Last Updated on December 1, 2025 by John Berry
The receiver threshold of reception is… well, just that. It’s the input to a receiver required to just produce the required communication result.
For most radio amateurs, the threshold they’re most used to is that for voice communications using SSB modulation.
Signal to noise
Simply, the signal to noise (S/N) ratio is the report of the ability of a receiver to determine wanted information. There’s a threshold ratio at or above which the information is available. Below the threshold, the information is not available, or available with greater than required error, and communication fails.
The signal to noise ratio is strictly an analogue ratio. It applies to analogue transmission systems. But importantly, and critically, it also applies to any digital system (such as FT8 and Q65) that is borne over analogue modulation like 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. Eb/N0 needs a depth of understanding that few radio hams have.
I will therefore dismiss Eb/N0 and concentrate on S/N.
Definition of receiver threshold
We can define the threshold as a voltage at the receiver input to cause a required signal to noise ratio. Usually we’d quote it in potential difference or PD. Some manufacturers do quote in electromagnetic force (EMF). Potential difference (PD) is half electromotive force (EMF). PD is when the voltage is measured across a 50Ω load impedance. EMF is the same but across an open circuit – hence twice PD. Academic – but important.
So, a properly quoted threshold in this case would be 0.25μV PD 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. It’s measured in a narrow audio slot and is considered more realistic. Bit error rate is used in digital signals (below).
Receiver threshold as reference
The threshold definition is incomplete without a definition of the communications system. For example, so far, we’ve just discussed threshold for a voice SSB system. This is typically the reference when radio amateurs discuss modulating a sub-carrier to send data as used in the FT8 system. The threshold improvement achieved by reducing the bandwidth and introducing coding can be stated with reference to the SSB value. I use this approach when discussing EME and other systems.
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.
Receiver threshold is always an input level quoted for a response in the receiver for a given communications system.
The receiver threshold forms one of the two parameters to calculate the System Value. The other is the power output from the corresponding transmitter. The System Value is the total loss permissible between transmitter output and receiver input.
The receiver threshold must always be quoted considering noise in the system. 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.
Digital systems receiver threshold
It’s worth a note on digital systems. In digital systems, the threshold might be -119dBm for a bit error rate of 10-4. That’s a signal power in a 50Ω system of -119dBm to cause the required bitstream output with one error in 10,000. If we wanted better BER, we’d likely have to put more signal in. Likewise, in the analogue system, we’d need more signal in for 20dB S/N (when compared with 12dB S/N). A signal power of -119dBm converts to about 0.25μV PD.