What Wobblewing says ^^^ is right. The problem with fast digital logic is that they create lots of spurious carrier waves, little transmitters that can interfere with your regular COM, your transponder and your GPS.
A clean analog local oscillator produces a single sine wave signal. This is usually not a big problem unless it sits exactly on the frequency you are trying to receive. A digital system on the other hand uses square waves. That means that besides the signal frequency, you will also find signals at multitudes of that signal frequency. Usually the odd multiples, so 3x, 5x, 7x etc. What that means is that if you have a clock somewhere at 1 MHz, you'll find signals at 3, 5, 7MHz etc. An entire comb of signals. In a DSP there are usually lots of clock-like signals, so lots of noise.
A good DSP (or any other digital) system will use proper shielding to keep that interference from escaping. Unfortunately the cheap consumer dongles use simple plastic casings and cheap cables that keep nothing in. The RPi itself is guilty as well.
What might help a bit is to use a metallic case that includes the dongle, and perhaps some ferrite beads on every wire coming out of that case. One of the other threads showed some 3D printed cases. Perhaps spraying them with metallic paint on the inside would be a good first start to fight interference.
