Abstract:
An antenna array system is disclosed which uses subarrays of slots and subarrays of dipoles on separate planes. The slots and dipoles respectively are interleaved, which is to say there is minimal overlap between them. Each subarray includes a microstrip transmission line and a plurality of elements extending perpendicular thereto. The dipoles form the transmission elements and the slots form the receive elements. The plane in which the slots are formed also forms a ground plane for the dipoles--hence the feed to the dipole is on the opposite side of this ground plane as the feed to the slots. HPAs are located adjacent the dipoles on one side of the substrate and LNAs are located adjacent the slots on the other side of the substrate. The dipoles and slots are tuned by setting different offsets between each element and the microstrip transmission line.

Description:
ORIGIN OF INVENTION 
     The work described herein was made in the performance of work under a NASA contract and is subject to the provisions of Public Law 96-517 (35 U.S.C. 202) in which the contractor has elected to retain title. 
    
    
     The present defines a novel antenna system which uses separate slot and dipole arrays in a multi-layered arrangement. 
     BACKGROUND AND SUMMARY OF THE INVENTION 
     Antenna design always requires a trade-off. Usually a larger and more expensive antenna will provide better operational performance. An antenna can be made smaller at the expense of higher cost, or at the expense of decreased performance. The present invention provides a small and potentially inexpensive antenna with excellent performance. 
     The specific application intended for the antenna of the present invention is the so-called Advanced Communication Technology satellite (&#34;ACTS&#34;) and Advanced Mobile Terminal (&#34;AMT&#34;) made by NASA&#39;s Jet Propulsion Laboratory. This system is intended to provide voice data and video communication from a mobile vehicle via a geostationary ACTS satellite operating using the K and Ka band frequencies. A critical component of the AMT is its antenna system which must establish and maintain the basic RF link with the satellite. 
     The ACTS/AMT system will initially provide a demonstration of voice, fax and compressed video transmitted from a van or car travelling along roads and highways to a geostationary satellite. A two-way communication is envisioned between the vehicle and the first location, to the ACTS satellite and to a base station in a second location. The antenna must have a sufficiently wide elevation coverage so that only azimuthal tracking is required. 
     The specific requirements of the system require a low-profile, compact antenna array with approximately 22 dBi gain that receives vertical polarization at 20 GHz and transmits horizontal polarization at 30 GHz. 
     The antenna of the present invention provides a novel approach to meeting these requirements by using a multi-layered/interleaved array antenna of microstrips, slots and dipoles. A first planar array includes series-fed linear arrays of slots and a second planar array includes series-fed linear arrays of dipoles. Each of the arrays is formed of elements that are transversely coupled to a microstrip transmission line. The dipole and slot radiating elements have similar radiation patterns but radiate orthogonal polarizations. 
     More specifically, the antenna of the present invention includes a first layer including a receive array and a second layer including a transmit array. Each of the arrays include a plurality of subarrays; each subarray formed from a layer of separated antenna elements and a microstrip transmission line layer coupling together the antenna elements. The subarrays collectively form a plane of series-fed-type linear arrays. Different aspects of the elements relative to the microstrip transmission line can be modified in order to change various characteristics of the antenna. According to one particular aspect of the antenna, one of the layers is used as the ground plane for the other of the layers. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     These and other aspects of the invention will now be described in detail with respect to the accompanying drawings wherein: 
     FIG. 1 shows a planar view of an active antenna array showing all of the layers in plane view; 
     FIG. 2 shows a cross-sectional of the layers forming the antenna along the line 2--2 in FIG. 1; 
     FIGS. 3A and 3B show the preferred feed configuration of the feed network; 
     FIGS. 4 and 5 respectively show a dipole subarray according to the present invention and the equivalent circuit modeling this dipole subarray; 
     FIGS. 6 and 7 show the slot subarray and its equivalent circuit, respectively; 
     FIG. 8 shows the layers and their arrangement for the dipole elements; 
     FIG. 9 shows the layers and arrangment for the slot elements; and 
     FIG. 10 shows an equivalent circuit to be used in modeling the RF characteristics. 
