EP2159878A1

EP2159878A1 - Stacked patch antenna array

Info

Publication number: EP2159878A1
Application number: EP08163188A
Authority: EP
Inventors: Michael Philippakis; David Moore; Dean Kemp
Original assignee: Era Technology Ltd
Current assignee: Chelton CTS Ltd
Priority date: 2008-08-28
Filing date: 2008-08-28
Publication date: 2010-03-03
Also published as: EP2359433A1; WO2010023454A1; ES2644589T3; EP2359433B1

Abstract

A radiating element board for use in a passive antenna, the radiating element board comprising a plurality of multi-layer radiating elements, each of which comprises a first patch (11) provided on a first layer for receiving signals within a first frequency band and a second patch (12) provided on a second layer for receiving signals within a second frequency band, wherein the plurality of radiating elements are arranged on the radiating element board such that each radiating element is sequentially rotated a predetermined angle from its neighbouring element, all elements being rotated in the same direction and in the same plane.

Description

This invention relates to an antenna that can be used with multiple satellite-based global positioning and navigation systems, such as both the NAVSTAR GPS and Galileo GNSS.
A number of satellite-based global positioning and navigation systems currently exist, such as the above-mentioned United States NAVSTAR Global Positioning System (GPS), which was developed by the U.S. military in the 1970's. Each system is based on the transmission of ranging signals in particular frequency bands.
Currently, in the NAVSTAR GPS, there are in excess of 30 GPS satellites orbiting the Earth. However, at any one time there are at least 24 functional GPS satellites orbiting the Earth in 12-hour orbits; four satellites in each of six orbit planes, each transmitting a ranging signal. A typical GPS receiver requires ranging signals from at least four GPS satellites to determine its position using geometry and trilateration.
The NAVSTAR GPS is based on transmission of ranging signals in frequency bands including L1 (1575 MHz) and L2 (1227 MHz), both having a bandwidth of 20 MHz. Other satellite navigation systems use different, wider, frequency bands. For example, the new GNSS called Galileo, which is to be deployed by the European Union, operates at frequency bands that are different to those used by the NAVSTAR GPS, including E5a (1166.45 to 1186.45 MHz) and E5b (1197.14 to 1211.14 MHz). Since the two E5 bands (E5a and E5b) are very close together, it can be assumed that both bands will be combined and have a combined bandwidth of 45 MHz.
There is therefore a need for an antenna, which can receive signals emitted in the multiple frequency bands of either system, whilst utilising existing GPS installations. This requirement is particularly relevant in airborne applications where a retrofit to legacy installation is needed to reduce cost without compromising airworthiness. Furthermore, a multi-standard antenna for both NAVSTAR GPS and Galileo GNSS would effectively be dual-band, with the upper frequency band from 1565 to 1585 MHz (L1) and the lower frequency band from 1166 to 1211 MHz (E5).
However, a problem exists in that, with antenna systems, the physical size of the antenna is related to the bandwidth of the system and, as the bandwidth of the Galileo GNSS is more than double that of the NAVSTAR GPS, the size of the respective antennas differs.
In addition, the radiation pattern for the multi-standard airborne GNSS antenna is expected to have advanced characteristics in terms of coverage and more stringent requirements on its polarisation purity as compared to single band GPS antennas of today.
Accordingly, an object of the present invention is to provide a multi-standard radiator for a GNSS antenna, which can receive ranging signals transmitted in multiple frequency bands from different GNSS, whilst remaining an overall physical size that ensures that the antenna assembly can be backwards compatible with existing GPS fixtures/docking stations and installations onboard a vehicle where it is to be used.
According to the present invention there is provided a radiating element board for use in a passive antenna, the radiating element board comprising a plurality of multi-layer radiating elements, each of which comprises:

a first patch provided on a first layer for receiving signals within a first frequency band; and
a second patch provided on a second layer for receiving signals within a second frequency band,
wherein the plurality of radiating elements are arranged on the radiating element board such that each radiating element is sequentially rotated a predetermined angle from its neighbouring element, all elements being rotated in the same direction and in the same plane.

