Radio (RD) function notes - AD82.60-P-2000-04A

VHF and VHF stereo reception in moving vehicles 

Since the introduction of stereo radio systems in passenger cars there have been various complaints of interference with stereo reception. However, this interference is not caused by a fault in the vehicle or radio or the reception system, as is assumed in practically all cases, but is attributable to the special peculiarities of ultra-short wave reception.

For these reasons we are providing you with information which will make it easier to evaluate complaints and should prevent unnecessary troubleshooting.

Field strength ratios 

Due to the special nature of ultra-short waves which propagate in straight lines, in theory only direct reception is possible between the transmitter and receiver. In practice the ultra-short waves follow the curvature of the earth, being broken or reflected by all types of buildings and ground elevations.

The result of this is that field strength ratios occur which are caused by transmitter shadowing. Whereas with stationary systems even field strength ratios can be expected, in the moving vehicle these fluctuate continuously at different intensity.

The field strength graph shows this type of field strength curve. In a city for example, differences in signals up to a ratio of 1:1000 can occur within a few meters.

The different field strength ratios are of particular importance for stereo reception, because a three-times larger frequency band is required than for mono reception.

From this it is clear that with stereo reception a three-times larger frequency band contributes to three-times more "noise" than with mono reception. In practice about ten times the field strength is therefore required for interference-free (noise-free) stereo reception.

In areas with unfavorable field strength ratios stereo reception can be interfered with much more easily than mono reception e.g. due to shadowing. It is also possible that a transmitter tuned in this way suddenly "disappears". The cause of such interference is not a defect or fault in the receiver but is due to the reception conditions.

Reflection 

Interference noise and distorted reception may be caused by reflections which are the most frequent reception fault. They occur when a direct and a reflected signal from a transmitter are received at the same time. The signals received on the tuned-in transmitter frequency are then a mixed product and, circumstances permitting, are phase-shifted due to differences in propagation times so that distortions of the LF signal which are clearly audible occur. As it only occurs under quite specific conditions, this process is generally very short but its acoustic perception is very marked and may be reproduced clearly by slowly driving through sections of road with reflections. The signals are superimposed and cancel each other out.

Other faults 

The vehicle antenna not only receives transmitter signals but also interference signals which can be caused by interference sources outside the vehicle or by the vehicle itself. If interference suppression on the vehicle itself is in order, a useful signal which is too weak may also cause interference. If the useful voltage at the receiver input is a multiple of the interference voltage, interference in the VHF range is completely suppressed.

If by contrast the interference voltage is about the same order of magnitude as the useful voltage or even higher, for example with the tuning of a weak transmitter, interference during reception is particularly apparent. There is no technical solution to this interference, you have to change over to another transmitter.

Radio interference due to short-term interference sources 

The expense required for effective interference suppression of short-term interference sources, e.g. signalling systems, solenoid valves, electrically adjustable exterior rearview mirrors and the windshield washer fluid pump bears no relation to the effect which can be achieved and is therefore not taken into account in the scope of interference suppression on the part of the plant. For interference due to these consumers it can be pointed out that they only occur anyway when the traffic situation calls for greater concentration and that for this period an impairment to the radio reception can be acceptable.

Retrofit interference suppression is possible in individual cases, but this work is very time-consuming and has to be paid for by the customer.

For further information, refer to the radio Operating Instructions.