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     The antenna system forming the preferred embodiment of the present invention is shown in FIG. 1. Its cross section along the line 2--2 is shown in FIG. 2. In brief, the antenna of the preferred embodiment is a dual-frequency antenna intended to receive vertical polarization at 20 GHz and to transmit horizontal polarization at 30 GHz. The antenna uses a multi-layered arrangement with different elements on different layers. Slot elements 100 are on a first layer 200, and the dipole elements 102 are on a second layer 202, overlying (with respect to the intended direction of mounting of the antenna) the first layer 200. The two layers form distinct and spaced planes. 
     The preferred embodiment transmits using a planar array of series-fed-type linear subarrays of electromagnetically-coupled microstrip dipole elements 102. A plurality of elements 102 on layer 202 are transversely coupled to a microstrip transmission line 104 on layer 204. Each of the dipole elements 102 is identical or nearly identical according to the preferred embodiment, although non-identical elements could alternately be used. For example, each of the dipole arrays could be formed as a Yagi type antenna, or any other type array antenna. 
     The desired power distribution along each linear array is set by offsetting each dipole element 102 with respect to the transmission line 104. FIG. 1 shows the use of ten dipole radiating subarrays for the transmit array, one of the elements being representatively shown by the reference number 102. It should be understood that any number of such dipole subarrays could be alternately used. 
     The receive array includes a plurality of series-fed-type linear subarrays of slot elements 100. Each linear subarray 110 includes a plurality of slots 100 forming layer 200, transversely coupled to a microstrip transmission line 112 forming layer 210. 
     The first and second sets of subarrays are interleaved, which is to say looking from the top as in FIG. 1, each dipole subarray is between two adjacent slot subarrays. The slot subarrays are between two adjacent dipole subarrays. This interleaving helps to reduce the amount of interference between the transmit and receive arrays. 
     The preferred embodiment uses the slotted plane 200, on which the slot radiating elements are formed, as one of the ground planes for the transmit array on layers 202 and 204. The receive array is backed by a ground plane layer 215 approximately a quarter wavelength from the slot element layer 200. Vertical shorting pins may be mounted on layer 215 to reduce undesired cavity-mode wave propagation. 
     Each linear subarray is optimized by selecting the offset of each element relative to the transmission line to achieve the proper power distribution. In short, each transversely coupled electromagnetically coupled microstrip dipole is modeled as a shunt impedance loading the microstrip transmission line as shown in FIG. 5. Each transversely coupled slot dipole is modeled as a series impedance loading the microstrip transmission line as shown in FIG. 6. The specific optimization will be discussed in more detail herein. 
     The preferred embodiment uses solid state monolithic microwave integrated circuits (&#34;MMIC&#34;) high power amplifiers (&#34;HPAs&#34;) representatively shown by element 120, and MMIC low-noise amplifiers (&#34;LNAs&#34;) 122. A plurality of HPAs 120 are mounted on the antenna, on a top side of the ground plane 200 (which forms the slotted elements) near the dipole radiating elements and on the same side thereof as shown in FIG. 2. Each HPA is preferably coupled to a single associated dipole array through feed 132. For example, HPA 120 is coupled only to array 102. 
     Similarly, each LNA 122 is mounted on a bottom side of ground plane 200, and is coupled to a single array 110 through feed 134. Integration of the MMIC amplifying elements onto the plane forming the antenna array itself minimizes the transmission line length between the amplifiers and the antenna elements. This keeps the losses low and the antenna efficiency and sensitivity high. The direct combination of one amplifier module to one radiating subarray allows for total signal amplification to be divided among the subarrays. This allows, for example, HPA 120 to provide only a fraction of the total transmitted power for the entire transmit array. In the preferred embodiment, each HPA must provide only approximately 0.1 watts, but of course the specific power depends on the requirements. 
     Alternatively, the preferred embodiment may eliminate the MMIC amplifiers (HPAs or LNAs or both) and be used in a passive manner. 
     Definition of Terms. 
     Prior to further discussion, a definition of the terms used throughout this specification will be provided. 
     An antenna element refers to one separated element of the antenna. 
     A subarray, or linear array, is formed by a series of antenna elements as coupled together by the respective transmission lines. For example, element 102 is one subarray, element 103 is another subarray, element 110 is a third subarray, etc. The subarrays can be further characterized as being either a transmit subarray such as 102, also called a dipole subarray or receive subarrays, also called slot subarrays, shown as 110, for example. 
