Abstract:
A Yagi antenna system consisting of a low noise amplifier (LNA) and a reflector co-located on the same printed circuit board (PCB) as the radiators and directors is disclosed. Furthermore, the balun cable is replaced by surface mount devices whose feed line is implemented in microstrip technology, all co-located on the same printed circuit board.

Description:
FIELD OF THE INVENTION 
     The present invention relates to a Yagi antenna system wherein the active and parasitic elements of the antenna can be co-located on one printed circuit board (PCB) with a low noise amplifier (LNA). Furthermore, surface mount devices (SMDs) can replace the balun that is conventionally used for impedance matching between the symmetrical radiator impedance and the asymmetrical LNA input impedance. 
     BACKGROUND OF THE INVENTION 
     Yagi antennas are used in high frequency applications such as satellite radio transmission. There presently exists a population of 4 billion people that are generally dissatisfied and underserved by the poor sound quality of short-wave or terrestrial radio broadcast systems. This population is primarily located in Africa, Central and South America, and Asia. FIG. 1 shows an overview of a satellite broadcast system  10  comprising various broadcast stations  2  for transmitting multiple audio signals, for example, to a satellite  4 , which in turn transmits these signals to the receivers  9 . The satellite broadcast system  10  is particularly useful for providing high-quality broadcast programming to users in Africa, Central and South America, and Asia. The present invention relates to a low-cost antenna that can be mounted on a portable radio receiver  9  for reception of satellite radio transmissions. This invention is particularly useful for the reception of satellite signals where a receiver antenna gain on the order of 9 dbi together with a noise figure on the order of 1 dB are required due to the low power flux density available at the receiver location. 
     Yagi antennas generally consist of three types of elements: reflector, radiator, and directors. The radiator (e.g., a folded dipole) is an active element that receives the power concentrated by the parasitic elements. The reflector is a parasitic element with an inductive quality. The directors are also parasitic elements but with a capacitive quality. Yagi antenna systems use the parasitic elements in combination with active elements to control the direction and width of the beam. The Yagi antenna optimizes gain by using specific director lengths and spacing between the directors and the driven element (e.g., the radiator). 
     In addition, the Yagi antenna typically employs a balun (e.g., a half wavelength coaxial line) to achieve a 180 degree phase shift of the signal. Specifically, as seen in FIG. 2, a coaxial cable  32  is physically connected to the driven element (e.g., folded dipole)  40 . The inner sheath  38  is connected to one side of the folded dipole  40  and the feed cable  34 , and the opposite inner sheath  39  is connected to the opposite side of the dipole. The outer sheath  36  is connected to ground. As the signal travels around the inner sheath from  39  to  38  it becomes 180 degrees phase shifted from the original signal. This cable and dipole arrangement is cumbersome and prevents an antenna arrangement from being constructed on a simple printed circuit board. A need exists for a more compact means to drive the components of a Yagi antenna. 
     A compactly designed Yagi antenna is disclosed in U.S. Pat. No. 5,612,706. However, this antenna merely reduces the distance between two rods and is not well suited for radio receiver portability. It is more convenient to have a Yagi antenna that can be folded for transportation. Further, it would be advantageous to have a less costly implementation than the one disclosed in U.S. Pat. No. 5,612,706. 
     Removal of the balun is described in U.S. Pat. No. 5,898,410. A log periodic dipole array antenna system achieves impedance matching by adjusting the distance between a focusing element and one of several dipoles or driven elements. The antenna system therefore has plural active elements and, correspondingly, impedance matching requirements for each of these elements. A need exists for a low-cost antenna having a simple active element impedance matching design. 
     A performance limitation of the Yagi antenna is the signal loss caused by cables and connectors between the antenna feed point and the low noise amplifier input stage. There is currently a requirement to match the antenna feed point to a standard impedance (such as 50 ohms) which can be accommodated by off-the-shelf connectors and cables, and then again match the impedance to the low noise amplifier input stage. This sequential impedance matching requirement incurs line and connector losses, which in turn detrimentally affect the performance of the Yagi antenna. 
     As shown in FIG. 3, some patch and Yagi antenna systems  10  use dual circular polarization outlets which can be costly due to the type and number of components. For example, the system shown requires two outlets, that is, a right-hand circular polarization outlet  18  and a left-hand circular polarization outlet  16 , two low-noise amplifier (LNA) input stages  24  and  26 , an electronic polarization switch  14 , and at least two housing mounts  12  and  13 . 
