Hinged read-out antenna system

A novel antenna system disposed around a conveyor belt for the interrogation of objects passing through the antenna which overcomes centralized dead zone problems is disclosed. A frame antenna which is disposed around the conveyor belt achieves the interrogation and a separate reading antenna, which is disposed hingedly from the top of the frame antenna performs the reception.

FIELD OF THE INVENTION 
The invention relates to an antenna system for an interrogating station for 
the interrogation and identification--using high-frequency signals--of an 
object which is provided with a transponder and which is moved along an 
interrogation path, such as a conveyor, which antenna system is operable 
for transmitting interrogation signals and for receiving and 
identification code transmitted by the transponder. Such an antenna system 
is known in practice. 
BACKGROUND OF THE INVENTION 
Through such an interrogation station, the transponders fitted on the 
objects are illuminated or charged by means of high-frequency 
interrogation signals. Such a transponder is generally a passive 
transponder in which, on the basis of the charge produced by the 
electromagnetic field, a transmission of its own identification code then 
takes place. The antenna system used can have the function of transmitting 
or interrogating antenna and the function of receiving antenna for 
receiving or reading the identification code. The antenna can be in the 
form of a frame antenna which is fitted around a conveyor belt, but which 
as regards to reading of the transponders, has one or more dead zones for 
certain transponder antenna orientations. For example, there lies a dead 
zone across the very center of the conventional frame antenna. If a 
transponder is located within a window which is parallel to the belt and 
comprises some distance on either side of the center frame axis, and 
maintains this position throughout the read area of the antenna, the 
transponder would not be read. Moreover, the accuracy and completeness of 
this reading also decreases if several transponders are following one 
another in close succession during the movement along. When transponders 
are in close succession, because they are relatively far from the read 
antenna, the transponders appear to be the same distance from the read 
antenna and thus send back simultaneous transmissions. The result of a 
simultaneous transmission is an unintelligible identification code. This 
is particularly the case in a noisy environment, for which a shielding 
would then be necessary in receiving conditions. 
SUMMARY OF THE INVENTION 
The object of the invention is to eliminate the above-mentioned problems 
and to indicate an antenna system by which both the illumination and the 
reading are achieved with great accuracy. 
This is achieved in the case of an antenna system of the type mentioned in 
the preamble, in that for the interrogation, a frame antenna is disposed 
around the conveyor, and for the reception a separate reading antenna is 
provided, having a hinged suspension at the top side of the frame antenna. 
This design with the reading antenna in the center or near the center of 
the frame face produces a reading radiation pattern through which the 
aforementioned dead zone otherwise occurring in the center is avoided and 
problems with the reading of transponders following each other in close 
succession are also eliminated.

DESCRIPTION OF THE PREFERRED EMBODIMENTS 
FIG. 1 shows schematically the arrangement of a frame antenna 2 which is 
disposed around a conveyor belt 1 and is intended for interrogating and 
identifying objects transported by the conveyor. These objects can be 
pieces of luggage each provided with a passive transponder. If the frame 
antenna have both the function of illuminating or charging the 
transponders and that of receiving or reading the identification code 
transmitted by these transponders, it will be clear that for certain 
transponder orientations dead zones can occur in particular along the 
center frame axis. This problem of incomplete or poor reading occurs in 
particular when transponders are moved along in the center of the 
interrogation path in close succession. In order to eliminate this 
problem, it is possible, as in the copending Netherlands Dutch Patent 
Application No 9100110, to use several receiving antennae which are 
disposed equally around the periphery of the frame antenna. 
However, according to the invention, another way is followed and use is 
made of a separate receiving antenna 3 which is suspended hingedly at the 
top side of the frame antenna 2 in such a way that, on the approach of the 
objects, it can be moved away as indicated by the dashed line in FIG. 1. 
As the antenna moves away, the reading radiation pattern moves as well, 
thereby encompassing a much greater read area than would a stationary 
antenna. In this way a dead-zone free read area is established wherein 
every transponder orientation can be read. The separate hinged receiving 
antenna also solves the problem of trying to read a close succession of 
transponder antennas by hanging in the midst of the objects to be read 
such that a more real distance between transponders is achieved and only 
one transponder at a time will respond. 
The antenna coil can have several windings which are embedded, for example, 
in a plastic disc. This disc is suspended at the top side of the frame 
antenna by means of a plastic carrier in which or on which the antenna 
coil connection wires are accommodated. The hinged fastening 4 at the top 
side can be designed in different ways. This arrangement of the separate 
reading antenna ensures a good reading of the transponders, with the 
avoidance of dead zones. The separate reading antenna can in this case be 
arranged in such a way that it is suspended approximately in the center 
of--or a little above--the frame face. 
FIG. 2 shows an electrical diagram of the hinged coupling with the 
connections at one side to the receiving or reading antenna in the form of 
an air-core coil and at the other side to the receiver in the 
transmit-receive unit of the interrogating station. The embodiment shown 
in FIG. 2 comprises an inductive or contactless coupling with two coils, 
one of which is fixed, while the other is movable or hinged. The advantage 
is that no fatigue of connecting wires occurs, as in the case of an 
galvanic connection. 
FIG. 3 gives a cross-sectional view of this inductive and contactless 
coupling of FIG. 2. In order to make the magnetic coupling larger, two pot 
cores are used in said inductive coupling, one of which is fixed to the 
top side of the frame antenna, while the other can move along it rotating 
about the same longitudinal axis as that of the fixed pot core. As 
indicated, each pot core is provided with a coil. It goes without saying 
that other inductive or contactless couplings are also possible, for 
example with two ferrite rods. FIG. 4 shows an example of such a variant. 
If an electrical galvanic connection is still selected, there is the 
possibility of making the hinged suspension with an antenna conductor wire 
wound in spiral form, so that the connection from the reading coil by 
means of the conductor acting as suspension is fixed at the top side and 
is taken from there to the receiver. Standard wire can advantageously be 
used for this conductor wire, in order to counteract the influence of 
parasitic capacities.