Abstract:
A log-periodic dipole array system employs a structure for the transmitter and the receiver designed in a way such that they compensate for the non-linear characteristics of each other to realize linear phase characteristics as a pair. Radiation elements on the receiver are positioned with respect to its corresponding transmission line in an order opposite to the positioning of the radiation elements on the transmitter. Although neither the transmitter dipole array nor the receiver dipole array itself has linear phase characteristics, the overall dipole array antenna system can realize linear phase characteristic. The log-periodic dipole array system has the advantages that linear phase characteristics can be obtained without sacrificing high radiation efficiency and gain.

Description:
CROSS-REFERENCE TO RELATED APPLICATION 
       [0001]    This application claims priority under 35 U.S.C. §119(e) from co-pending U.S. Provisional Patent Application No. 60/951,668 entitled “Ultra-Wideband Log-Periodic Dipole Array with Linear Phase Characteristics,” filed on Jul. 24, 2007, which is incorporated by reference herein in its entirety. 
     
    
     BACKGROUND OF THE INVENTION 
       [0002]    1. Field of the Invention 
         [0003]    The present invention relates to Broadband/Ultra-wideband (UWB) antenna design. 
         [0004]    2. Description of the Related Art 
         [0005]    Ultra-Wideband (UWB) communication has been the subject of intense research over the last few years. The essence of UWB systems is the ability to transmit and receive UWB pulses, which occupy a bandwidth over several octaves. A UWB system needs an antenna that maintains good phase and amplitude linearity over a wide bandwidth. 
         [0006]    Broadband antennas have been studied in the past for short pulse applications. Basically, there are two ways to achieve broadband functionality in an antenna. One is to broaden the bandwidth of currently available antennas, i.e., using one radiation element to cover the entire UWB bandwidth. The other approach is to use an antenna array for UWB applications. The antenna array is made of several radiation elements, with each of which covering a relatively narrow bandwidth, with their sum of bandwidths complying with the UWB requirements. 
         [0007]      FIG. 1  shows a conventional 2-element Log-periodic Dipole Array (LPDA)  100  in schematic form. In general, an LPDA is a broadband, multi-element, unidirectional, narrow-beam antenna with impedance and radiation characteristics that are regularly repetitive as a logarithmic function of the excitation frequencies. The individual radiation elements in LPDA are dipole antennas. In a LPDA, there are several radiation elements or dipoles (for example, radiation element  1  ( 102 ) and radiation element  2  ( 104 )), each of which covers a narrow bandwidth, and the LPDA  100  uses a single transmission line  108  to connect all the radiation elements (e.g., the two elements  102 ,  104 ) in order to achieve broader bandwidth. 
         [0008]    Assume that element  1  ( 102 ) has a resonant frequency f 1 , and that element  2  ( 104 ) has a resonant frequency f 2 . If signals  106  with frequencies f 1  and f 2  are fed into the LPDA  100  at the same time, signals with frequency f 1  will be radiated into free space by element  1  ( 102 ) while signals with frequency f 2  will move along the transmission line  108  further since frequency f 2  is not the resonant frequency of element  1  ( 102 ). Signals with frequency f 2  will experience some additional delay caused by the transmission line  108  until it is radiated into the free space by element  2  ( 104 ). Obviously, such a radiation mechanism would introduce a non-constant group delay, i.e., non-linear phase characteristics. 
         [0009]    Such non-linear phase characteristic will be even worse if a pair of LPDAs is used for UWB signal transmission and reception.  FIG. 2  shows an example of using the LPDAs  100 ,  130  as the transmitter and receiver, respectively. Note that the elements  122 ,  124  in the LPDA  130  on the receiver side are arranged in orientation to the transmission line  128  identically to the way the elements  102 ,  104  in the LPDA  100  on the transmitter side are arranged in orientation to the transmission line  108 . Because of the non-linear phase characteristics, signals with frequency f 1  are radiated first and signals with frequency f 2  are radiated later with a delay caused by the transmission line  108 . As a result, the signal with frequency f 1  arrives at the receiver LPDA  130  earlier than the signals with frequency f 2 . In addition, signals with frequency f 2  travel further along the transmission line  128  until it reaches its signal output  120 , adding an extra delay between the signals with frequency f 1  and the signals with frequency f 2 . Therefore, the original signals cannot be recovered. 
         [0010]      FIGS. 3 and 4  show another conventional antenna array  300 , referred to as Independently Center-fed Dipole Array (ICDA), for ultra-wideband applications, in schematic form. The ICDA also uses several narrowband radiation elements (e.g., two radiation elements  302 ,  304 ) in order to cover a broad bandwidth. However, the feed network  308  in the ICDA is different from that in LPDAs. Instead of having all the dipole elements serially connected by a transmission line, each element  302 ,  304  in the ICDA is fed independently through its own transmission line  320 ,  322 , and all the transmission lines  320 ,  322  are connected at a splitting point  314  to the common transmission line  308  coupled to the input signal source  306 . In other words, a broadband signal would travel on transmission line  308 , be split up at the splitting point  314 , and then fed into all the dipole elements  302 ,  304  via separate transmission lines  320 ,  322 . By using the same transmission line  308  for both elements  302 ,  304  and then splitting up to separate transmission lines  320 ,  322  with equal length at the splitting point  314 , all frequency components of the signal will be simultaneously fed into and radiated out by the corresponding active elements  302 ,  304 . 
