It's too bad the word "ground" got in our electrical vocabulary. Electrical circuits require two conductors, one to deliver the current from the source, and one to return it to the source. Because all conductors have resistance, when current flows in the conductor, there will be some voltage drop, however small. When two separate loads are attached to the same conductor, any voltage drop due to the current of one load will show up on the other; this is referred to as "common-mode coupling". This most often shows up on "ground" circuits since the voltage circuits are usually separate, using separate fuses and wires, whereas "ground" appears to be common to everything! To prevent this type of signal coupling from getting into sensitive circuits such as in a radio receiver or transmitter, the manufacturer will often designate different circuit returns as radio ground, power ground, digital ground, chassis ground, GND, GRD, etc. If the instruction manual with the radio does not distinguish the connections to their separately-labeled grounds, it is best to check with them on what they require so as not to have squeels and noise later on. Your microphone circuit is especially sensitive to unwanted pick-up, and the wiring to the microphone often shares the PTT circuit. One good way of getting interference is to wire the PTT switch with un-shielded wires with one of them connected to some airframe ground rather than back to the radio. 'Hope this dissertation gives a little in-sight into good wiring practices!
