LOOP ANTENNA

This invention relates to a loop antenna, and in particular to a method of feeding a loop antenna, and to a loop antenna having a particular feed arrangement.

Loop antennas are known, in which a loop of conductive material is positioned in front of a reflector, and is fed through a feed line. A balun typically needs to be inserted between the feed line and the driven loop, in order to achieve the intended impedance matching.

In some wireless communications applications, for example in the case of a Global Positioning System (GPS) transceiver, it is necessary to receive and/or transmit circularly polarized radio signals. The antenna for such a transceiver is often formed as a patch antenna, because it is convenient to feed a patch an antenna in such a way as to achieve the required circular polarization.

However, patch antennas have the disadvantage that they have a relatively narrow axial ratio bandwidth.

According to an aspect of the present invention, there is provided a loop antenna, that can be fed in such a way as to achieve circular polarization, while maintaining a desirable operating bandwidth.

More specifically, according to an aspect of the present invention, there is provided an antenna, comprising: a reflector; a loop of conductive material, the loop being spaced from the reflector by means of a solid dielectric material; and a feed for the loop of conductive material, wherein the solid dielectric material defines a cavity, located inside the loop of conductive material.

This has the advantage that a relatively small antenna can be used to provide good operating characteristics.

Figure 1 is a schematic diagram, showing an antenna in accordance with the present invention.

Figure 2 is a schematic diagram, showing a second antenna in accordance with the present invention.

Figure 3 is a schematic diagram, showing a part of an antenna in accordance with the present invention.

Figure 4 shows an antenna arrangement in accordance with the present invention.

Figure 5 shows a second antenna arrangement in accordance with the present invention.

Specifically, Figure shows an antenna taking the form of a loop antenna 10. A reflector 12 is formed, for example by providing a conductive coating on a substrate. A conductive loop 14 is then formed in front of the reflector 12, with a dielectric insert 16 between the reflector 12 and the loop 14, so as to space the reflector 12 from the loop 14. Spacing the conductive loop 14 from the reflector 12 by means of a dielectric insert 16 formed from a material having a relatively high dielectric constant allows the use of a generally smaller antenna.

In this illustrated embodiment of the invention, the dielectric insert 16 is shaped in the form of a cylinder having a square cross section. However, a cylinder of any cross- sectional shape can be used. Further, it is not necessary that the dielectric insert 16 be in the form of a cylinder.

However, the cylindrical form illustrated in Figure 1 has the advantage that the entire upper surface can be coated with a conductive material, typically silver, in order to form the conductive loop 14 on top of the dielectric cylinder. This makes the manufacturing process easier. For example, the loop can be printed directly onto the upper surface of the dielectric. However, the loop can be printed onto the upper surface of the dielectric even if the desired shape of the loop is not the same as the shape of the upper surface of the dielectric insert.

Figure 1 also shows the radio frequency electronic circuitry required to process a signal received by the antenna 10. The following description therefore relates to a situation where the antenna is being used in a radio receiver, for example a Global Positioning System (GPS) receiver, that is required to receive circularly polarized radio signals.

The primary energised loop 14 is able to induce electrical signals in two smaller loops 18, 20.

More specifically, the first smaller loop 18 takes the form of a half loop, formed from a conductive material, printed onto a first inner surface 22 of the dielectric cylinder 16. One end of the half loop 18 is shorted to the reflector 12 at a shorting point 24, while the other end of the half loop 18 is connected to a first signal output 24. The second smaller loop 20 takes the form of a half loop, formed from a conductive material, and printed onto a second inner surface 28 of the dielectric cylinder 16. The second inner surface 28 of the dielectric cylinder 16 is one of the inner surfaces adjacent to the first inner surface 22. One end of the second half loop 20 is shorted to the reflector 12 at a shorting point 30, while the other end of the half loop 20 is connected to a second signal output 32.

The first and second signal outputs 26, 32 are then connected to radio frequency receiver circuitry.

In this illustrated embodiment, with the feed loops 18, 20 printed on the inner surfaces of the dielectric cylinder 16, it is advantageous that the receiver circuitry is located within the dielectric cylinder 16, as this reduces the overall size of the receiver device.

As mentioned above, where the received signal is circularly polarized, the signal connection points 26, 32 for the two loops are connected to a 90° combiner 34.

As is generally conventional for a GPS receiver, the output from the combiner 34 is connected to a first low noise amplifier 36, a bandpass filter 38, a second low noise amplifier 40, and a further filter 42, allowing the resulting signal to be transmitted over a coaxial cable (not shown in Figure 1) to further signal processing components. It will be appreciated by the person skilled in the art that the receiver circuitry could include other components and combinations of components. In any case, it will be appreciated

that locating some or all of the receiver circuitry within the dielectric cylinder 16 has advantages in terms of size reduction.

In the illustrated embodiment of the invention, the receiver circuitry is mounted onto a substrate 44, which is, in turn, mounted on the reflector 12 along with the dielectric cylinder 16. The dielectric loop 16 could be glued to the reflector 12 or, alternatively, its base could be coated in the same conductive material used to form the energised loop 14, and it could then be soldered to the reflector 12.

Although the coated lower surface of the dielectric loop 16 could be soldered directly to the reflector 12, a printed circuit board (PCB) could alternatively be mounted on the reflector 12. In that case, the PCB contains a footprint to which the dielectric cylinder 16 is soldered, and has two connection points for each of the two half loops 18, 20. One of these connection points can then be connected to the receiver circuitry, including the circuitry for ensuring the intended phase relationship between the signals. The other of the connection points can be shorted to the reflector 12.

