Radio frequency band
Radio broadcasts use the radio frequency bands shown in the table below.
The service areas of AM and FM broadcasts are vastly different. Sometimes an AM broadcast can be received very clearly but an FM stereo broadcast cannot. FM stereo has the smallest service area, and is prone to pick up static and other types of interference such as noise.
Radio reception problems
In addition to static, other problems such as "phasing", "multipath" and "fade out" exist. These problems are not caused by electrical noise, but by the radio signal propagation method itself.
AM broadcasts are susceptible to electrical interference and another kind of interference called phasing. Occurring only at night, phasing is the interference created when a vehicle receives 2 radio wave signals from the same transmitter. One signal is reflected off the ionosphere and the other signal is received directly from the transmitter.
Multipath is a type of interference created when a vehicle receives 2 radio wave signals from the same transmitter. One signal is reflected off buildings or mountains and the other signal is received directly from the transmitter.
Fade out is caused by objects (buildings, mountains and other such large obstacles) that deflect away part of a signal, resulting in a weaker signal when the object is between the transmitter and vehicle. High frequency radio waves, such as FM broadcasts, are easily deflected by obstructions. Low frequency radio waves, such as AM broadcasts, are less likely to deflect.
Technicians must have a clear understanding about each customer's noise complaint. Use the following table to diagnose noise problems.
|Radio Frequency||Noise Occurrence Condition||Presumable Cause|
|AM||Noise occurs in a specific area||Foreign noise|
|AM||Noise occurs when listening to an intermittent broadcast||An identical program transmitted from multiple towers can cause noise where the signals overlap|
|AM||Noise occurs only at night||Signal phasing|
|FM||Noise occurs while driving in a specified area||Multipath resulting from a change in FM frequency|