The present invention relates to an information transmis¬ sion system for the transmission of information through communication via electromagnetic waves, especially micro¬ waves, comprising a l:st communication unit with a 1 :st transmitter and a l:st receiver, and a 2:nd communication unit with a 2:nd receiver and a 2:nd transmitter, the 1 :st communication unit being mounted on a stationary stand and the 2:nd communication unit being mounted on a vehicle, which is movable along a path, the antenna lobes of the l:st and 2:nd transmitters and of the list and 2:nd recei¬ vers being designed and oriented in such a way that commu¬ nication between the 1:st and 2:nd communication units is possible, when the vehicle is located within a relatively short distance from the stand.

Such an information transmission system can e.g. be used at car tolls, i.e. such car tolls where automatic payment is effected, which permits the vehicle to travel a certain length. The antennae or antenna, for they can be combined into one, of the transmitters and receivers of the 1:st communication unit are placed at a certain height above the road with the antenna lobe so oriented that communica¬ tion is made possible with the 2:nd communication unit, in this case called a transponder, i.e. the antennae or the mutual antenna of its transmitter and receiver. This transponder is suitably located inside the front window of the vehicle.

It is desirable that the communication axis is not too vertical, partly because certain vehicles have front win¬ dows close upon the vertical position, partly because a suitably inclined communication axis gives an increased length of communication with the vehicle and thereby the

possibility of transmitting more data without increased demands on the data transmission speed.

The orientation of the antenna lobe of the l:st communica¬ tion unit must be such that communication with the 2:nd communication unit is possible irrespective of where on the path the vehicle is driven and irrespective of where behind the front window the 2:nd communication unit is located.

A problem hereby arising is that the wave between the an- tenna of the 1:st transmitter and the antenna of the 2:nd receiver can extend partly directly, and partly via a re¬ flection in the vehicle bonnet. Hereby, interference can occur between the wave that goes directly and the one that goes via a reflection. The problem is that the combined signal fades when the vehicle passes the l.st communica¬ tion unit with the interference giving an addition or fade-out as is well known in the radio technique.

A known way to solve this problem with reflection is to provide said transmitters and receivers with circularly polarized antennae whereby the polarization of the re¬ flected wave is inversed in the reflex point and can thereby no longer interfere with the direct-going wave. For example, the reflex from a right-polarized wave will be left-polarized after reflection and thereby not receivable by a right-polarized antenna.

A circularly polarized system is, however, relativly com¬ plex and thereby expensive. This is due to the fact that the creation and reception of a circularly polarized wave require circuits for generating/receiving two linearly po- larized waves with a mutual phase angle difference of plus or minus 90° depending on whether the polarization is to be right- or left-oriented. A known construction solution

for circular polarization, compared with a linearly pola¬ rized solution, requires a power divider, a phase changer and an additional feed point on the radiating element. There are, per se, other ways with thinner band to achieve circular polarization with maintained polarization qua¬ lity, take up manufacturing differences and admit diffe¬ rent frequency channels in the system.

It is also technically difficult to design circularly po¬ larized antennae so that their polarization quality (ellipticity) is acceptable over the entire range of di¬ rection.

The object of the present invention is therefore to achieve an information transmission system of the kind mentioned in the introduction, which is simple in con- struction and which admits a relatively wide variation in orienting the antenna lobe of the l.st transmitter, and yet admits a safe information transmission despite the presence of more or less horizontal or vertical reflecting surfaces, primarily the vehicle bonnet.

An information transmission system according to the in¬ vention is primarily characterized in that the 1:st transmitter is arranged to transmit via its antenna a linearly polarized wave, whose polarization vector (E vector) in the polarization plane forms principally an angle α with the horizontal component in the polarization plane, cc being preferably 45°, the 1:st and 2:nd recei¬ vers and the 2:nd transmitter being arranged to receive and transmit, respectively, a wave with corresponding po¬ larization, so that disturbing reflexes in existing prin- cipally horizontal and vertical surfaces on the vehicle are avoided.

