It has been mentioned by some that they only use a single insulted wire for their PTT circuit rather that shielded wire and it's never caused a problem. This sort of puts me in mind of a friend who discovered that he had not put safety wire on his prop bolts his previous inspection and found that the bolts were still torqued. Since safety wiring is a knuckle-busting adventure, he decided not to use safety wire again because the bolts were always torqued.
Here's the situation. A single wire on the PTT switch obviously won't work. The switch must also have a ground. Where is this coming from? Do you ground it to the control stick? That really gives a great ground through all the Heim joints! The ideal situation is to ground it to the radio ground which is where the shielded wire works out. The single wire, not being shielded, can act as an antenna and pick up noise from many sources. But if you use a ground from some other part of the plane, which may have noise relative to the radio ground, then both of those noises may couple to the mic lead since they run together through the three-wire mic-ptt-ground cable and can put the noise on the mic lead which is a very low signal level. You won't know about it when you transmit since you get sidetone in your headset, but others can, and it will give a scratchiness to your transmissions. A word to the wise!
Here's the situation. A single wire on the PTT switch obviously won't work. The switch must also have a ground. Where is this coming from? Do you ground it to the control stick? That really gives a great ground through all the Heim joints! The ideal situation is to ground it to the radio ground which is where the shielded wire works out. The single wire, not being shielded, can act as an antenna and pick up noise from many sources. But if you use a ground from some other part of the plane, which may have noise relative to the radio ground, then both of those noises may couple to the mic lead since they run together through the three-wire mic-ptt-ground cable and can put the noise on the mic lead which is a very low signal level. You won't know about it when you transmit since you get sidetone in your headset, but others can, and it will give a scratchiness to your transmissions. A word to the wise!