Propagation formulae or programs need the value "required signal to noise" for one of the paramater inputs. The equations below can be used to figure out this value, according to the service (modulation) in use.Required signal to noise is the expected signal quality for the circuit.

The equations to use are the following: where a is mod index ( 0 to 1 ),  Note: * means SQUARED. You will see that AM is very inefficient compared to CW. A mod index or 1 is merely 100% modulation, a certainty in SSB and CW.

SNR out=2[(a/2+a*)]*[Pt /(NoW)]* ---- for threshold signal.  Pt is signal power, No is 1 hz noise, W is transmitted bandwidth, 3 times the baseband frequency for CW, W=baseband freq for AM and SSB. Baseband is simply the frequency of the modulating signal. For large Pt/NoW, remove the * in the second bracket ie. [Pt/NoW].

When S/N out is 1, the signal is no longer heard, so it has to be higher, like 2 or more. Take 10Log of S/N out for Db value. S/N out is what is heard at receiver output.

If S/N out is 2 the signal would be very weak, but audible, like S-1. Required S/No for 1 baud CW would be about like 12 db. For 12 wpm CW its approx 20 db S/No, For SSB its about 45 db S/No, for AM like 51 db S/No.Broadcast quality AM runs like 73 db for that.  No is the 1 hz noise power. This is the required S/No input for VOACAP. SN out is numerical, take 10 Log for db value, ie 2 = 3 db S/N out ( what you actually hear ).

S/No is Signal to 1 Hz noise.

Equations courtesy R.Tainter and W. Ziemer,

"Principles of Communication"

Updated May 17-2020

The Voacap Blog has something here about Req Signal to Noise also.

http://voacap.blogspot.com/2018/04/minor-changes-in-reqsnr-values-and-new.html

The Main Page:

https://www.voacap.com/