Earth Loops and Bike Powering
Bike Powered Radios
If you only use your radio on the bike it makes sense to power it off the bike so you never need to charge the battery. This can be done using a battery eliminator which sits in place of the radio battery and converts the bike 12V down to around the 8v required by the radio.
Bike powering can sometimes cause problems when the radio is connected to another device which is also powered off the bike, an intercom for instance. The problem reveals itself by a buzzing or alternator whine heard over the radio or headsets.
An explanation of if its cause is rather technical to go into here but it is commonly known as an 'earth-loop' and it can usually be cured by one of several methods each in turn more technical than the last. The simplest cure fortunately will work for nearly all users:
The power for both the radio and the power for the other powered device should be taken from exactly the same location on the bike. Both the ground wire and the 12V wire (via fuses) must be wired to the same common point.
The ideal location is direct from the battery terminals however this is usually not practical since you would like the devices to be powered off when the bike is off. If that is the case take the 12v power off the relevant key-powered bike circuit or fit a toggle switch to the battery so the devices can be switched off after use.
Fuses are there for one purpose only. To prevent your bike catching fire if a wire short circuits. They are to protect the wiring (not the device). Use a fuse ( typically 2 to 5amp max) for the devices we are talking about. The fuse should fitted as near the battery or bike power source as possible, not near the device.