Although Figure 1 shows the two half loops 18, 20 printed on respective inner surfaces of the dielectric cylinder 16, they could be formed in other ways. For example, they could alternatively be printed on outside surfaces of the dielectric cylinder 16.

In addition, while Figure 1 shows the use of a 90° combiner 34 in order to achieve the required phase difference between the signals received in the loops 18, 20, Figure 2 shows an alternative arrangement for achieving this.

In Figure 2, elements having the same functions as elements of the antenna shown in Figure 1 are indicated by the same reference numerals. In the antenna 60 shown in Figure 2, however, the connection points 26, 32 of the loops 18, 20 are connected to a common connection point 62 by means of respective conductive paths 64, 66.

The conductive paths 64, 66 are printed on the respective surfaces of the dielectric cylinder 16, and their lengths are such that they introduce a 90° relative electrical phase difference into their respective received signals. Thus, in the illustrative example shown in Figure 2, the conductive path 64 between the connection point 26 and the common connection point 62 introduces a phase delay at the frequency of interest that is 90° greater than the phase delay introduced by the conductive path 66 between the

connection point 32 and the common connection point 62. This allows a circularly polarized signal to be received by the antenna 60 or, equivalent^, allows the antenna 60 to be used for transmitting a circularly polarized signal.

As shown in Figure 2, the common connection point 62 is provided on the inner surface of the cylinder 16. In an alternative arrangement, as shown in Figure 3, the common connection point can be provided as a connection pad, and the connection pad can be provided on an underside of the dielectric cylinder 16.

Figure 3 is a schematic diagram showing the location of the connection pad in such an arrangement. Specifically, Figure 3 is a partial view of the inner surface 28 of the dielectric cylinder 16 in the antenna 60 of Figure 2. As described above, the dielectric cylinder 16 is mounted on a reflector 12, and has a conductive loop 14 on its upper surface. A feed loop 20 is printed on the inner surface 28 of the dielectric cylinder 16, and a conductive path 66 is also printed on the inner surface 28 of the dielectric cylinder 16.

A slot 72 extends partly or entirely through the dielectric cylinder 16, and a connection pad 74 is provided on an upper surface of the slot 72. The required RF circuitry can then be connected to the connection pad 74. It will be apparent that the slot 72 serves to space the connection pad 74 from the conductive surface of the reflector 12.

This arrangement can be used in the device of Figure 3, as described, or can be used to form the connection points 26, 32 in the arrangement of Figure 1.

It will be appreciated that, as is usual, the antenna of the present invention can be used in combination with one or more other antennas. For example, the combination of antennas can be used to form an antenna array. Alternatively, or additionally, an antenna in accordance with the present invention and having a particular operational frequency range can be used in combination with one or more other antennas having a different operational frequency range. The one or more other antennas can be in accordance with the present invention, or can take any other convenient form.

The operational frequency ranges of the antennas can then be chosen to be adjacent or to be spaced apart from each other. The resulting combination can then act as a multiband antenna, or as an antenna having an enhanced bandwidth.

Figure 4 shows an antenna arrangement 80, in which there are two antennas in accordance with the present invention, each on the same reflector 81, although it will be understood that more than two such antennas can be used. A first antenna 82 comprises a dielectric cylinder 84, with a conductive loop 86 printed on its upper surface, and with feed loops 88, 90 printed on two of its inner side surfaces. A second antenna 92 comprises a dielectric cylinder 94, with a conductive loop 96 printed on its upper surface, and with feed loops 98, 100 printed on two of its inner side surfaces. The feed circuitry connected to the feed loops of the two antennas is not shown, but can for example be as described above with reference to the previous figures.

The two dielectric cylinders 84, 94 can then be mounted on the same PCB, or can be formed from a single piece of dielectric material.

The antennas 82, 92 have different operational frequency ranges, such that the resulting antenna arrangement 80 can act as a multiband antenna.

Figure 5 shows a further antenna arrangement 108, including a first antenna in accordance with the present invention. Specifically, the first antenna includes a reflector 110, on which is mounted a dielectric cylinder 112, with a conductive loop 114 printed on its upper surface, and with feed loops 116, 118 printed on two of its inner side surfaces. The feed loops 116, 118 are connected to suitable RF transmitter and/or receiver circuitry 120, which is located within the dielectric cylinder 112.

Also located within the dielectric cylinder 112 is a second antenna 122. In this illustrated embodiment, the second antenna 122 is a patch antenna, and is also located within the dielectric cylinder 112.

The second antenna 122 has a different operational frequency range from the first antenna, such that the resulting antenna arrangement 108 can act as a multiband antenna.

The RF circuitry 120 can then process received signals and/or signals for transmission using either or both of the two antennas.

The invention has been specifically described herein with reference to its use in a GPS receiver. However, it will be apparent that it is also applicable to many other wireless communication systems. For example, in the case of the antenna shown in Figure 1 , it will be appreciated by the person skilled in the art that the operation of the antenna as a transmit antenna will be exactly analogous to its operation as a receive antenna, and hence that, with appropriate transceiver circuitry, a signal for transmission can be fed to the two half loops 18, 20 to cause the antenna 10 to act as a transmit antenna. If the antenna 10 is then to act as a transmit antenna in a system using circular polarization, a transmit signal can be connected to the two half loops 18, 20 in such a way as to ensure that the phase difference between the two antenna ports 26, 30 is 90°. This phase difference can be achieved in many ways, as will be known to the person skilled in the art.