Antenna system for ground based applications

Abstract: An antenna system (200) is disclosed having a plurality of vertically-distributed elements (202) (204) (206) (208) (210) (212) (214). The orientation of the elements provides an improved linear array pattern covering the upper hemisphere with a sharp cut-off at a relatively small angle above the horizon (24). Each of the elements are distanced from each other by, for example, αλ/2, wherein the unitless constant, α, is less than unity.

Description: Field of the Invention The present invention relates generally to an antenna system and, more particularly, to an improved antenna system for ground applications. Background of the Invention The operation of Remote Satellite Measurement Units (RSMUs) with Satellite Landing System (SLS) ground stations is particularly susceptible to multi-path errors. Such errors typically originate from the illumination of the antenna by rays that are reflected from the earth or surface objects and structures. Thus, it is desirable to design a ground-based system would acquire emissions that originate above the horizon, but reject rays that arrive from below the horizon. Antenna system that exhibit such a sharp radiation cut-off pattern are typically very large. Previous attempts to reduce multi-path errors have employed L-band antenna designs. These efforts have met with limited success. Early trials involved patch antennas and quadri-filar helix designs. To help improve the performance of these antennas, choke rings were introduced around the bases of the basic antenna elements in an attempt to reduce the response to signals that reflect from the earth and other objects below the horizon. In addition, "lift kits" have been installed with patch antennas to raise the patches to various heights above the choke ring base of the antenna. While some of these trials have met with limited success, none have satisfactorily eliminated the multi-path errors. The latest attempt to reduce multi-path errors uses a large array having a vertical array of vertically polarized dipoles and a second antenna which is a heli-bowl mounted above the vertical dipole array. The vertical dipole array provides coverage of lower elevation angles and cuts off sharply below an elevation of approximately 5 - 10 degrees. Furthermore, the vertical dipole array also cuts off at higher elevation angles in the range of about 35 - 40 degrees above the horizon. As may be appreciated, coverage of elevation angles near the zenith would be fundamentally limited with the vertical dipole array as the vertical dipole elements do not radiate or receive in the vertical direction. Regardless of the array construction, coverage typically will not be provided for a direction in which the basic elements do not radiate or receive. Therefore, two antenna sections are configured for coverage of the low elevation angles and the high elevation angles. More specifically, the vertical dipole array is provided for the low elevation angles and the heli-bowl is provided for the high elevation angles. A two antenna configuration, including the heli-bowl and vertical dipole array combination, typically requires the use of two separate receiver channels. The signals from the two antenna sections cannot be combined into a single analog or digital signal prior to signal detection because at some elevation angles, the summation of two Radio Frequency (RF), Intermediate Frequency (IF), or digital signals will result in a signal aiding or cancellation in the common region where the radiation patterns of the two sections overlap. This results in undesirable pattern nulls and peaks commonly referred to as grating lobes. While the situation involving peaks in an antenna pattern due to signal aiding is not generally considered to be a problem, nulls resulting from signal cancellation are undesirable due to a reduction in coverage volume. In addition to the disadvantages associated with signal cancellation in a two antenna configuration, the required use of two receivers for this antenna type imposes a cost penalty. For example, a single SLS ground station is typically outfitted with three RSMUs. Therefore, a two antenna configuration would typically require six receivers for each SLS ground station. In addition, synchronization between multiple RSMUs at each site must be resolved, and a switching threshold algorithm is needed to select the proper receiver output based on elevation angle, signal quality, or some other appropriate parameter. U.S. Patent No. 5,534,882, issued Jul. 9, 1996 to Lopez, discloses an antenna system having upper hemisphere coverage close to the zenith. Referring to Figure 1, a computer-generated plot of antenna gain versus elevation angle for the Lopez system is illustrated. As shown, the gain is uneven from the horizon to the zenith (0 to 90 degrees). There is a sharp cutoff at the horizon with the sidelobes approximately 10 dB+ down. In view of the foregoing, an antenna array is desired that is constructed with basic elements and provides improved isotropic coverage of the upper hemisphere while rejecting signals that arrive from below a suitable threshold above the horizon (i.e., upper hemisphere coverage with a sharp cut-off near the horizon). In addition, it is desirable to stabilize the gain from the horizon to the zenith. Summary of the Invention Accordingly the present invention provides an antenna system as detailed in the appended claims. Various embodiments of the present system overcome the prior art problems by providing an improved antenna comprising a plurality of vertically-distributed element arrays configured to cover the upper hemisphere while providing a sharp cut-off at a relatively small angle above the horizon. In accordance with a further aspect of the present invention, an antenna includes a plurality of element arrays distanced by at least αλ/2, wherein α is an unitless constant and λ is the wavelength. Brief Description of the Drawings These and other features, aspects, and advantages of the present invention will become better understood with