If you replace the radio signal source at the radio receiver input with a shielded resistive terminator, you no longer have any attack channel (because now the radio receiver has only an output, but no input).
For this purpose, radio receivers are just very sensitive amplifiers. Any very sensitive amplifier will output noise when its input is terminated on a resistor, both amplified resistor noise and additional noise from sources internal to the amplifier.
When you use much less sensitive amplifiers than radio receivers, e.g. audio amplifiers, a resistor at the input may be not enough and you must put there a source of greater noise, e.g. an avalanche breakdown diode (a.k.a. Zener diode).
For this purpose, radio receivers are just very sensitive amplifiers. Any very sensitive amplifier will output noise when its input is terminated on a resistor, both amplified resistor noise and additional noise from sources internal to the amplifier.
When you use much less sensitive amplifiers than radio receivers, e.g. audio amplifiers, a resistor at the input may be not enough and you must put there a source of greater noise, e.g. an avalanche breakdown diode (a.k.a. Zener diode).