Today is the last and third part of the subject of antennas. It is impossible to imagine windscreens, backlights and sidelights without glass antennas. But the complexity and amount have greatly changed over time. For example, four antennas are installed in the backlight of the VW ID3: 2x FM and 2x DAB. Mobile radio and Car2X are possible due to the roof antenna. As we can see based on this example, the installed antennas don’t necessarily have to be different antennas. For DAB reception, two antennas are often required in order to get the best signal through antenna diversity.
But how are antennas installed in vehicle glass?
The antennas are placed inside the PVB film like heating wires. The shape and length vary depending on the area of application. To be able to use the antennas for different frequencies, impedance (resistance) transformers are installed in the vehicle.
To filter the useful signal of the backlight antenna from interference signals from the vehicle electrical system, interference suppression chokes are fitted to the plus and minus sides of the backlight heating. These consist of a capacitor and a coil connected in parallel. Capacitors and coils act as reactances in the high-frequency (HF) range and help to electrically "decouple" the backlight. However, this isn’t a galvanic isolation. On-board power supply fluctuations such as generator harmonics and HF interference from data bus systems can have a negative effect on the antenna signal.
Technically speaking, most backlight antennas are similar to a monopole/rod antenna, as the vehicle mass is used as a counterweight. Dipoles are also occasionally used.
My radio plays interfering signals - what can I do?
If the FM antenna is installed in the backlight, a simple supporting tool can be made from an H7 light and a 1 m long wire. Solder 50 cm to each of the contacts on the bulb. To do this, the wire must be completely exposed.
But why 50 cm? In frequency modulated (FM) reception, an average frequency of 150 MHz is assumed, and the physical wavelength is approximately 2 meters. A dipole is created by attaching the wire at both ends. This works best with a setup according to λ/2. As there are 2 poles, the wavelength of 2 meters must be divided by 2 (= 1 meter). Due to the setup, the wavelength of 1 meter is divided by 2 again. This results 50 cm.
How do you use the tool?
The H7 lamp and the 50 cm wires function as a replacement for the backlight heating.
The backlight heating is disconnected, and the tool is connected. The H7 lamp thus serves as a load and ensures that there is no short-circuit. The wires serve as a replacement for the antenna.
Next, the radio is switched on and then the "replacement" backlight heating (support tool). How does the radio signal behave now?
Variant 1) If the radio signal is better after switching on, the fault is in the installed backlight.
Variant 2) If the interference signal is still present, the interference suppression chokes may be the source of the fault.
Variant 3) If the radio signal is poor even when the "replacement" backlight heating is switched off, the problem lies elsewhere. Possible sources of error:
• Backlights, especially with radio antennas, must not be installed with conductive glue. Exception: The manufacturer explicitly specifies the use for backlights with heating.
• If conductive tinting films are used, there is a risk of short-circuiting the glass heating. Then, the Antenna reception is also no longer available.
The subject of antennas in vehicles is very extensive and can be very misleading under certain conditions. Please feel free to contact me if you have any questions or problems.
I hope you enjoyed this short excursion into antennas, and I look forward to the next and for now last article.