CW vs SSB signal to noise ratio advantage

Discussion in 'Ham Radio Discussions' started by WL7PM, Dec 12, 2019.

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  1. WL7PM

    WL7PM Ham Member QRZ Page

    "CW gets through when conditions will not support SSB"
    We've all heard that for decades, and I do not doubt it a bit.

    Can we quantify the dB advantage of CW over SSB for weak signal reception ?
    ie, what is the CW carrier power level that will be just EQUAL to 1500 watts PEP SSB signal?
  2. KL7AJ

    KL7AJ Ham Member QRZ Page

    That depends on the bandwidth of the receiver as much as anything.
  3. SM0AOM

    SM0AOM Ham Member QRZ Page

    It is very hard to do, as the ear-brain signal processing represents a highly individual
    signal matched filter which will influence where the copying thresholds are placed for both aural Morse ("CW") and SSB reception.

    However, some guidance may be found in the ITU-R Recommendation F.339-8, where required normalised SNR:s for various modulation schemes are tabulated:



    Here it can be found that A1A (Morse) requires 31 dB/Hz SNR, while J3E (SSB) requires 47 dB/Hz SNR to be "Just usable", a difference of 16 dB.

    The required SNR:s for both Morse and SSB are highly dependent on the operator proficiency, but the stated numbers may serve as a first approximation, so Morse requires about 16 dB or a factor of 40 lower power than SSB.

    SWL37632, K3EY, WA1GXC and 1 other person like this.
  4. HB9PJT

    HB9PJT Ham Member QRZ Page

    My tests showed that it depends a lot of the SSB modulation quality. Frequency response and good speech processor are important. My tests indicated 7 to 13 dB difference.

    73, Peter - HB9PJT
  5. SM0AOM

    SM0AOM Ham Member QRZ Page

    An age-old "rule of thumb" has been 13 dB advantage for Morse over SSB.
    It was derived and used in an 1990s doctoral dissertation by otolaryngologist Peter SM7CMY (SK):
    "Signal Detection in Noise, with special reference to telegraphy".

    SWL37632, K0UO and WA9SVD like this.
  6. WB2WIK

    WB2WIK Platinum Subscriber Platinum Subscriber QRZ Page

    I agree it's at least 10 dB and likely more than that -- and considerably more if the RX bandwidth can be effectively selected to be very narrow for CW reception.

    Of course CW reception requires some operator skill, but then so does "hearing very weak voice signals" and one can have pretty bad hearing and still copy CW fine while they may not be able to decipher anything on SSB (or AM or FM either).
    K0UO, N0TZU, W4NNF and 1 other person like this.
  7. K9STH

    K9STH Ham Member QRZ Page

    When the bandwidth of the receiver is added, the effective difference becomes even greater. For example, comparing an SSB 6 dB filter 2.4 kHz wide versus a CW filter 200 Hz wide, at least another 10 dB, and probably even more, is added to the effective reception in favor of CW.

    Glen, K9STH
    Last edited: Dec 15, 2019
    G3ZPF likes this.
  8. N1FM

    N1FM Ham Member QRZ Page

    Yes. Huge difference between a weak CW signal at 250 Hz vs an equivalent strength SSB signal at 2 kHz.
  9. N1OOQ

    N1OOQ Ham Member QRZ Page

    This is why QRP CW is no big deal. It's very easy to make contacts with 5W and narrow bandwidths. Almost like running an amp.
  10. K0RGR

    K0RGR Premium Subscriber QRZ Page

    I can never find the issue, but there was an article in QST a few years back that compared all the popular modes, including JT65, which was the 'hot' new digi mode at the time. Of course, the proponents of the different modes all stood up and called the other ones liars.

    All I can tell you is that I have had many JT65 contacts where the other station was inaudible in the noise. The same is true of Olivia contacts. I particularly enjoyed calling CQ on 'dead' bands and getting answers - both 6 and 10 meters are good hunting grounds. One of our locals made WAS on 6 THIS YEAR that way, using FT8, I believe.

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