Electrical Isolation for Radios

[Harold Lanfear]

Most FM or other car stereo components are chassis grounded, even if they have a separate ground wire. If you need to isolate a FM or other radio, most hardware stores carry flanged nylon bushing and either nylon or rubber washers. I bought some that fit an 8-32 screw which fit well in a hold made with a #5 drill.

Push the bushing in, put a washer on the other side and use a razor knife to trim the bushing to length. If you need to ensure other portions of the system don't touch metal, the plastic top of a coffee can (some are clear- you just have to teach your wife to like the 'right' coffee) will make very nice washers.

You can also buy nylon screws if strength isn't important, but the method above uses standard screws with full strength. Don't forget to test isolation with an ohmeter when done.

[Sprig]

Hail VAF Harold,

Originally Posted by Harold Lanfear

Most FM or other car stereo components are chassis grounded, even if they have a separate ground wire. If you need to isolate a FM or other radio...

I really can't imagine any reason WHY you would want to do this...

This method will "float" the radio ground at a different potential then the airframe. I've been taught why this might cause all number of problems, but never had anyone teach me why this might solve even one.

I'm still trying to figure, if an external antenna to this radio is airframe grounded (with a coax length => ground cable length) if the radio chasis could acheive a negative voltage with respect to airframe ground (resulting in under-voltage to said radio).

And, after some thought, if the static discharge off the airframe is NOT adequate, I think this situation may be possible.

I'd sure like to see the instruction manual printed with this radio! I'd pay a fast 20 to the first person that showed me it says, "Make 100% certain the chasis is NOT grounded!!! (especially in an areoplane)"

Anyways, in MY opinion, this is generally a bad idea. It is your experimental areoplane, and I respect your opinion to experiment.

[Harold Lanfear]

I believe you misunderstood me. By isolating the chassis and then terminating the ground wire coming from the radio at the avionics ground with the other avionics, you may should avoid a ground loop and the potential for associated noise. This is based on my understand from Bob Nuckolls class and books; but I'm not trying to solve any issues, just offered a method to do so if appropriate.

[gmcjetpilot]

Originally Posted by Harold Lanfear

I believe you misunderstood me. By isolating the chassis and then terminating the ground wire coming from the radio at the avionics ground with the other avionics, you may should avoid a ground loop and the potential for associated noise. This is based on my understand from Bob Nuckolls class and books; but I'm not trying to solve any issues, just offered a method to do so if appropriate.

I understand what you are saying but I have to agree, it is probably not necessary to isolate the metal case of most avionics from the metal frame of the aircraft. It's the way they are designed. Any ground isolation is internal to the case.

You may be thinking of the audio inputs (mic) and output (phones) where the radios or intercom isolate the mic and phone jacks. This is to protect from ground loops. Audio is low volts and very prone to ground loop, so the manufactures provide discreet grounds, which all ground at one spot in the radio or intercoms amplifier. This is very important.

I could be wrong and also not understand, but I think this is an unnecessary step, but if it's called out, sounds like a good idea. Just referring to most installation manuals for avionics, you will see that the metal case can be grounded. If there is any isolation needed it is done internally on "the board". In some cases they recommend the device chassis be grounded, in addition to the ground wire.


When I read isolation of radios I think of power supply. In critical commercial aerospace and military equipment, sometimes they are isolated from the airframe and the power is feed by isolation DC-DC power supplies. Basically it's regulated protected power, so for examples a lightning strike surge does not get into the equipment thru the power leads or airframe. All this is great on a nuclear sub, but a little beyond our needs. When in doubt follow the manufactures recommendation.