FM Multipath Cancellation and Capture, Fading and Static

FIGURE 21:

FM MULTIPATH CANCELLATION

^ FM multipath cancellation is caused by "line-ofsight" characteristics. This condition exists when radio waves are reflected from object or structures. The reflected signal cancels the normal signal, causing your antenna to pick up noise and distortion. CANCELLATION effects are most prominent in metropolitan areas, but can also become quite severe in hilly terrain and depressed roadways (Figure 21 ).
FM STRONG SIGNAL CAPTURE AND OVERLOAD

^ FM capture is an unusual condition that occurs when traveling in the vicinity of a broadcast tower, a stronger station up or down the radio dial may capture the weak station. This switch to the stronger station occurs without changing the radio dial. As you pass the tower, the station may switch back and forth a few times before returning to the station that you were listening to originally. When several broadcast towers are present several stations may overload the receiver resulting in considerable station changing, mixing and distortion. This condition will not harm the receiver (Figure 21).

FIGURE 22:

FADING

^ AM fades under freeway viaducts. Fades on distant stations at night. Fades in downtown areas with many tall buildings (Figure 22).
^ FM does not fade under viaducts. Fading and noise occur on distant stations. Fading is caused by reflections from buildings and hills (Figure 22).

FIGURE 23:

STATIC

^ AM static is caused by power lines and electric fences, particularly noticeable in rural areas where only weak stations are available. Traffic lights and electric signs can cause static. Static from thunderstorms can make AM unlistenable (Figure 23).

^ FM static is not caused by Power lines, electric signs and fences, traffic lights, or lightening. However, there are other sources of static that could bother FM.