Radio Reception
RADIO RECEPTIONAntenna Position
The length of the fixed antenna is preset to provide the best FM reception.
- FM stereo has a range of about 32 km (20 miles) before interference noises are heard.
- This means that in concern areas with tall buildings or hills, it is necessary to select the strongest possible station.
- The electronic radio chassis automatically tunes to the center of any given station, eliminating the need for manual fine tuning.
Antennas and Mobility
Although an automotive radio chassis will give outstanding mobile reception, it cannot provide the continuous reception of home audio components.
- The home receiver is not limited by the vehicle operating characteristics and certain geographical effects as is the mobile unit.
- For example, for the best automotive FM reception, the antenna system should be designed like a television antenna and pointed in the direction of the station.
- The best AM antenna is a long piece of wire; the higher the wire the better the reception.
- However, because of design necessity, the automobile antenna is restricted in size, height and direction and must receive both AM and FM stations. This means that a limited amount of the station's signal reaches the vehicle radio chassis.
Tuning
Fine tuning is not required for the radio chassis.
- The electronic radio chassis automatically tunes to the center of the station where it will obtain minimum noise and best possible reception.
Tone Control
Reducing the amount of treble can improve sound quality when receiving a weak signal.
FM Flutter
Flutter can best be described as repeated pops and hissing bursts heard in the radio speaker during an otherwise good broadcast.
- Usually this condition exists while traveling in the fringe area of the station.
- Flutter will become more severe beyond approximately 40 km (25 miles) of the station.
- The signal loss becomes greater away from the station, until finally noise takes over and reception becomes impossible.
- Flutter may also be noticed near the station because of the line-of-sight characteristics of FM radio waves.
- This condition can happen when a building or large structure is between the radio chassis and the station being received.
- Some of the FM signal bends around the building, but certain spots have almost no signal.
- Some of these signal losses are only a few inches wide. If the vehicle is parked in one of these dead spots, you will only hear noise from the radio speaker.
- After moving out of the shadow of the structure, the station will return to normal.
- Flutter will not occur on AM because the AM radio waves are much longer than FM waves.
FM Multi-Path Cancellation
Another effect caused by the line-of-sight characteristic is called cancellation.
- This condition exists when the radio waves are reflected from objects or structures.
- The noise produced by cancellation is similar to flutter, with the addition of distortion in the program.
- A more familiar description of cancellation is its similarity to the multiple ghosts and picture jumping that occurs on television when a low flying plane passes.
- The same condition exists in the vehicle, except that the vehicle is moving and the reflecting structure is stationary.
- The reflected signal cancels the normal signal, causing the 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.
FM Strong Signal Capture and AM Overloading
FM capture is an unusual condition that occurs when traveling in the vicinity of a broadcast tower.
- If listening to a weak FM station when passing the broadcast tower, a stronger station may interfere without changing the tuning control.
- When passing the tower, the station may switch back and forth a few times before returning to the station originally tuned.
Overloading is a localized condition which will not harm the receiver.
- When several broadcast towers are present (common in metropolitan areas), several stations may overload the receiver resulting in considerable station changing, mixing and distortion.
- Some overloading may also be noticed on AM, but usually to a lesser degree.
Receiving FM Stereo
Because more data is carried in the FM stereo waves than in the monaural FM broadcasts, flutter, cancellation and capture are even more noticeable.
- The FM stereo noise-free broadcast range is approximately 8 km (5 miles) less than that received with the monaural FM radio.
- The AM/FM stereo radio chassis may never encounter any of these troublesome conditions because they are more prominent in metropolitan areas, hilly terrain and depressed roadways. However, when diagnosing FM stereo concerns, tune to the strongest FM stereo station.
Stereo Indicator Inoperative or Flickering (AM and FM Reception Are OK)
- Verify that customer is listening to stereo stations.
- A weak or distant signal may cause the stereo chassis indicator to flicker. Tune radio chassis to a nearby FM stereo station. If reception is good but stereo indicator is still intermittent, remove radio chassis for service.
Interfering Noises
There are many possible sources of interference:
- One such source is the vehicle ignition system. This high-voltage switching system produces a radio frequency electromagnetic field that radiates at AM, FM and OB frequencies. Although components have been designed into the vehicle to minimize this concern, the noise is more noticeable if the radio chassis is tuned slightly off channel when listening to FM programs.
- Vehicle electrical accessories and owner add-on accessories may also contribute to radio chassis interference.
- There are many noise sources outside the vehicle such as power lines, communication systems, ignition systems of other vehicles and neon signs.
Ignition Noise
The most effective method for evaluating ignition noise is to compare the radio chassis performance with the engine on, versus engine off. If ignition noise is present with engine running:
- Check to see that the ignition wires are the suppressor type and that the spark plugs are the correct resistor type.
Missing or Damaged Noise Suppression Components
- Noise suppression components may be damaged or missing.
- Check radio frequency interference suppression bond grounding effectiveness by wedging a large file between metal parts to make sure of proper ground, such as between the tail pipe and body or between the fender and frame while radio chassis is playing and engine is running.
- Listen for a decrease in the objectionable radio chassis noise. If a reduction in radio noise is noted, first try tightening body and exhaust system clamps and brackets. Then, if necessary, install a new radio frequency interference suppression bond between the two metal parts to make sure of proper ground.