System Value

The system value is the maximum total loss between the transmitter output and the receiver input for a given response in the receiver.

The system value is a fixed figure for that system technology (comprising modulation and coding). There are differing system values for narrow band FM voice, SSB and each narrow band data mode such as FT8. It applies to all communications systems but is most used in VHF/UHF/SHF communications.

The system value matters because it represents the core of the link budget. The link budget is described in another blog. The link budget determines if a given link between two stations will work. The system value is limited by the physics of the technology.

TechnologyOccupied bandwidthTypical usable receiver sensitivity, PrTypical transmitter power output, PtSystem value, Pt-Pr
SSB voice3kHz (Bssb)-146dBW20dBW166dB
FT850Hz (Bft8)-166dBW*20dBW186dB
System Values for SSB and FT8 at 144MHz

*A decoding threshold of -20dB below the SSB threshold is assumed. This may be pessimistic with some commentators stating -24dB with a priori decoding. The -20dB comprises bandwidth advantage and a coding advantage. The bandwidth advantage is calculated from {bandwidth advantage = 10log10 (Bssb/Bft8)}.

The bigger the system value, the more loss there can be in the link budget. As a result, greater path loss is possible. Greater distance is therefore possible between stations. And some of the more esoteric propagation modes become possible – such as sporadic E Region reflection and meteor scatter. Ordinarily the received signal levels in such modes would be below threshold for typical paths and geometries.

At HF, the link budget must include environmental noise. This acts to reduce the effective receiver sensitivity but it is considered outwith the system value calculation.

So, in line with the science, by narrowing the bandwidth, and adding some fancy coding, FT8 has a significantly bigger system value than SSB for the same transmitter power output and receiver sensitivity. As a result you can do more with it, albeit while passing little information.