Last Updated on May 13, 2026 by John Berry
The receiver threshold defines the weakest signal your equipment can successfully detect. Every receiver faces a physical limit caused by internal electronic or thermal noise. You cannot detect signals that fall below this noise floor. So the receiver threshold is the signal level at which this limiting signal-to-noise ratio occurs.
Understanding this signal level or threshold helps you optimise your station for weak-signal communication.
Signal to noise
Simply, the signal to noise (S/N) ratio is the report of the ability of a receiver to determine wanted information.
The signal to noise ratio is strictly an analogue ratio. It applies to analogue transmission systems. But importantly, and critically, it is used in many digital systems (such as FT8 and Q65) which are borne over analogue modulation such as SSB.
The equivalent for pure digital systems using digital modulation directly applied to the carrier, is the energy per bit over normalised noise, or the Eb/N0 ratio. It is possible to work in Eb/N0. But if the designers such as Joe Taylor K1JT express their systems like Q65 with respect to the analogue S/N threshold, it’s easier to use S/N.
I will therefore dismiss Eb/N0 and concentrate here on S/N. For more on Eb/N0, see my discussions on the Shannon limit.
Thermal noise and bandwidth
Heat causes electrons to move randomly within electronic components. This movement creates a constant background hiss called thermal noise. We calculate this noise power using a formula.
The formula is P = kTB.
In this equation, P is the noise power in Watts. The letter k represents Boltzmann’s constant (1.38 x 10-23 Joules per Kelvin). T is the absolute temperature in Kelvin. Usually, we assume a standard room temperature of 290 Kelvin. Finally, B is the receiver bandwidth in Hertz.
So noise power depends on bandwidth.
How bandwidth affects threshold
You can control bandwidth to set the threshold. A wider bandwidth lets more noise into the system. This extra noise raises (degrades) the receiver threshold. Consequently, weak signals become harder to distinguish from the background.
For example, a standard SSB filter is 2,400 Hertz wide. A typical threshold sensitivity might be -149dBW for 10dB signal to noise. A CW filter might only be 500 Hertz wide. Using the narrower filter reduces the noise power significantly. You will hear weaker signals when you narrow your bandwidth but the amount of information you can convey becomes limited too.
A narrow band data signal might only occupy 50Hz. This sort of signal has a threshold or minimum discernible signal approaching the Shannon limit a few dB above the narrow band noise.
Often in ham radio the data threshold is quoted as dB below the noise: like -24dB for example.
This is confusing since in many cases, the noise is considered for a 2.4kHz bandwidth while the signal is assessed in a significantly narrower channel. Quite naturally, the ratio is negative. Despite being confusing, it’s a convention that has been adopted.
Definition of receiver threshold
We can define the threshold as a voltage or power at the receiver input needed to cause a minimum signal to noise ratio. Usually we’d quote it in potential difference or PD, or in signal power.
So, a properly quoted threshold in this case would be 0.25μV PD or -149dBW for 10dB S/N for an SSB communication.
(measured in voltage or power considering the receiver input impedance)
The receiver threshold is always quoted for a given communications system as an input that will cause the required output from the receiver. SINAD in the above diagram stands for signal-to-noise-plus-distortion to noise-plus-distortion. SINAD is considered a more correct ratio. As I note above, bit error rate is used in digital signals.
Receiver threshold as reference
You will see that the threshold definition is incomplete without a definition of the communications system. The receiver threshold is the reference which is modified by noise, low noise amplifier noise figure (and gain), feeder losses, and antenna gain when calculating the ability of a radio amateur radio station to communicate with others.
The receiver threshold forms one of the two parameters to calculate the System Value. The other is the power output from the corresponding distant transmitter. The System Value is the total loss permissible between transmitter output and receiver input.
At low frequencies (LF, MF and HF) environmental noise may degrade the threshold significantly, making the receiver site and its antennas unusable. Generally environmental noise reduces with frequency.