Why does input attenuation reduce receiver overload on the lower frequency HF bands with little or no impact on signal-to-noise ratio?

On the lower HF bands (such as 160, 80, and 40 meters), the dominant source of noise is usually external, originating from atmospheric static and man-made interference. This external noise floor is typically much higher than the internal thermal noise generated by the receiver's own circuitry. When you turn on an input attenuator, both the incoming signals and the external noise are reduced by the exact same amount. Because the external noise is so high to begin with, it usually remains well above the receiver's internal noise floor even after being attenuated. As a result, the ratio between the signal and the noise (SNR) remains largely unchanged. However, the absolute voltage of all signals entering the receiver is significantly lower, which successfully protects the front-end mixer and amplifiers from being overloaded by strong local stations.