Good work
yes, lower coil voltage = lower spark voltage
radio noise is fairly easy to identify
Does noise change with engine RPM?
If yes then alternator, coil or spark plug wires are the issue.
Any electric motor like blower fan or fuel pump can cause radio noise
Antenna's base must be properly grounded to body, and so to the Radio chassis, or the antenna is just doing what is suppose to do, pick up RF waves, but thru the BODY of the truck.
So this is the very first thing to check.
Cold engine
Loosen fan belt and take it off alternator
Start engine, see if radio noise is gone, if not then alternator is not the source.
If noise is gone then you need to add suppressor to alternator
For the rest read here:
https://www.howacarworks.com/accessories/identifying-and-suppressing-radio-interference
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