regard to the following description, appended claims, and accompanying drawings where: Figure 1 is a computer-generated antenna pattern illustration of the prior art; Figure 2 illustrates an antenna system in accordance with one embodiment of the present invention; Figure 3 illustrates an antenna system in accordance with an embodiment of the present invention; and Figure 4 is a computer-generated antenna pattern in accordance with an embodiment of the present invention. Detailed Description of the Exemplary Embodiment Referring now to Figure 2, an antenna system 16 in accordance with various aspects of the present invention includes a mast 20 that is substantially normal to the horizon 24. The mast 20 supports a linear array of isotropic radiating (or receiving) elements formed of multiple vertically oriented elements 28, 32, 36, 40, 44 and 48. Each vertically oriented element (i.e., 28, 32, 36, 40, 44, 48) generates a propagation ray 60. It should be appreciated that the angle from zenith () (e.g., as illustrated in Figure 1, =52°) is the complement of the elevation angle (i.e., the angle between propagation rays 60 and horizon 24). The vertically oriented elements being separated a distance ( d ) from each other. The vertically oriented elements, in accordance with the present invention, are configured to be circularly polarized in the zenith direction and become elliptically polarized at the lower elevation levels while maintaining satisfactory axial ratio values. The orientation of the elements provides a linear array pattern covering the upper hemisphere with a sharp cut-off at a relatively small angle above the horizon, such as about 5°. The exemplary embodiment illustrated as antenna system 16 includes six vertically oriented elements 28, 32, 36, 40, 44, 48 forming the linear array of isotropic radiating elements. However, it will be appreciated that the number of vertically oriented elements may be increased or decreased without departing from the scope of the invention. The total coverage volume is the vertical dipole array and heli-bowl antenna combination previously described above. Due to the isotropic nature of elements 28, 32, 36, 40, 44 and 48, equal response theoretically exists in all directions. However, it should be appreciated that each element radiates electromagnetic energy at an amplitude and phase which depends on the RF power and phase of the drive signal applied to the element. Furthermore, the net electromagnetic field at a distant observation point is typically the sum of all the fields from the individual elements (it is assumed that the observation point is sufficiently far from the array that the propagation paths can be approximated as being parallel). In addition, because the relative distances of propagation are dependent on the elevation angle with respect to the observation point, the distance traveled by each individually propagated signal is different and corresponding phase delays are the result. At the observation point, the phase of an individual component of the electromagnetic field is advanced or delayed relative to the phases of the signals generated by the other elements. Accordingly, it is desirable to design the physical dimensions of the array to produce the necessary relative propagation distances as a function of elevation angle. Additionally, the individual elements are powered with RF signals such that the electromagnetic fields at the distant observation point add for elevations in which signal coverage is desired and subtract/cancel for elevations in which signal rejection is desired. In one embodiment, the antenna array is configured to receive signals from about 5° to 10° and upward, and to reject signals at and below the horizon by about 40 dB. The number and spacing of the vertically orientated elements, as well as their relative amplitudes and phases, may be optimized in accordance with the disclosure. One particular embodiment in accordance with the present invention is illustrated in Figure 3. An antenna 200 includes seven isotropic elements (i.e., m = -5, -3, -1, 0, 1, 3, and 5; elements 214, 212, 210, 208, 206, 204, and 202 respectively). In this embodiment, the elements are distributed along a mast 220 symmetrically with respect to element 208. Elements 206 and 210 lie at a distance αλ/2 from element 208, where α is an unitless constant, and λ is the signal wavelength. The remaining elements, 202, 204, 212 and 214, are distanced at αλ intervals. It should be appreciated that the antenna configuration shown in Figure 3 is effectively an eleven-element design (having elements distanced at αλ/2 intervals) that has been "thinned" to seven elements. This configuration allows elements which may be driven at diminishingly low levels to be removed without significantly altering the performance of the antenna. Antenna elements 202-214 are crossed, orthogonal, inverted-vee dipole elements fed in quadrature. This configuration produces circular polarization in the two directions perpendicular to the plane of the dipoles (e.g., upward and downward for horizontal dipoles). However, the axial ratio in such systems degrades in directions away from the perpendicular axis and becomes linearly polarized in the plane of the dipoles. This embodiment having crossed, inverted-vee dipoles does offer a desired lower degradation. The individual elements are driven at specific amplitude and phases to achieve suitable cancellation of signals below a threshold elevation angle. The antenna array illustrated in Figure 3 includes a feed network (not shown) to drive each element. The network suitably includes signal couplers configured to establish the correct amplitudes and delay lines (transmission lines, e.g., microstrip, stripline) to produce the correct phases for each of the individual elements. The network further incorporates the necessary quadrature feed for the crossed inverted-vee dipoles. For further background information regarding antenna theory, see, e.g., Constantine Balanis, Antenna Theory Analysis and Design (1982). In one exemplary embodiment of the antenna