         [0011]    Although the ICDA has linear phase characteristics, it also has low radiation efficiency.  FIG. 4  shows an ICDA with N radiation elements. Referring to  FIG. 4 , the input signal  310  would travel on transmission line  308 , and then be split up at junction  314  to N waves on separate transmission lines  320 ,  322 , and propagate to each port corresponding to each radiation element ( 302 ,  304  . . . ). Thus, each radiation element would receive only a small portion of the original incident wave  310 . For example, the incident wave  312  that is transmitted to element  1  ( 302 ) is only a small portion of the original incident wave  310 . Thus, radiation efficiency is low in ICDAs. 
       SUMMARY OF THE INVENTION 
       [0012]    Embodiments of the present invention include a dipole array antenna system, comprising (i) a transmitter dipole array including at least a first radiation element and a second radiation element coupled to a first transmission line, the first radiation element positioned on the first transmission line at a first distance from a signal input to transmitter dipole array and the second radiation element positioned on the first transmission line at a second distance from the signal input, the second distance being larger than the first distance, and (ii) a receiver dipole array including at least a third radiation element and a fourth radiation element coupled to a second transmission line, radiation characteristics of the third radiation element and the fourth radiation element being substantially same as radiation characteristics of the first radiation element and the second radiation element, respectively, and the third radiation element positioned on the second transmission line at a third distance from a signal output from the receiver dipole array and the fourth radiation element positioned on the second transmission line at a fourth distance from the signal output, the third distance being larger than the fourth distance. In one embodiment, a difference between the first distance and the second distance is substantially same as a difference between the third distance and the fourth distance. 
         [0013]    According to the dipole array antenna system of the present invention, the first radiation element is configured to radiate a first frequency signal, the second radiation element is configured to radiate a second frequency signal, the third radiation element is configured to receive the first frequency signal, and the fourth radiation element is configured to receive the second frequency signal. The first frequency signal is transmitted by the first radiation element at a first timing and the second frequency signal is transmitted by the second radiation element at a second timing delayed by a first time delay with respect to the first timing. The first frequency signal is received by the third radiation element at a third timing and the second frequency signal is received by the fourth radiation element at a fourth timing delayed by a second time delay substantially the same as the first time delay. The first frequency signal is transmitted on the second transmission line during the second time delay and combined together with the second frequency signal at the signal output at substantially the same time, with linear phase. In other words, the first frequency signal and the second frequency signal will experience the same total delay when reaching the signal output. Therefore, although neither the transmitter dipole array nor the receiver dipole array itself has linear phase characteristics, the overall dipole array antenna system can realize linear phase characteristic. The dipole array system of the present invention has the advantages that linear phase characteristics can be obtained without sacrificing high radiation efficiency and gain. 
         [0014]    The features and advantages described in the specification are not all inclusive and, in particular, many additional features and advantages will be apparent to one of ordinary skill in the art in view of the drawings, specification, and claims. Moreover, it should be noted that the language used in the specification has been principally selected for readability and instructional purposes, and may not have been selected to delineate or circumscribe the inventive subject matter. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0015]    The teachings of the embodiments of the present invention can be readily understood by considering the following detailed description in conjunction with the accompanying drawings. 
           [0016]      FIG. 1  shows a conventional 2-element Log-periodic Dipole Array (LPDA) in schematic form. 
           [0017]      FIG. 2  shows an example of using the conventional LPDAs as the transmitter and receiver. 
           [0018]      FIG. 3  and  FIG. 4  show a conventional Independently Center-fed Dipole Array (ICDA). 
           [0019]      FIG. 5  shows a 2-element ultra-wideband log-periodic dipole array (transmitter and receiver), according to one embodiment of the present invention. 
           [0020]      FIG. 6  shows how the signal is transmitted and received in the pair of ultra-wideband log-periodic dipole arrays, according to one embodiment of the present invention. 
       
    
    
     DETAILED DESCRIPTION OF EMBODIMENTS 
       [0021]    The Figures (FIG.) and the following description relate to preferred embodiments of the present invention by way of illustration only. It should be noted that from the following discussion, alternative embodiments of the structures and methods disclosed herein will be readily recognized as viable alternatives that may be employed without departing from the principles of the claimed invention. 
         [0022]    Reference will now be made in detail to several embodiments of the present invention(s), examples of which are illustrated in the accompanying figures. It is noted that wherever practicable similar or like reference numbers may be used in the figures and may indicate similar or like functionality. The figures depict embodiments of the present invention for purposes of illustration only. One skilled in the art will readily recognize from the following description that alternative embodiments of the structures and methods illustrated herein may be employed without departing from the principles of the invention described herein. 