     Manufacturing costs are also a contributing factor to the expense of the receivers  9 . It is known in the art to use coaxial cables  20  and  22  to connect the LNA input stages  24  and  26  to the antenna outlets  18  and  16  to achieve impedance matching. However, as mentioned in U.S. Pat. No. 4,518,968, balanced low impedance feeders have been recommended, but have not often been adopted in practice. This is because such feeders, when engineered for dipole and Yagi-Uda array matching impedances, are dimensionally awkward to manufacture and install. Further, since the folded dipole and the director elements are separate from the low noise amplifier (LNA), two fabrication procedures are needed, thereby increasing the likelihood of problems due to manufacturing tolerances. Thus, a need exists for a low cost Yagi antenna design that is easily mass-produced with a low error tolerance. 
     It is known, for example, from U.S. Pat. No. 5,272,485, to use antennas embedded in substrates in microwave frequency applications where a feedpoint and via are used as an input to a low noise amplifier, thereby obtaining optimum impedance matching. However, these diagonally-fed electric microstrip dipole antennas are patch antennas that are constructed on at least two layers of a dielectric substrate. These types of patch antennas cannot be designed for high gain without using an array of patches, thereby incurring a negative effect on complexity and size. 
     Accordingly, a need exists for a more simple means of impedance matching of a Yagi antenna with only one driven element. A need also exists for an active antenna system that is low cost and readily mass-produced while providing reasonably high gain, directivity and noise performance. A foldable design is desirable to keep the antenna compact for travel. 
     SUMMARY OF THE INVENTION 
     These needs and others are satisfied by the Yagi antenna system of the present invention which, in a preferred embodiment, comprises an LNA, reflector, radiator or driven element, and at least one director all located on the same printed circuit board. Therefore, the present invention can eliminate the need for two separate housings, that is, one containing the LNA and the other containing the radiator and the directors. 
     An object of the present invention is to provide a low cost antenna that allows for simple and cost-effective mass manufacturing. This is possible because the antenna system of the present invention can be located on one printed circuit board, thus allowing for tighter tolerances during mass production. 
     Another object of the present invention is to eliminate the need for a balun cable. Since all the elements of the antenna can be located on the same printed circuit board, signal losses caused by coaxial cables and connectors and by the impedance matching between the LNA and the driven element are minimized as well. 
     Yet another object of the present invention is to provide a simple means of achieving the 180 degree phase shifting requirement for the feed to the opposite dipole side. The present invention eliminates the need for a signal cable in front of the LNA because of the preferred single circuit board design. Further, to avoid the balun cable, the phase shifting can be accomplished by means of surface mount devices located on the same printed circuit board. 
     Still another object of the present invention is to allow for the ability to fold the antenna for transportation purposes. The present invention allows for the elimination of electrical connections, such as cables, required between the parasitic and active elements. Therefore, the antenna design can comprise two flexibly connected plates which can be folded together during transport The front plate can contain an array of directors printed on a printed circuit board. The directors can comprise metallic rods or stripes inserted into a front plate compartment in a preferred embodiment of the present invention. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     These and other objects, advantages and novel features of the present invention will be more readily appreciated from the following detailed description when read in conjunction with the accompanying drawings, in which: 
     FIG. 1 is an overview of a satellite broadcast system; 
     FIG. 2 is schematic representation of a prior art coaxial cable connected to a half wave dipole director element; 
     FIG. 3 is a schematic representation of a prior art circularly polarized antenna system; 
     FIG. 4 is a schematic representation of a Yagi antenna receiver system in accordance with an embodiment of the present invention; 
     FIG. 5 is a schematic representation of the signal and phase shift feeding to the folded half wave dipole director element in accordance with an embodiment of the present invention; and 
     FIG. 6 is a polar graph illustrating the antenna beam pattern of the Yagi antenna constructed in accordance with an embodiment of the present invention. 
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     FIG. 4 illustrates an integrated Yagi antenna and low noise amplifier system  100  in accordance with a preferred embodiment of the present invention. The entire system  100  is preferably located on one epoxy glass fiber printed circuit board  130 . The printed circuit board  130  can be manufactured with any suitable material and is not limited to epoxy glass fiber. The system  100  consists of an F-connector  120  which attaches to the antenna transmission line of the receiver such as the radio receiver  9 . The Yagi antenna system  100  is comprised of a reflector combined with a low noise amplifier (LNA) indicated at  118  which are implemented using surface mount device technology. The reflector and LNA combination  118  are in turn connected to the radiator  122 . The reflector and the LNA are co-located on the same printed circuit board. This design eliminates the cables and connectors used with conventional Yagi antennas which produce signal loss in front of the low noise amplifier  118  and reduce antenna sensitivity. 
