Last Updated on April 17, 2026 by John Berry
The system value is the maximum total loss between your transmitter output and the threshold of the distant receiver.
It is a fixed figure for your chosen system technology (comprising modulation and coding). There are differing system values for all technologies including narrow band FM voice, SSB and each narrow band data mode such as FT8. It applies to all communications systems but is used most in VHF/UHF/SHF communications. As shown below, the system value belongs to the equipment, not the path. It can be thought of as a figure of merit for the equipment.
The figure matters because it represents the core of the link or path budget. The link budget is described in another page. Because it is limited by the physics of the technology, it determines if a given link between two stations will work.
You calculate the system value by subtracting the receiver threshold from the transmitter output power:
System Value [dB] = PTX [dBW] – PRX Threshold [dBW]
Example system values
| Technology | Occupied bandwidth | Typical usable receiver threshold, PRX Threshold | Typical transmitter power output, PTX | System value |
| SSB voice | 3kHz (Bssb) | -146dBW | 20dBW | 166dB |
| FT8 | 50Hz (Bft8) | -166dBW* | 20dBW | 186dB |
*In FT8, a decoding threshold of -20dB below the SSB threshold is assumed. I may be pessimistic here, 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 bandwidth advantage relates to the Shannon limit that I describe elsewhere on this site.
Practical use
The bigger your 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.
As I discuss in another page on this site, both threshold degradation and threshold impairment must be considered. Both are outwith the system value, but inside the path budget. Threshold degradation effectively acts to degrade the figure by adding noise at the receive end of the link. Threshold impairment effectively reduces the figure by adding noise from interferers. In both cases, the possibilities available by having a good system value are reduced upon real-world installation.
The figure for a technology does not include fade margins to enable communications for a high percentage time or percentage of locations. Adding fade margins would then move from system value to system reliability. Typically radio amateurs are more interested in communications for small percentages of time (DX).
Summary
Each technology chosen has a unique system value. It’s a figure of merit . It forms the core of the path or link budget. Improve the figure by adding a power amplifier or an improved receiver and you increase the communications possibilities. The figure is somewhat dynamic – in professional wireless systems, it is is improved by decreasing the data carried and the necessary bandwidth. This improvement overcomes path constraints thereby maintaining service under difficult conditions.