system of Figure 3, constant (α) is approximately equal to 0.90 which results in the following suitable drive levels: Exemplary Drive Coefficients m DB Relative phase 5 -26 -90 3 -20 -90 1 -10 -90 0 -6 0 -1 -10 90 -3 -20 90 -5 -26 90 Referring now to Figure 4, an exemplary computer-generated antenna pattern in accordance with the values of Table 1 is shown. The ultimate antenna pattern is the array factor multiplied by the antenna pattern of the individual elements. The center element (m=0) is driven at -6 dB and α ≅ 0.90. Figure 4 demonstrates the improved uniformity in gain from the horizon to the zenith (0 to 90 degrees) over the prior art pattern illustrated in Figure 1. In addition, below the horizon all sidelobes are indicated to be at least -20dB down from the horizon. The unitless parameter α may be varied in accordance with the particular application. In the illustrated embodiment, α is a real number less than unity, preferably in the range of 0.90-0.99. However, it should be appreciated that other amplitude values may be suitable depending upon particular design requirements. The present inventors have found that scaling the distance between the elements by α significantly improves the pattern of the antenna system. Those skilled in the art will clearly recognize the improved antenna pattern in accordance with the present invention with a comparison of the antenna pattern of the prior art (Figure 1) and the exemplary antenna pattern in accordance with the present invention (e.g., Figure 4). Each element, namely 202, 204, 206, 208, 210, 212, and 214, in the illustrated embodiment are substantially isotropic. Ideally, it is desirable to use elements as nearly isotropic as possible, however, in practice, a truly isotropic radiation pattern is generally rare. In one embodiment, the antenna polarization is right-hand circular polarization (RHCP). In this embodiment, the individual elements radiate (and receive) RHCP electromagnetic signals. An antenna array in accordance with one embodiment, having individual elements which radiate nearly isotropically and near the zenith of the upper hemisphere, is not limited to linearly polarized elements and provides an improved antenna pattern design over the upper hemisphere. Reception (radiation in general) near or below the horizon is reduced by the array factor which reduces the response (by field vector cancellation) near the horizon and in the lower hemisphere. The large array configuration offers improved signal rejection near and below the horizon due to superior pattern shaping characteristics over smaller aperture antennas. The present invention has been described above with reference to exemplary embodiments.

Claims: An antenna system (200) having an antenna pattern comprising: a plurality of vertically orientated elements (202,...,214), one of said elements (208) being the centre element, spaced along an axis (220) substantially normal to a horizontal plane, each of said elements being placed αλ/2 intervals apart and having a corresponding drive coefficient; a feed network to drive each of said elements, said feed network comprising signal couplers configured to establish an amplitude and a delay line for each of said elements, and characterised in that α is a value in the range of 0.90 to 0.99, and more preferably about 0.90 and is predetermined to optimize the antenna pattern for reducing multipath errors. The antenna system of claim 1 or claim 2 comprising at least 7 elements (202) (204) (206) (208) (210) (212) (214). The antenna system of claim 1, wherein said center element (208) comprises a drive coefficient approximately equal to -6 dB. The antenna system of any preceding claim having a substantially circular antenna polarization. The antenna system of any preceding claim, wherein said plurality of elements comprises crossed, orthogonal, inverted-vee dipoles. The antenna system of any preceding claim, wherein said plurality of elements are configured to be substantially circularly polarized in a zenith direction and substantially elliptically polarized near the plane. The antenna system of claim 1 wherein said plurality of elements are configured to reject signals at or below said horizontal plane. The antenna system of claim 7 comprising: a first element (208) defined as the center element having a drive coefficient approximately equal to -6 dB; a second element (206) and a third element (210) spaced αλ/2 from said first element in a substantially vertical direction on either side of said first element (208); a fourth element (204) and a fifth element (212) spaced αλ from said second and said third elements (206) (210) respectively in a substantially vertical direction; and a sixth element (202) and a seventh element (214) spaced αλ from said fourth and said fifth elements (204) (212) respectively in a substantially vertical direction. The antenna system of any claim 1, wherein each of said elements creates an electromagnetic field at a propagation distance corresponding to each of said elements, said propagation distance being a function of an elevation angle relative to said horizontal plane. The antenna system of claim 9, wherein said feed network is configured to drive each of said elements such that at a distant observation point a net electromagnetic field, which is defined as the combination of all of said electromagnetic fields, adds for said elevation angles in which signal coverage is desired and subtracts for said elevation angles in which signal rejection is desired. The antenna system of any one of claims 9 to 10, wherein said plurality of elements is configured to receive signals from 5° elevation angle upwards. The antenna system of claim 11, wherein said plurality of elements is configured to reject signals at and below said horizontal plane. The antenna system of claim 12, wherein said rejection is by about 40 dB relative to signals at said horizontal plane.

Inventors: NYHUS ORVILLE K, NYHUS, ORVILLE K.
Applicants: HONEYWELL INT INC, HONEYWELL INTERNATIONAL INC.
Application year: 2000
Main classification: H01Q21
IPC codes: H01Q21