Antenna Arrangement
An antenna for use in a direction finder system, said antenna including a microstrip patch layer with a stripline transmissionline layer below. The microstrip layer and the stripline layer share a common groundplane. In this groundplane is formed a number of apertures. Each aperture couples a patch to a radiation point in the stripline feed layer. The patches and transmissionlines are designed from a lossy material. The substrates in the microstrip and stripline layers may be designed from materials with lossy dielectric properties.
The present invention discusses a novel structure for an antenna unit in a direction-finder system for detecting and localising radio frequency emitters. The present solution makes use of an antenna structure which is extremely stable and which gives a very accurate direction to any number of emitters.
Radio frequency emitters (radars, satellite uplink stations, cell-phone base stations, relay links) can be detected, analysed, and geo-referenced from a remote observation platform. This is achieved using a sensor with an antenna system for detecting the radiation, connected to a receiver and processing system. These systems can be deployed from satellites, aircraft, UAVs, ships vehicles or mounted in masts.
Typical solutions employ radio receiver systems operating in the frequency bands 1 through 12 GHz. These systems employ multiple receiving antennas and multiple receivers to derive a course direction to the emitters.
In general, existing systems either gives high accuracy or wide angular coverage, and have the following limitations:
Angular accuracy is limited (typically 1 deg) Limited ability to handle multiple emitters Large physical sizePoor capability to operate in adverse conditions (e.g. Sun radiation, vibration etc.)
Use of multiple separately supported antenna structures Expensive manufacture SUMMARY OF THE INVENTIONThe overall object of the present invention is to provide an antenna system for use in direction-finding systems, which it is optimised for direction finding accuracy and ability to separate individual emitters.
Another object of the inventive antenna system is to provide both high accuracy and wide angular coverage.
Another object is to provide an antenna system with high fidelity wave front pickup and optimized phase linearity.
Still another object is to provide an antenna system with optimal mechanical, thermal and electrical stability.
The objects above are achieved in an antenna system as claimed in the appended patent claims.
The invention will now be described in detail in reference to the appended drawings, in which:
Initially, we will give an overview of the direction-finder system as a background for the present invention. As shown in
The antenna is organised as 4 sub-panels mounted in a 2×2 configuration with a spacing of approximately one quarter of a wavelength.
Typical dimensions of the antenna sub-panels is 1-5 wavelengths (15 cm-75 cms at 2 GHz)
The antenna has an electrical layout optimized to achieve an accurate response in terms of phase errors and mutual coupling effects. To minimize the impact of thermal and mechanical distortions, e.g. warping due to sun heating, the antenna has been designed as a layered composite structure with a stiff support layer. This restricts the electrical solutions that are available for this design. Of many possible designs that have been considered, two constructions have been found advantageous in this connection: Patch antenna array with stripline feed, and slot antennas fed from slotted waveguides.
The first option has been found easiest to implement in a layered design, and is the preferred embodiment of the invention. We will describe this solution, illustrated in
In order to optimize the patches for high fidelity wave front pickup and wide bandwidth, the patches are designed to be as wide as possible, room permitting, and being of a lossy conductive material.
The patches may be widened by thickening the substrate while lowering the substrate relative permittivity. In this way the value of the impedance of the patches against the groundplane is preserved.
The patches are also designed to transform the impedance of free space (370 ohm) into the impedance of the feed-line transferring energy from the patches to the RF unit 2.
The patches, and the whole feed structure are designed to introduce ohmic losses into the signal path. The preferred approach for introducing losses, is to cover the copper conductors in patches, groundplanes and transmission lines with a lossy material. As an alternative, or in addition to the former approach, the substrate may have dielectric losses. Still another alternative is to produce the patches, groundplanes and transmission lines from a semiconducting material instead of copper.
Typical values for conductivity of the lossy material are about 0.2 Siemens/m or less. As for the substrates, the loss angle should typically be above 0.1 radian.
The introduced attenuation tends to swamp internal and external reflections in the structure, and is an important feature for obtaining the desired response. The attenuation also masks any residual impedance mismatch against free space, and effectively couples the patches to this impedance. This gives a hereto superior electrical response in terms of phase and amplitude accuracy. The penalty of this is that the antenna has an insertion loss of 2-5 dBb, which is of no consequence for passive reception of signals. In fact, a gain in dynamic range of about 40 dB is obtained instead.
Below the patch layer, is a stripline feed layer composed of a number of narrow flat conductors 14 lying between upper 13 and lower 15 ground planes. The upper ground plane 13 is also acting as the ground plane for the microstrip patches 11.