An example of the present invention will now be described with reference to the following figures, in which:

Figure 1 is an exploded view of a multi-standard radiator comprising a radiating element board according to the present invention;
Figure 2 shows the plurality of radiating elements that comprise the radiating element board;
Figure 3 shows the patches of the radiating elements with the dielectric removed;
Figure 4 is a cross-sectional view of the patches with the dielectric removed;
Figures 5(a) to (d) show examples of different feeding structures for the patches;
Figure 6 is a schematic of a polarization forming network;
Tables 7(a) and 7(b) show minimum and maximum gain, respectively, above the horizon; and
Figures 8(a) and 8(b) show an antenna radiation pattern template and close-up of the details along the horizon, respectively.

Figure 1 is an exploded view of a multi-standard radiator 1, comprising a radiating element board 2 according to the present invention. The radiating element board 2 is attached to a polarization forming network circuit board 3 and enclosed within a radome 4, which seals against a base-plate 5 that has grooves to accommodate components of the polarization forming network circuit board 3. The radome 4 may be made from "PTFE", "Nylon" or a similar material, approximately 2.5mm thick. The radome (4) is bonded, using an epoxy resin bond film, to the base-plate 5, which is ideally aluminium. A gasket 7, preferably neoprene or plastic, may be provided to ensure a good seal between the radome 4 and the base-plate 5. Alternatively the radome can be directly glued on the base plate with structural adhesive. An output 9 to the multi-standard radiator 1 is provided on the underside of the base-plate 5.
Alternatively, the radome may be a foam-filled radome, which is formed around the elements to keep the weight of the antenna low whilst ensuring high mechanical stability, at the same time preventing humidity ingress as well as mitigating adverse differential pressure effects.
Figure 2 shows an array of four passive radiating elements 10, which form a radiating element board 2 according to the present invention. The elements 10 are arranged to form a substantially square radiating element board 2, wherein each element 10 is rotated 90 degrees from its neighbour and all elements 10 are rotated in the same direction. The arrangement shown in Figure 2 provides right-hand circular polarisation although it will be appreciated that lefthand polarisation could also be achieved were the elements orientated in the other direction.
Each passive radiating element 10 comprises two patches 11,12 separated by dielectric 13, which is preferably an RF quality plastic material, either single or in a stratified form. Alternatively the space between the patches could be air or foam, as described above. The first patch 11, provided for the NAVSTAR GPS and Galileo (L1) frequency band, is arranged to be on the outside of each radiating element 10. A second, physically larger, patch 12 is provided on an inner layer of each element 10, with dielectric 13 either side of it, and covers the GPS and Galileo (both E5A and E5B) frequency bands simultaneously. The passive radiating elements 10 are constructed using standard multi-layer printed circuit board (PCB) technology, i.e. several substrate layers bonded together to create a thick multi-layer assembly.
Dielectric 13 is provided on the radiating element 10 between the patches 11,12 and on either side of the inner patch 12 to ensure that proper operation in terms of antenna voltage standing wave ratio (VSWR) is achieved, by resonating the antenna to the bands of interest, as well as to ensure that the overall physical size of the antenna 1 is backwards compatible with existing GPS fixtures/docking stations and installations onboard vehicles, such as commercial aircraft, where it is to be used. For instance, currently many aircraft employ GPS only antennas with fixings that suit the particular form as dictated by the ARINC743A standard. Any deviation from the details outlined in this standard is likely to be unacceptable in terms of installation cost and overall airframe performance. The increased height is necessary so as besides the bandwidth the group delay of the antenna is overall acceptable. It is well known that the Q factor of an antenna is inversely related to the volume it occupies. As it is also well known that the Group delay response of the antenna and its variation across the band is proportional to the Q of the underlying antenna cavity. Therefore the bandwidth response of the antenna is crucial in maintaining the required group delay performance.
Figure 3 shows the arrangement of radiating elements 10 in a radiating element board 2 according to the present invention with the dielectric 13 removed. Both the outer patches 11 and inner patches 12 of each radiating element 10 are short circuited by wires 14 passing through one of their edges, the wires 14 connecting both patches 11,12 to ground in order that a quarter-wave operation is achieved and to make sure that the overall mechanical size is physically controlled. These shorting wires are preferably formed using plated through-holes, or vias. Of course, it will be appreciated that alternative means of short circuiting the patches 11,12 to ground can also be used and specifically engineered for a particular application. For example, plating the edge of the PCB with a solid metal layer, or using a series of flat metal strips instead of wires. The number of wires 14 and their diameters also need to be accurately controlled. The wires can be thought as a series additional inductive loading of an otherwise resonant cavity formation. Therefore a suitable choice for their number and diameter can affect both the precise resonant frequencies for a given antenna element size and arrangement as well as its bandwidth at resonance. In addition other means of short circuiting can be used in terms of strips of metal.
In Figure 3 it can also be seen that the individual patches 11,12 also have a serrated edge, preferably provided on the side opposite to the side that is short circuited, to help in the tuning process where, in a controlled fashion, some teeth 15 can be physically removed up to the point that the frequency operation fits the required frequency bands.
Figure 4 is a cross-sectional view of Figure 3, showing each element 10 having a feeding structure comprising an individual feed pin 16 passing through it. In this example, the feed pin 16 originates at the outer patch 11, passes through a hole 17 provided in the inner patch 12 and then out through the base of the radiation element board 2. The feed pins 16 therefore connect each individual element 10 to the circuit board 3.
A number of other feeding structures are possible, as illustrated in Figures 5(a) - (d). For example as shown in Figure 5(a), a feed signal may be fed into the element via a cylinder 18 provided in the lower portion of an element 10, with a feed pin 16 passing through the cylinder to connect it to the lower portion of the element and hence inner patch 12, as the feed pin 16 passes up through the hole 17 provided in the inner patch 12 to connect with the outer patch 11.
