_Howard

Actually FT8 bases it’s SNR reading on 2500Hz not 3000Hz

so using your 50Hz filter for CW

Then the gain advantage of FT8 is 2500/50=50 or 17dB or 3 S units 

so you are doing very good copying CW when the readings on FT8 are -15dB