Listening to shortwave on a low-cost software defined receiver (SDR), you hear several stations known to operate on much higher frequencies. What type of filter could help?

Low-cost Software Defined Receivers (SDRs) often suffer from front-end overload or 'aliasing', where strong signals from higher frequencies bleed or mix into the lower frequency bands you are actively trying to listen to. Shortwave frequencies are relatively low, typically falling between 3 and 30 MHz. To fix this interference, you need to block the strong higher-frequency signals (like local FM or TV broadcast stations) while allowing the lower shortwave frequencies to reach the receiver. A low-pass filter achieves exactly this by letting the lower target frequencies pass while aggressively attenuating the unwanted higher frequencies.