Radio Ground

[trib]

The schematic for my intercom shows the headset jacks with one lead to the "radio ground". It also shows a representation of the radio with one lead to this ground. Should the radio ground be any different than the common ground used for the rest of the airplane, i.e. to limit noise? I have a 50 tab fast-on ground connection which is bolted to the airframe and wired to the firewall and engine ground. Everything is grounded to this and I was going to use it for the radio and intercom also.

[Rivethead]

All grounds end up in the same place.

[elippse]

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!

[Greg Arehart]

My understanding (but I'm not an electrical engineer) of radio grounds (i.e., plug jacks) is that they should be different from other electrical grounds. Typically, the "radio ground" is just a return wire to the radio or audio selector and is not grounded to the airframe. That's why there are insulating washers on phone jacks - to keep the radio circuit separate from the rifraff of other electrical stuff.

greg

[osxuser]

All the avionics should have a common ground point or ground bus to reduce overall background noise. In fact, I think that the whole airplane should have one.

[f1rocket]

Originally Posted by Rivethead

All grounds end up in the same place.

Yes, eventually. It's just a matter of how much "noise" they will pick up in the process of getting to the same place.

I can't explain the details like an electrical engineer, but I've always run the ground for the jacks back to the radio to include it in the gound feeds from the radio. Eventually, this ground wire makes iis way to the common ground block.