     The amplifiers used according to the present invention include HPAs and LNAs. The term amplifiers is intended to be generic to both the transmit HPAs and the receive LNAs. 
     A combination of one HPA connected to one antenna subarray is called herein an amplified subarray. The entire set of dipole subarrays together form the transmit array. Similarly, the entire set of slot radiating elements together form the receive array. 
     In general, an active array antenna refers to any array antenna in which each element or subarray is associated with active, or nonlinear, components such as amplifiers or phase shifters. Typical active arrays use electronic scanning capabilities and are called phased arrays. However, electronically steerable antennae use a relatively large number of extremely expensive components. Hence the antenna of the present invention trades off the ability of unlimited steering by providing, at a lower cost, a main beam which is fixed in elevation but mechanically steered in azimuth. 
     In order to reduce costs, a number of relatively small power amplifiers are provided according to the present invention, one amplifier being associated with each antenna subarray. A choice of different subarrays can therefore be made depending on the direction of the main beam and the loss considerations. Fixed beam scanning can be accomplished by a progressive phase shift, related to the desired scan angle, among the array elements. 
     Each antenna also includes a transmit feed network 130 and a receive feed network 140. The feed networks are required for both the transmit and receive arrays to divide/combine the input/output power among the linear arrays. A series-type divider, while suitable for passive array operation, has proved undesirable for this active array application, since it does not provide isolation among the subarrays. If one amplifier were to fail, the resultant impedance change could be severe enough to cause damage to the other amplifiers. 
     A corporate-type beam forming network using power dividing hybrids is shown in FIG. 3A. This uses so-called Wilkinson power dividers and provides isolation among the active linear arrays. The amplifiers are formed from MMIC packages of commercially available types but may be fully integrated with the fabrication of the arrays. Typically these amplifiers have a phase variation of ±5% and an amplitude variation of ±1 dB. 
     The MMIC amplifier is preferably fed to the linear array at the end or ends of each subarray. 
     Both the dipoles and slots are offset to achieve the proper power distribution. 
     The stacked configuration of the system is shown in FIG. 2, which as includes a top layer 202 including dipole elements 102. A second layer 204 is directly below layer 202 and includes the transmission line elements 104 which extend in a transverse direction to the direction of the microstrip dipoles. The slotted ground plane 200 is below the transmission line layer 204 and includes slots. A second layer of transmission lines for the slots is embodied as layer 210, which includes second transmission lines. A second ground plane 215 forms the final layer. 
     The HPAs 120 and LNAs 122 are shown on the proper respective sides, as are the transmit and receive feed networks 130 and 140 respectively. 
     The feeds to the dipoles are shown as element 132 and feed to the slots are shown as 134. Moreover, because the feed lines are on opposite sides of the ground plane 200, coupling of surface waves is minimized. 
     A basic dipole subarray is shown in FIG. 4. The basic dipole array includes dipole element 102 and microstrip transmission line shown in FIG. 8. The offset δ is defined as the distance between the center of the microstrip transmission line 104 and the center 502 of the dipole element 102. Each offset for each element can be different. 
     Preferably, each dipole element is of approximately the same length, in the preferred embodiment approximately half a wavelength in length L and less than L/20 in width t. The offset, according to the preferred embodiment, varies between 0.1 L to 0.7 L. The system is modeled according to the equivalent circuit of FIG. 5, where the normalized resistance (with respect to the transmission line characteristic impedance) and the resonant frequency both as a function of offset can be obtained theoretically or experimentally. 
     The basic slot subarray is shown in FIG. 6. The basic slot array includes slot element 100 and transmission line 112 as shown in FIG. 9. The offset δ is defined as the distance between the center of the microstrip transmission line 112 and the center 500 of the slot element 100. Each offset for each element can different. 
     Preferably, each slot element is of approximately the same length, in the preferred embodiment approximately half a wavelength in length L and less than L/20 in width t. The offset, according to the preferred embodiment, varies between 0.3 L to 0.7 L. The system is modeled according to the equivalent circuit of FIG. 7, where the normalized resistance and resonant frequency both as a function of offset can be obtained either theoretically or experimentally. 
     Linear Array Design 
     To achieve a scanned beam θ 0  measured off broadside, or normal, to the antenna, the following phase relationship between the slot and dipole radiating elements in the elevation direction, as shown in FIG. 1, should be satisfied ##EQU1## where d is the spacing between elements and λ 0  is the free space wavelength. 
