Before revision: Without an adequate RF filter, any signal S2 (green) at the image frequency is also heterodyned to the IF frequency along with the desired radio signal S1 (blue) at , so they both pass through the IF filter (red).
After revision: radiofrom the antennaWithout an adequate RF filter, any signal S2 (green) at the image frequency is also heterodyned to the IF frequency along with the desired radio signal S1 (blue) at , so they both pass through the IF filter (red).