Alternatively, one or two discs 19 may be provided in the upper and lower portions, respectively, of the element 10 that the feed pin 16 passes through, thereby connecting the outer patch 11 and the inner patch 12 to the circuit board 3.
Another option is to feed a signal to one or more tuning posts 20 provided at the base of an element 10, at the side of the hole 8 that the feed pin 16 passes through to enter the element 10.
Yet another option is to provide a slot 21 in the base of an element 10, the slot 21 being substantially perpendicular to a slot 22 provided in the circuit board 3 that the element is mounted on. In this example, when a feed signal is fed into the slot 22 in the circuit board 3, electromagnetic radiation 23 propagates up through the element 10 to the inner and outer patches 12, 11.
The particular arrangement of the radiating elements 10 on the board 3, according to the present invention, creates the effect of circular polarization. Accordingly, an antenna 1 incorporating the radiating element board 2 of the present invention will operate through circular polarization. The example shown in the accompanying figures provides for right hand circular polarization, although a skilled person will recognise that a simple rearrangement of the elements 10 will provide left hand polarization.
When the radiating element board 2 is in use in a GNSS antenna 1, the individual feed pins 16 connect each of the elements 10 to a circuit board 3, which is part of a circular polarization-forming network such as the one shown in Figure 6. The network combines the input from each of the elements 10 which is rotated 90 degrees from its neighbour, to provide a common output of the antenna 1, preferably via a TNC connector 9 provided on the antenna base 5, as shown in Figure 1. The antenna 1 can be connected to an external pre-amplifier box (not shown) by means of a short RF quality cable which is ideally, but not essentially, less than 300mm in length. Alternatively, the pre-amplifier can be directly integrated with the structure, preferably with the polarization-forming network board.
Many physical implementations are possible but any network compatible with that shown in Figure 6 will suffice, provided it can cover the necessary frequency bands.
Low dispersion implementations are always important for ensuring the group delay of the antenna 1 is not compromised. The intimate connection of a suitable network with the radiating element board 2 is important for proper function.
The advantages of forming the radiating element board 2 as described above are two-fold. Firstly, it allows a single feed per element 10 yet a good quality RHCP is produced when the elements 10 are fed with signals with a progressive phase sequence; and, secondly, a radiation pattern with significant coverage can result for directions close to the horizon, as shown in Tables 7(a) and 7(b). The good coverage at directions close to the horizon is in addition to the outcome of the precise sequential array arrangement of the elements 10 and the fact that the effective points where radiation is emanating is confined only in the space between adjacent elements 10 and not through both opposite edges for each elements 10 as in conventional GNSS patch antennas.
Figures 8(a) and 8(b), which shows the circled area of Figure 8(a) in more detail, show the measured radiation patterns performance in relation to the EUROCAE template for future multi-standard GNSS antennas. These templates on one hand ensure that the antenna is sensitive over a broad angular range in the upper radiation hemisphere and on the other hand the radiation is not stronger than required in order to minimise the susceptibility of the GNSS system to unwanted strong emissions from external interference sources. As can be seen, the present arrangement performs well on both aspects. However the antenna 1 is expected to conform to other GNSS system pattern templates in present or evolved forms.
The radiating elements 10 of the present invention are required to have a thickness that depends on the desired frequency and bandwidth requirements for an antenna. However, for many applications, the elements 10 are required to be of a thickness that cannot be manufactured as a single block by conventional methods. Therefore, two separate blocks are manufactured to provide an upper layer with a patch 11 provided on it and a lower layer with a patch 12 provided on it. These layers can then be fixed together preferably using conductive adhesive, although other suitable conductive joining means could be used, to form a radiating element 10 having the necessary thickness. When conductive adhesive is used it is important that it has substantially the same thermal expansion properties as the radiating element 10 material. The upper layer, although having substantially the same width as the lower layer, has a shorter length than the lower layer, thereby providing a step on which a portion of the inner patch 12 can be exposed when the two layers are connected to form the radiating element 10. Furthermore, in addition to having the outer patch 11 printed on its top surface, the upper layer also has a dummy lower patch printed on its underside, which is used to match the upper layer up with the inner patch 12 provided on the lower layer when fixing the two layers together. In this way the short circuit vias forms a continuous low resistance path conductively connecting the upper patch 11, the lower patch 12 and the base plate 2. Similar effects are ensured for the feed via or wires.
As explained above, the elements 10 are arranged on the circuit board 3 with each element 10 being rotated 90 degrees from its neighbour, with all elements 10 being rotated in the same direction. In addition, there is a gap between each element 10, which enables teeth 15 on the outer and outer patches 11,12 to be easily removed to fine tune the apparatus. An additional benefit of these gaps is that they allow radiation to escape between the elements 10, rather than only from around the outer edges, and this results in the formation of good radiation patterns.
The antenna 1 is fabricated using several multi-layer PCB elements. Since the bandwidth requirement is very large, the PCB is, in this instance, created from three PCBs that are bonded together using a patterned conductive bond film. The PCB is made as one complete block, but with slots machined to separate the four radiating elements and to expose the tuning teeth 15 on the lower patch.
An alternative construction approach would be to make the whole element board 2 from a single multi-layer PCB with plated via holes through the entire element board 2. This depends on the capability of the PCB manufacturer and is dependent on factors such as the specific thickness of the PCB and the aspect ratio of the plated via holes.
The antenna described herein is designed to operate at GPS band L1 and Galileo bands E5a and E5b. However, a person skilled in the art would understand that simple tuning of the antenna design would allow it to operate at alternative GNSS bands, for example L1/L2 or L1/E6.

