Radio Wave - Basics

RADIO WAVES

Radio waves move at the speed of light 186,000 mi./sec. (300,000 km/s). Depending upon the wave-length, these waves are affected to differing degrees by the weather, mountains, buildings, tunnels, trees and other obstacles.

AM RADIO WAVES

AM = amplitude modulation (changing the amplitude of the transmitter frequency as a function of the transmitted speech and music).

This method is used in the long wave, medium and short wave bands

AM radio waves have relatively low frequencies (approx. 150 kHz to approx. 6 MHz). If certain conditions are met, the path of these waves follows the curvature of the earth and the waves are reflected by various layers of the atmosphere.

At these frequencies, the range of a transmitter is considerably greater than is the case for FM radio waves (particularly during the evening and night).

FM RADIO WAVES

FM = frequency modulation (changing the frequency of the transmitter signal as a function of the transmitted speech and music). This method is used in the very high frequency (VHF) band.

FM radio waves have much higher frequencies (approx. 87.5 MHz to approx. 104 MHz). They follow along straight lines, do not follow the curvature of the earth and are not reflected by various layers of the atmosphere. In addition to the dependence on the wavelength of the signal, the signal strength received is affected by the height of the transmitting antenna, the transmitted power and the conditions between the transmitter and receiver.

Reception is best when the car antenna can see: the transmitting antenna. However, this is almost never the case. Many times, the signal is reflected by stationary objects, subjected to interference and only then received by the vehicles antenna. In this manner, the signal is delayed and weakened.

SUSCEPTIBILITY TO INTERFERENCE OF AM AND FM RADIO WAVES

Since the music and speech information are contained as changes in amplitude of the transmission frequency in the entire transmitter signal and since interference pulses mainly affect the amplitude of a transmitter signal, the susceptibility to interference of AM radio waves is considerably greater than that of FM radio waves.

FADING

Fading is a typical AM phenomenon characterized by irregular fluctuations in volume in conjunction with distortion. This phenomenon occurs if the distance between the transmitter and receiver is very large and if the vehicles antenna has not been completely extended. Fading is compensated to a large degree by the automatic fader control provided in all radios.

OVERMODULATION

Receiver overmodulation manifests itself as distorted reproduction as well as multiple reception of the same transmitter. Overmodulation occurs in the vicinity of high-power radio transmitters.

INTERFERENCE SUPPRESSION

In the case of interference on a car radio, differentiation must be made between interference with a physical cause (affected by the propagation conditions) and radio interference.

Interference with a physical cause cannot be eliminated by means of interference-suppression measures on the vehicle or by receiver technology.

Radio interference coming from the vehicle itself can be eliminated to a large extent by means of pinpoint interference-suppression measures.

Interference suppression means short-circuiting the interference energy to the chassis via the shortest possible path or to impede the propagation of this energy, to block it.

The effectiveness of interference suppression depends largely on the following basics:

1. Mount the interference-suppression component as close as possible to the source of interference. The connecting leads between the source of interference and the interference-suppression component should be kept short.

2. Proper electrical connections of the interference-suppression components, the positive terminal of the source of the interference, and/or chassis ground. This only applies to interference-suppression capacitors, interference-suppression filters and ground straps.

3. The antenna should be installed as far away from the source of interference as possible.

4. The ground contact of the antenna has a decisive effect on the interference component in the transmitter signal. The base of the antenna which is to be connected to the chassis without resistance if at all possible has the task of short circuiting to ground interference acting on the antenna lead via the shield connected to the base. For this reason, if the interference is present, the ground contact of the antenna must be checked. This means removing paint, body sealer, and corrosion if present.