What would cause a pronounced static on AM radio ? Especially distant AM radio stations ? It is in sync with the engine speed. What is the cure ? Stock am/fm radio in a 2003 Blazer LS 2 door with 4 WD.

 

I've noticed that before. On my blazer and my daily driver Cobalt. It's not all the time, I'm wondering if atmospheric conditions can lead to it. Only seems to happen at night, or early morning. I think AM stations have to lower their power output at night, though

 

In general, I think You may have two options:
- (less possible) a power supply noise on the battery charging system that is not accurately separated off the radio system. Possible sources: alternator, voltage regulator, aging power filter inside the radio. I bet there are the low-pass filtering circuits inside the radio, like filter made of capacitors and chokes. The electrolytic capacitors are subject of aging. Alternator is rpm dependent. Voltage regulator may be a source of noise too (oscillations of the alternator excitation current). All this can be traced using scope or a quality multimeter.
- (more possible) a noise from the ignition system. Creating a spark discharge is usually related ti a high emission of EM field around, that can interfere with the RF circuits inside the receiver. That is also RPM dependent.

 

It is surely related to atmospheric conditions, distance and even a daytime.
There are tons of books on radio signal propagation https://en.wikipedia.org/wiki/Radio_propagation
For example, late night I can hear the AM stations from a very distant China, up to the sunrise. Then there is a dead spot up to the evening hours.

Long story short, You can think of a radio signal that goes out of the antenna and bounces (refracts) in between earth surface and the atmosphere layer. Depending on the wave frequency, a different atmosphere layer is used to bounce back to the earth. It can bounce quite a distance - You may transmit the wave, that goes round the world and get back to Your receiver.

In general, the higher the frequency is, the higher the layer is located.
Some of the highest layers (like ionospheric) may - or may not - develop at the time, so You have to be lucky.
At the edge of high frequency, the waves are not bouncing, but get dispersed in the air (therefore the FM stations have a horizontal range).
A curiosity here - there is a frequency range, that is pretty useless at a short distance, but works well on a long one.

With regards from the ham radio op SP5BYK.

 

Mike, what is the 'noise from the ignition' most likely to be : a bad spark plug wire, a worn rotor or distributor cap, or something else ?

 

2 ways for noise to come into your radio. Through antenna or through 12V power. Disconnect antenna to see which it is. Here's an article that might help. https://www.popularmechanics.com/car...o/a84/1272491/

 

Sorry, it doesn't work that way. Not for AM stations. The exterior antenna You have is for FM band. The AM band antenna is usually a ferrite shaft with high magnetic coefficient and is located inside the receiver. Not easy do disconnect.

I've checked how it's on mine. Same story as in Yours case. Although here nobody listens to AM stations, as they - by definition - have a poor audio quality, so I haven't check till today. In fact, the EM noise signals are common in industrial areas, and that also affects the quality of reception.

If You want to split the problem between ignition EMI and the power supply line, You may want to supply the receiver directly and solely from additional battery and see if the problem goes away. If so - it is related to the power supply line signal. Finding a source of EMI is a difficult. If You have a nerdy attitude, You may use one of those SDR receivers with a compact antenna, and move it across the engine bay to see where the EMI comes from....

 

The source is an electric discharge effect. The truth is that explains all and says nothing in particular. You'd have to find the source then You can say.

 

I am the OP. What I was looking for was the Secondary Ignition components ;Secondary ignition includes: Ignition coil, distributor cap & rotor, spark plug wires, and spark plugs.`I am not saying these are the only items that can cause static, Just what are the common ignition parts.

 

Yep, any of these can be the possible source of EMI.
But, so far we cannot say what's the nature of this distortion. Does it come with a noisy power line (non-EMI)? Or it is an EMI interference? Therefore I'd suggest getting an additional battery to supply the radio and see if the interfering signal disappears. That could at least rule out the power supply line problems.