If a plane, linearly polarized wave with the E field com¬ ponent parallel with a reflecting plane, such as a metal plate, incides upon it, the wave is reflected with the E field component 180° phase shifted. If the H field compo- nent of the incident plane wave is parallel with the re¬ flecting plane, the phase of the E field component does not change. Since the E field component of a linearly po¬ larized wave can be divided in two components, one paral¬ lel with the reflecting plane and the other perpendicular to the first component, this means that if a plane, linearly polarized wave incides at a gradient of 45° on the line which characterizes the direction of the hori¬ zontal component in the reflecting plane, the polarization of the plane, reflecting wave will be perpendicular to the polarization of the incident plane wave.

If the receiving antenna, i.e. in this case the 2:nd re¬ ceiver, is arranged to receive a wave with the described polarization, it will not be capable of receiving, and thereby being disturbed by, a wave whose polarization has been turned 90° relative to the wave it is arranged to re¬ ceive.

In a suitable embodiment of the information transmission system according to the invention the 2:nd communication unit, the transponder, is mounted at the front window of the vehicle and the 1:st communication unit is mounted on said stand at a level essentially higher above the plane of said path than the 2:nd communication unit.

When applied at a car toll the 2:nd communication unit is suitably arranged to reflect the incoming wave, preferably after modulation of the frequency of the wave, for example through phase shift.

The information transmission system should be so designed that both reading and writing in data from the 1:st commu¬ nication unit from and to, respectively, the 2:nd communi¬ cation unit is possible through coding the wave.

The invention will now be described in more detail with reference to the accompanying figures, of which:

Figure 1 shows schematically a side-elevation of an information transmission system according to the invention;

Figure 2 shows a diagram of the amplitude of a transmitted signal partly a) without interference from a reflected wave, and partly b) with interference from a reflected wave.

Figure 3 shows schematically the information transmission system in Fig 1, seen from the front;

Figure 4 shows schematically the antennae of a 2:nd commu- nication unit.

In figure 1 a 1:st communication unit is designated by 1, mounted on a stand 2. In a vehicle 3, driven on a path 4, a 2:nd communication unit 5 is mounted in the upper end behind a front window 5. The l:st communication unit 1 is provided with l:st transmitters and l:st receivers, not shown in detail. The antenna of the l:st transmitter, which can be one with the antenna of the l:st receiver, shows an antenna lobe, which incorporates the antennae of the 2:nd receivers and 2:nd transmitters of the 2:nd com- munication unit, which antennae 2 can also be one mutual antenna. In the figure there is shown a direct-going wave from the l:st transmitter to the antenna of the second re¬ ceiver. The path of a reflected wave is designated by 7.

In figure 2 it is shown how the amplitude of the signal transmitted with the wave changes on the vehicle passing the l:st communication unit, i.e. with time, the graph A showing the undisturbed signal, and the graph B showing the fade-out that takes place, if a wave reflected in the vehicle bonnet 8 interferes with the direct-going wave.

From figure 3 it is evident how a plane, linearly polari¬ zed wave inclined at 45° to a line, which characterizes the direction of the horizontal component of the wave in the reflecting plane, i.e. the vehicle bonnet, is trans¬ mitted from the antenna of the l:st transmitter of the communication unit to the antenna of the 2:nd communica¬ tion unit. The figure also illustrates how the signal, re¬ transmitted from the communication unit through the 2:nd transmitter, goes back to the l:st receiver, α = 45° gra¬ dient to the reflecting plane is the optimal angle, but a good interference suppression is obtained also at a cer¬ tain deflection from this angle, however less good the farther from α = 45°.

In figure 4 the antenna arrangement is shown schematically in a 2:nd communication unit with the antenna 9 of a 2:nd receiver and antenna 10 of a 2:nd transmitter, the former arranged to receive a plane, linearly polarized wave, inclined at 45° according to the above-mentioned defini- tion, and the latter arranged to transmit a thus polarized wave.