Signal Interference, FM Reception

FIGURE 24:

SIGNAL INTERFERENCE

^ AM "crosstalk" (two stations being received at the same time) is often heard when driving near towers. AM towers are usually not located as close to each other as FM towers (Figure 24).
^ FM "Capture" or station changing, and distortion is often heard when driving near towers. FM towers tend to be more "clustered" in urban areas causing concentrated "bad" areas (Figure 24).
FM Reception in the Automobile

Several factors affect the FM signal to cause the signal to drop below this clipping or limiting level. These factors occur more frequently in a car radio than in a home set. At home the only source of static would be the electrical appliances in the home. In an automobile, the radio reception is subject to interference from a 25,000 volt ignition system located only a maner of inches from both radio and antenna. The auto ignition system in vehicles passing and being passed radiates interference. These and other noises cause interference patterns that are constantly changing.

The antenna system used with a home radio can be as elaborate or high as necessary to get desired reception. Elaborate antenna systems on a car would not only be impractical but very unsightly. Antenna configurations are limited in both design and height to something compatible to the styling of the car.

Another problem in FM auto radio reception is mobility. A home set can be placed in one spot, and the antenna oriented for maximum reception; the auto radio enjoys no such luxury. It is expected to play with minimum reception disturbance at many different distances from
the station, behind buildings and hills and through areas of high electrical interference. These conditions are seldom constant, yet the radio must be designed to keep the interference to a minimum. This involves continuous engineering and development in circuit design, output power, tuning accuracy, selectivity, sensitivity and shielding. It is common today to go from very near a station to 50 miles away from it within an hour. The radio must be able to compensate for these changes.

FM signals can be thought of in the same manner as light. It will not bend around corners but it will reflect. When the sun goes down beyond the horizon, the light is cut off. Therefore, the listenable range of an FM station is the horizon which in the case of a high transmitting tower is at most 25 miles. Even though a building is between you and the sun, enough light reflects from surrounding objects to allow you to see. FM is similar-in that even though a building is between you and the transmitting antenna, enough signal is reflected behind the building into its shadow to allow you to pick up the station on your receiver. The lower the sun gets, the darker the shadow is. Similarly, the farther away the station the lower the power of the station, the weaker the signal is in the "shadow" of the building.