Principles of Automotive Antenna


The antenna can be said to be the radio signal’s “doorway” to the radio, and is thus an important element to reproduce good sound.
2. Types of antenna
The following two types of radio antennas are used for the vehicle: the pole antenna and the print type rear window antenna.
The pole antenna can be separated into the type installed on the front or rear fender and the type installed on the middle back of the roof.
A motor type antenna can automatically move up/down by switching on and off.
A print type rear window antenna has conductive paint on the rear window glass. The characteristic of this antenna is that there is no need to move the pole up and down like a pole antenna, there is no wind noise, and it is durable because it does not fold or rust.
3.Antenna and receiving sensitivity
The radio wave picked up by the antenna becomes extremely low-amperage electrical signals and is transmitted to the radio through the core electric wire called the coaxial cable (feeder line).
To pick up the radio wave for the antenna, its length should be half of the wave length of the radio wave. (For example, when the broadcast station whose frequency is 1,300 KHz of AM broadcasts, 115 m of antenna is needed.)
It is impossible to install such a long antenna on the vehicle, but an antenna should be used that is as long as possible.
In the case of a pole antenna, to listen to the radio with a good quality of sound, the antenna should fully be extended.
In the case of a print type rear window antenna, even a tiny break in the imprint will cause the sensitivity to worsen.
4.Antenna and noise
The electrical signal picked up by the antenna arrives at the radio via the core wire of the coaxial cable. If any noise other than the broadcast radio wave is entered, noise occurs in the radio and listening to the program will be difficult.
The electrical equipment on the vehicle such as the ignition system, charging system, and motor produces various noises.
To prevent the noise from entering the core wire of the coaxial cable is covered with netted electric wire called shield wire. This shield wire blocks the noise and grounds it to prevent the noise from entering into the core wire.
When the shield wire cannot ground well, noise is produced.
5.Glass printed antenna
This antenna gets imprinted on the rear window glass like the pattern shown in the figure. (The patterns vary depending on the models.)
The electrical signal picked up by this antenna should be amplified by the booster amplifier due to the following reasons.
Sensitivity is worse than the pole antenna
The distance between antenna and radio is long and the received signal weakens.
In the case of a print type rear window antenna, something called choke coil is connected to the rear window defogger circuit as shown at the left.
This prevents the noise from entering the radio by grounding the noise current contained in the power supply of the rear window defogger.
6.FM diversity antenna
This is an antenna system that maintains the good receiving condition by combining the two antennas to prevent reception problems such as fade out. Generally, this antenna has a main antenna and a sub antenna. When the sensitivity of the main antenna becomes weak, the sensitivities between the main antenna and sub antenna are compared and the better one is used.
7.Roof antenna
The roof antenna is approximately 1/8 long of the normal pole antenna so that it does not touch in the tunnel or in the indoor parking lot.
As a device to improve the sensitivity, an antenna amplifier is installed at the base of the antenna so that it can receive the radio wave not inferior level to the normal pole antenna.
This type is removable by rotating the antenna counterclockwise when the antenna is damaged.

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