     As described below in connection with FIG. 5, surface mount device (SMDs) are used to facilitate the connection between the symmetrical dipole feed points  112  and  114  of the radiator  122  and the LNA  118 . The SMDs are indicated generally at  50  in FIG.  4 . This allows for the two dipole feed points  112  and  114  to be driven approximately 180 degrees out of phase with respect to each other. As this method avoids the use of a balun cable, the complete antenna and LNA system  100  of FIG. 4 can now be implemented on one substrate  132  and can be enclosed in one housing  134  having a single mount for connection to a receiver such as the radio receivers  9 . The SMDs can be mounted on the printed circuit board  130 , specifically substrate  132 , of FIG.  4  and encased using a plastic material, for example, to more easily accommodate folding of the antenna. The encasing material is not limited to plastic, but can be any material that is appropriate. Such a design is more compact than a conventional Yagi antenna having coaxial sheath and core connections to the respective dipole feed points  112  and  114 . The preferred embodiment of the present invention employs a folded dipole as the driven element for ease with impedance matching, but an open dipole design could be used as well. Further, the type of driven element is not limited to an open or folded dipole design, and any appropriate design can be employed. 
     FIG. 5 illustrates the SMDs  50  which are preferably two capacitors  52  and  54  and an inductor  56 . The capacitors  52  and  54  each have one terminal connected to opposite terminals of the inductor  56  and the other terminal connected to ground  62  (e.g., the backplane of the substrate  132 ). The feed line  60 , which is preferably implemented using micro-strip technology on the substrate  132 , is connected to the feed point  114 . 
     The SMDs  50  allow for the signal on the feed line  60  directly connected to one of the dipole feed points  114  to be approximately 180 degrees out of phase with respect to the signal on the opposite feed point  112  of the folded dipole. The SMDs  50  are useful for antennas in a satellite broadcast system  100 , since such systems preferably use a limited bandwidth of about 1432-1512 Mhz or in the S-band range of 2630-2655 Mhz. The same design can be utilized in applications with higher or lower bandwidths, but the number of SMDs is adjusted to correlate to the bandwidth. 
     To further reduce the cost of the antenna and LNA system  100 , antenna operation is preferably linear, as opposed to circular, polarization. FIG. 3 shows the circular polarization technique. As mentioned above, circular polarization employs costly and duplicate components to process the left-hand and right-hand polarized signals. In order to meet the need for a low cost receiver, many components are eliminated or at a minimum reduced in number in the present system  100  by employing a linear reception mode for the circularly polarized signals. Benefits of a linear technique are the need for only one low noise amplifier input stage, and the elimination of the polarization switch and control logic to switch between the right-hand and left-hand polarized signal channels. In addition, one linear antenna can feed multiple receivers. The preferred embodiment of the present invention is able to employ this linear signal processing mode for left-hand and right-hand circularly polarized signals because the satellite broadcast system  100  does not require cross-polar separation. In addition to the difference in the polarization mode, the individual signals are displaced in frequency, thereby permitting the receiver tuning and selectivity to opt for either the right-hand or left-hand signal. 
     Since only one component of the circular radiation field is used, there is a signal loss of 3 db. This loss is compensated for by increasing the gain of the antenna by adding parasitic elements, thus enlarging the size of the antenna This is less expensive than using, alternatively, the configuration of FIG.  3 . Addition of these parasitic elements or directors is easily accommodated by the foldable design. The metallic axis within the fold of the antenna can be used as one of the magnetically coupled directors. This design allows for more room on the substrate  32  for placement of additional directors if needed. 
     As an additional benefit, this higher directivity offers better protection against interference, especially in the case of a linear interfering signal where the antenna can be decoupled by orienting it accordingly. Furthermore, users generally do not experience difficulties with antenna pointing, as the antenna lobe is still rather wide as shown in FIG.  6 . The 3 dB gain reduces the lobe width typically to 70%. 
     FIG. 6 shows the polar rotational diagram of a beam pattern for the antenna and LNA system  100 . The beam pattern demonstrates the ability of the antenna to deliver quality signals despite sub-optimum orientation by the user and further how much gain the antenna delivers if the antenna is turned slightly. For example, at optimum pointing, or 0 degrees, the antenna achieves a 9 db gain This does not drop off to 8 db until approximately 15 degrees away from optimum pointing. Furthermore, the antenna gain reaches 0 db at approximately 40 degrees. This figure shows the antennas tolerant gain despite the users error during antenna orientation. 
     The antenna system described herein offers many advantages since all of the components of the antenna and LNA system  100  can be placed on one printed circuit board. There is no need for bulky cables or connectors or for impedance matching. This allows for a simple design that facilitates portability of the radio receiver. Since the antenna can be placed on one printed circuit board, the present invention realizes reduced cost, and reduced likelihood for manufacturing tolerances and faults, allowing for the capability of excellent mass production. The preferred embodiment of the present invention employs linear signal processing as opposed to circular polarization processing to further reduce cost due to a reduction in the number of components in the system. 
     Although the present invention has been described with reference to a preferred embodiment thereof, it will be understood that the invention is not limited to the details thereof. Various modifications and substitutions will occur to those of ordinary skill in the art. All such substitutions are intended to be embraced within the scope of the invention as defined in the appended claims.