     For some series-fed linear arrays, the desired phase distribution among the elements may be obtained simply by adjusting the path length of the interconnecting transmission lines to the appropriate length. Described below are the required path lengths needed to obtain a beam at scan angle θ 0 . 
     Consider a symmetrically fed linear array where the spacing between elements is d and the length of the interconnecting transmission line is l≧d. 
     To direct the main beam at the proper angle, the phase (using e j ωt time dependence) must satisfy 
     
         Ψ=-Bl=±k.sub.0 d sin θ.sub.0 =2mπm an integer 
    
     where β=k 0  √ε r ,eff, k 0  =2π/λ 0 , and e r ,eff is the effective dielectric constant. Therefore, the path length of the interconnecting line must satisfy ##EQU2## To minimize E, i.e., minimize line loss, we solve the above equation when l=d to obtain ##EQU3## 
     The choice of sign in the above equation relates to which excitation end of the linear array is used to obtain the desired elevation beam. 
     Dipole Linear Array Design 
     The transmit antenna has linear arrays of series-fed-type dipoles electromagnetically coupled to a microstrip transmission line. Such a linear array, having N E  dipole elements, is shown in FIG. 4. 
     In an equivalent circuit model, each dipole i represents a shunt impedance, Z i  =R i  +jX i , to the transmission line as shown in FIG. 7, where Z i  is a function of the dipole offset, length, and width. It is desirable to operate each dipole at resonance, such that X i  =0. 
     A unit cell of the array equivalent circuit may be defined as shown in FIG. 10. 
     The transmission line of length l is characterized by its characteristic impedance Z 0  and its propagation constant k where ##EQU4## k 0  =2π/λ 0  is the free space wavenumber, λ 0  is the free space wavelength, e r ,eff is the effective dielectric constant, and α is the attenuation constant. 
     The voltage and current on the transmission line of the unit cell are given by ##EQU5## We obtain the ratio of V i+1  /V i  ##EQU6## Note that the above is simply a recursive expression to obtain the voltage at successive points along the array. 
     The power dissipated in each dipole radiator is given by ##EQU7## A recursive expression for the dipole resistances R i  is given by ##EQU8## Therefore, for a given amplitude distribution, √P i  (i=1,2, . . . ,N e ), a given transmission line length kl, R 1 , and V 1 , the resonant resistances R i  are given by the above expression. For θ 0  ≠O, R 1  =Z 0 . 
     Once the desired resonant resistances R i  are determined, design of the linear subarray is accomplished using design curves of resonant resistance and resonant frequency both as a function of offset, obtained either theoretically or experimentally. 
     Slot Linear Array Design 
     The receive antenna consists of linear arrays of series-fed-type slots transversely coupled to a microstrip transmission line. Such a linear array is shown in FIG. 6. 
     In an equivalent circuit model, ignoring mutual coupling between slot elements, each slot i represents a series impedance, Z i  =R i  +jX i , to the transmission line, as shown in FIG. 7, where Z i  is a function of the slot offset, length, and width. It is desirable to operate each slot at resonance, such that X i  =0. 
     A unit cell of the array equivalent circuit may be defined as shown in FIG. 10. 
     Similar to the dipole case, the voltage and current on the transmission line of the unit cell are given by ##EQU9## 
     We obtain the ratio I i+1  /I i  as ##EQU10## Note that the above is simply a recursive expression to obtain the current at successive points along the array. 
     The power dissipated in each slot radiator is given by ##EQU11## A recursive expression for the slot resistances R i  is given by ##EQU12## Therefore, for a given amplitude distribution, √P i  (i=1,2, . . . ,N E ), a given transmission line length kl, R 1 , and I 1 , the resonant resistances R i  are given by the above expression. For θ 0  ≠0, R 1  =Z 0 . 
     Once the desired resonant resistances R i  are determined, design of the linear subarray is accomplished using design curves of resonant resistance and resonant frequency both as a function of offset, obtained either theoretically or experimentally. 
     Although only a few embodiments have been described in detail above, those having ordinary skill in the art will certainly understand that many modifications are possible in the preferred embodiment without departing from the teachings thereof. 
     All such modifications are intended to be encompassed within the following claims.