Antenna for radio and television reception in motor vehicles

Details Antenna for radio and television reception in motor vehicles Antenna for radio and television reception in motor vehicles

1998-03-23

2001-05-22

Wong, Don (Department: 2821)

Communications: radio wave antennas

Antennas

With vehicle

C343S704000

Reexamination Certificate

active

06236372

ABSTRACT:

BACKGROUND OF THE INVENTION
The invention relates to a television and radio antenna in motor vehicles in the meter (high) and decimeter (very high) frequency ranges. The invention is based on a multi-antenna system for creating an antenna diversity system.
The Prior Art
Radio and Television multi-antenna systems are described, for example in European Patent EP 0 269 723; German Patents DE 36 18 452; DE 39 14 424; DE 37 19 692; P 36 19 704; and may employ different types of antennas such as a rod, windshield, windowpane or similar antennas. One problem with these patents is that with an adequate HF-decoupling of the antennas, reception interferences occur in the receiving field when the vehicle moves into different positions. Such reception interferences occur in connection with transient drops in the reception level because of multi-path propagation of the electromagnetic waves. This effect is explained by way of example in EP 0 269 723 with the help of
FIGS. 3 and 4
.
To overcome these problems, a scanning antenna diversity system is used to switch from one antenna to another when a reception interference occurs in the operating antenna. These diversity antennas provide an additional antenna to keep the number of level drops or signal breaks leading to reception interferences in a predetermined receiving field as low as possible on the receiver input. Diversity antennas are extensively effective, but require an indicator for the interference taking place, equipment for changing over the antennas, as well as two antennas. Unfortunately, the interference indicator and the required change-over equipment can be quite expensive. On the other hand, it is desirable to raise the receiving quality as high as possible, especially when an antenna diversity system is employed.
When trying to overcome these breaks in reception, statistical modeling by Rayleigh has been used to map the signal paths of radio and television waves. From these statistical models of the electromagnetic waves incident on the vehicle, it is known that locally limited level breaks of the receiving signal occur with each antenna present on the vehicle. When driving, these level breaks cause short-term reception interferences which are perceived as extremely annoying when receiving with only one antenna.
SUMMARY OF THE INVENTION
Therefore, it is an object of the invention is to obtain the highest possible receiving quality for a car antenna.
Another object of the invention is to provide a diversity antenna system that is simple in design, easy to operate and install.
Essentially, the invention relates to a diversity antenna that receives electromagnetic waves from a radio or television station from all azimuthal space directions with a similar probability especially in urban areas and hilly and mountainous regions. Therefore, the plot of the reception level of each of the antennas during a drive over time is practically independent of the shape of the relative azimuthal directional diagram. In addition, because the individual antennas have different directional diagrams, different positions, and different designs, breaks or drops in the receiving level of the individual antennas do not occur simultaneously.
Therefore, since the present antenna provides inconsistent drops in reception, it can provide consistent readings for reception. Accordingly, the probability curves for the antennas exceeding their threshold level are practically overlapping. However, there is a slight shift in these curves which is caused by a difference between the mean-time values of the logarithmic receiving levels of two antennas (S
meddb2
−S
meddb1
). The sensitivity of the receiving installation is measured based on its inherent noise, and the actual signal
oise (S/N) spacing (or separation). This spacing is determined by the ratio of the effective value of the useful level received on the antenna output to the effective value of the inherent noise level of the receiving installation based on the receiver input. For interference-free reception, the antenna may be required to exceed a defined minimum value SNR
min
. The probability “p” for falling short of this value when driving in a region with a mean or median value S
med
of the receiving level, and a noise level N, with a median value S
med
/N resulting therefrom, can be stated as follows:
p=1−exp (−SNR
2
min
/(S
med
/N)
2
)  (1)
Both of these values are expressed as usual in the logarithmic measure of dB, which is obtained by the following formula:
SNR
mindB
=20* log (SNR
min
) and (S
med
/N)
db
=20* log (S
med
/N)  (2)
The probability “p” for falling short of the minimum requirement SNR
min
in dB when driving through a region or area the following is obtained from the following formula:
p=1−exp(−10
(SNRmin
db
−Smed/N)
db
)10
)  (3)
This probability for the occurrence of interference occurs when the transmitted signal falls short of the minimally required signal
oise ratio. In this case, the probability of interference is synonymous with the relative interference time, with the proviso that the interference time is measured in percent of p%=p*100.
In the past, reception quality or Q was measured based upon the probability of interference by the following formula:
Q=1/p  (4)
Q
dB
′ can be precisely expressed also by the logarithmic measure with
Q
db
=−20* log [1−exp(−10
(SNRmin
db
−Smed/N)
db
)10
)]  (5)
This logarithmic value for signal quality is shown in
FIG. 1
c
with a completely statistical wave field according to Rayleigh. While the signal quality is independent of the form or shape of the directional diagram of the antenna. In practical applications of the antenna, there is only a minor deviation from this natural law.
For example, in rod-type and windowpane antennas, there is practically no deviation from the above conformity to natural law. In practical life, especially the typical “indent” of the azimuthal directional diagram over an angle range of up to 30° as it is frequently found with antennas, has hardly any notable negative effect because of the Rayliegh wave field. However, there have been recent efforts to obtain omnidirectional azimuth diagrams even though this criterion is not suitable for evaluating the receiving quality. For example, U.S. Pat. No. 4,260,989 is cited here as an example of such efforts, where azimuthal directional diagrams are shown in FIGS. 28
a
to 29
e
without notable “fading”, which are obtainable with the bizarre antenna structures specified in said reference.
However, because of the Rayleigh wave distribution, level breaks, fading or indents occur when driving with these antennas because they cancel the incident waves in various locations and lead to level indents when all of these waves are evaluated with a round azimuthal diagram. This shows that the demand or call for a diagram without indents is of little help. It has been shown that such a demand opposes optimization of the receiving quality as described above, especially when antennas are designed for a complete (overall) radio frequency range. The possibility for optimization is inadmissibly narrowed down by such a demand.
Therefore, contrary to the opinion frequent heard, which is that azimuthal roundness of the antenna diagram is the only important antenna property for VHF radio reception, the fact is that the expression
 (S
med
/S
min
)
dB
=(S
med
/N)
dB
−SNR
mindB
,  (6)
which inserted in equation 5 for the receiving quality results in
Q
dB
=−20* log [1−exp(−10
−(Smed/Smin)dB/10
)]
which represents the decisive or relevant feature for the receiving quality where S
min
is the minimum value of the signal level required in order to satisfy the requirement of a defined value for the signal/interference ratio SNR
mindB
. The connection or relation between receiving quality Q
db
and the mean value of the logarithmic protective signal spaci

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