The layout of stripline conductors 14 is illustrated in
In the stripline pattern, there is one “radiation point” for each patch 11, located below the patches 11, in a one-to-one relationship. In the upper groundplane 13 there are a corresponding number of apertures 18 transferring RF energy from the patches 11 to the associated radiation points. This design, with aperture coupled patches, works to isolate the patches from each other by restricting spreading of RF energy between adjacent patches.
As in the patch layer, the conductors 14 and/or the dielectric material between the groundplanes 13, 15 and the groundplanes themselves should be lossy.
In order to bring stable thermal properties to the panel, the layers in
An alternative embodiment of the invention is shown in
In order to diminish thermal heating from the sun, the antenna may be covered with a PVC film with anti-glare properties. When used in space, the antenna may be mounted behind a window of a similar material.
The antenna will when exposed to temperature stress be exposed to the following sources of deformation:
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- Antenna stretching (in-plane homogenous dimension change)
- Antenna bending (out-of-plane structural change)
- Antenna surface irregularities (out-of-plane)
In addition, depending on the mechanical interface to the surrounding structure, the antenna may experience
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- Out-of-plane multi-axis bending (Warping)
- In-plane irregularities (multiple local in-plane dimension changes)
The various forms of distortions are illustrated in
As mentioned above, the introduction of lossy materials/lossy dielectrics is important for achieving the is desired phase linear properties of the antenna. In the examples mentioned so far, the loss has been more or less continuously distributed in the antenna. However, it is also possible to use lumped resistive elements instead, forming attenuators in the signal path.
The panels should also be designed to be symmetrical about a central point in the 2×2 configuration.
Claims
1-10. (canceled)
11. An antenna arrangement in a direction-finder system, said arrangement including a first layer containing a multitude of radiation elements, a second layer containing at least one feed-line, said at least one feed-line being coupled to said radiation elements and being terminated in a connection point, said at least one feed-line being adapted to transfer energy from said radiation elements to said connection point, characterized in that said radiation elements and/or said at least one feed-line includes attenuation elements.
12. An antenna arrangement as claimed in claim 11, characterized in that said attenuation elements includes lumped resistive elements.
13. An antenna arrangement as claimed in claim 11, characterized in that said radiation elements and/or said at least one feed-line are designed of lossy materials.
14. An antenna arrangement as claimed in claim 13, characterized in that said radiation elements and/or said at least one feed-line are designed in copper with a lossy covering layer.
15. An antenna arrangement as claimed in claim 13, characterized in that said radiation elements and/or said at least one feed-line are designed in semiconducting materials.
16. An antenna arrangement as claimed in claim 11, characterized in that said radiation elements and/or said at least one feed-line includes dielectric substrates designed in materials with dielectric losses.
17. An antenna arrangement as claimed in claim 11, characterized in that
- said first layer consisting of a layer of microstrip patches including a number of individual patches on a dielectric substrate on a first groundplane,
- said second layer consisting of a stripline transmission line layer located below said microstrip patch layer, said stripline transmission line layer including a number of conductors sandwiched between said first groundplane and a second groundplane, with layers of dielectric substrates interleaved between said conductors and said groundplanes,
- said conductors including a number of radiation points and lines transferring radio frequency signals from the radiation points to said connection point,
- said first groundplane including a number of apertures, each aperture being located between an associated patch and an associated radiation point.
18. An antenna arrangement as claimed in claim 17, characterized in that said conductors are arranged in a branching pattern with the lines from each radiation point to the connection point being of identical length, and preserving an equal phase relationship between signals arriving from the individual radiation points.
19. An antenna arrangement as claimed in claim 18, characterized in that the lines are organised in two balanced branches, each branch being terminated in one of two connectors in said connection point, the connection point being connected to a balanced port of a balun, the unbalanced port being connected to an unbalanced transmission line conducting the signals to a receiver.
20. An antenna arrangement as claimed in claim 11, characterized in that
- said first layer consisting of a slot radiator layer including a number of slot radiators,
- said second layer consisting of a number of waveguide elements located below said slot radiator layer, said waveguide elements including a number of series fed waveguide slots transferring radio frequency signals from the slot radiators to said connection points.
Type: Application
Filed: Dec 30, 2004
Publication Date: May 15, 2008
Applicant: Telefonaktiebolaget LM Ericksson (publ) (Stockholm)
Inventor: Jens Fredrik Hjelmstad (Lillestrom)
Application Number: 11/569,030
International Classification: H01Q 1/38 (20060101); H01Q 1/50 (20060101); H01Q 9/04 (20060101);