Claims

A radiating element board for use in a passive antenna, the radiating element board comprising a plurality of multi-layer radiating elements, each of which comprises:
a first patch provided on a first layer for receiving signals within a first frequency band; and

a second patch provided on a second layer for receiving signals within a second frequency band,
wherein the plurality of radiating elements are arranged on the radiating element board such that each radiating element is sequentially rotated a predetermined angle from its neighbouring element, all elements being rotated in the same direction and in the same plane.
The radiating element board according to claim 1, wherein the first and second patches on each radiating element are separated by dielectric material.
The radiating element board according to claim 1 or 2, wherein the predetermined angle that each element is rotated from its neighbouring element is 90 degrees.
The radiating element board according to any preceding claim, wherein each patch is short-circuited to ground by a plurality of plated through-holes provided along an edge of the patch so that multi-band radiation can only escape from a virtual slot at the centre of the structure.
The radiating element board according to claim 4, wherein the plurality of plated through-holes are connected between layers by means of lamination with a conductive adhesive.
The radiating element board according to any preceding claim, wherein each element further comprises a feed structure connecting it, in use, to a polarization-forming network circuit board.
The radiating element board according to any preceding claim, wherein the first patch receives NAVSTAR GPS signals.
The radiating element board according to any preceding claim, wherein the first frequency band is 1565-1585 MHz (L1).
The radiating element board according to any preceding claim, wherein the second patch receives Galileo GPS signals.
The radiating element board according to any preceding claim, wherein the second frequency band is 1166-1211 MHz (E5).
The radiating element board according to any preceding claim, wherein each patch has serrated teeth provided along at least one edge for broadband and tuning purposes.
The radiating element board according to any preceding claim, further comprising a pre-amplifier connected to the polarization forming network.
The radiating element board according to any preceding claim, which is sealed inside a radome.
A radiating element board according to claim 13, wherein the radome is foam-filled around the elements.
A radiator comprising a radiating element board according to any preceding claim.