If you wire the P-Lead and Shield as mentioned above, (at both ends) you will end up with a ground loop.
Try to think of them as two different things. Shield and Conductor.
To avoid this...The P-lead...meaning the actual conductor wire INSIDE the shielded wire, ( the one you will put a ring terminal on) goes from panel-bussed ground, to one pole of your mag switch via ring terminal. The other end of the conductor wire goes to your magneto via ring terminal or splices to the old cigarette on old mags, as the source to activate or deactivate your mag. The other Pole of the mag switch, can be a non-shielded piece of wire going from your bussed ground, to the switch itself.
When you turn your mag switch ON...you are breaking the ground connection...and the Mag will run. When you turn the switch OFF...you are completing the circuit to GROUND and shutting the mag off or grounding it.
The SHIELD....DOES NOT connect to both the Panel-Bussed ground AND a screw on the mag....That is a ground loop. (because the engine is grounded and the panel/bus is grounded...so electrical noise flows in, on and around this loop.
INSTEAD:
You terminate the shield, 2 inches or so, from the end of the P-Lead wire going to the mag. I do this by using a solder sleeve and pull the pig tail out of it and throw it away. OR...you can do it in reverse...where you connect it to the mag screw end and leave it un-connected at the switch end...I like the cleaner first method.
ONLY ONE END OF THE SHIELD IS CONNECTED TO Ground at one end or the other but not both. The second end of the shield just stops, connecting to NOTHING. But the conductor wire is connected to Mag, then switch, then ground, when off.
The Ground Loop is formed, when you connect your grounded engine and mag, to your panel grounded buss, via the shield...All the electrical noise on the shield, can and will flow to your radio and intercom, via the panel bus, which feeds your radio gear and which electrically, you connected to your mag, by hooking the shield up to your buss and engine.
This is what you DO NOT WANT.
Sometimes, it can sound choppy, sometimes just a whine. Electro Magnetic Interference...likes loops. No amount of filters, will help, since you created a significant path of least resistance, if you hooked both ends of your shield to bonded grounds.
I hope that helps.
Steve