         [0023]      FIG. 5  shows a 2-element ultra-wideband log-periodic dipole array system (transmitter and receiver), according to one embodiment of the present invention. The ultra-wideband LPDA of the present invention can be used for ultra-wideband applications while keeping high radiation efficiency. Unlike conventional LPDAs used as the transmitter and the receiver, the LPDA of the present invention is designed to have different structures for transmitter and receiver. 
         [0024]      FIG. 5  shows both structures of the transmitter  100  and the receiver  550 ). Both the transmitter  100  and the receiver  550  use several narrowband radiation elements or dipoles (e.g., elements  102 ,  104  and elements  502 ,  504 ) to cover a wide bandwidth. Radiation element  102  on the transmitter side  100  and radiation element  502  on the receiver side  550  are identical and have substantially the same length, i.e., substantially the same radiation characteristics. Likewise, radiation element  104  on the transmitter side  100  and radiation element  504  on the receiver side  550  are identical and have substantially the same length, i.e., substantially the same radiation characteristics. In the examples of  FIG. 5  and  FIG. 6 , assume that radiation elements  102 ,  502  are configured to have resonant frequencies consistent with the excitation frequency f 1  of the input signal  106  and that radiation elements  104 ,  504  are configured to have resonant frequencies consistent with the excitation frequency f 2  of the input signal. Since transmitter  100  and receiver  550  are both LPDAs, radiation element  102  and radiation element  104  have different lengths, with impedance and radiation characteristics that are regularly repetitive as a logarithmic function of the excitation frequencies f 1  and f 2  of the input signal source  106 . Likewise, radiation element  502  and radiation element  504  have different lengths, with impedance and radiation characteristics that are regularly repetitive as a logarithmic function of the excitation frequencies f 1  and f 2  of the input signal source  106 . In the example of  FIG. 5 , radiation element  102  is longer than radiation element  104 , and radiation element  502  is longer than radiation element  504 . Radiation elements  102 ,  104  on the transmitter side  100  are connected via transmission line  108 , and radiation elements  502 ,  504  on the receiver side  550  are connected by transmission line  508 . 
         [0025]    Radiation element  102  on the transmitter  100  is positioned on the transmission line  108  at a distance  520  from the input signal source  106 . Radiation element  104  on the transmitter  100  is positioned on the transmission line  108  at a distance  522  from the input signal source  106 . Radiation element  502  on the receiver  550  is positioned on the transmission line  508  at a distance  532  from the signal output receiver  506 . Radiation element  504  on the receiver  550  is positioned on the transmission line  508  at a distance  530  from the signal output receiver  506 . In one embodiment, the length  524  of the part of the transmission line  108  between radiation elements  102 ,  104  on the transmitter side  100  (i.e., the difference between distances  520  and  522 ) is designed to be substantially the same as the length  534  of the part of the transmission line  508  between radiation elements  502 ,  504  on the receiver side  550  (i.e., the difference between distances  530  and  532 ). In one embodiment, distances  520  and  522  are substantially same as distances  530  and  532 , respectively. 
         [0026]    According to embodiments of the present invention, the signal input on the transmitter side  100  of the LPDA system is at an end different from the signal output on the receiver side  550  of the LPDA system. More specifically, referring to  FIG. 5 , the signal input source  106  is connected to the end of transmission line  108  closer to element  102  to feed the radiation elements  102 ,  104  of the transmitter side with the input radio frequency signal to be radiated. On the other hand, the signal output receiver  506  is connected to the end of the transmission line  508  closer to element  504  rather than element  502 . Thus, if a signal including frequency components f 1  and f 2  is fed into the transmitter  100  from input signal source  106 , it will reach element  1  ( 102 ) first and element ( 104 ) later on the transmitter side  100 . On the other hand, on the receiver side  550  the received signal will reach element  1  ( 502 ) first and element  2  ( 504 ) later. Note that this is opposite from the conventional LPDA shown in  FIG. 2 , where both the signal input source  106  and the signal output receiver  110  are connected to the end closer to elements  102 ,  122 . 
         [0027]      FIG. 6  shows how the signal is transmitted and received in the pair of ultra-wideband log-periodic dipole arrays, according to one embodiment of the present invention. On the transmitter side  100 , an input signal including frequency components f 1  and f 2  is fed from input signal source  106  into the transmitter  100 . The frequency component f 1  is transmitted on the transmission line  108  and reaches its corresponding radiation element  102  (with resonant frequency f 1 ) first, while the frequency component f 2  is transmitted on the transmission line longer and reaches its corresponding radiation element  104  (with resonant frequency f 2 ) later with a delay. Thus, frequency component f 1  will be radiated from the transmitter  100  into the free space first, and the frequency component f 2  will be radiated from the transmitter  100  into free space next, after a delay caused by the part  524  of transmission line  108  between radiation elements  102 ,  104 . 
         [0028]    On the receiver side  550 , the frequency component f 1  is picked up by radiation element  1  ( 102 ) first. However, because the length  524  of the inter-element transmission line  108  between the radiation elements  102 ,  104  on the transmitter side  100  is substantially the same as the length  534  of the inter-element transmission line  508  between the radiation elements  502 ,  504  in the receiver  550 , the frequency component f 1  will experience the same delay that the frequency component f 2  experienced on the transmitter side  100 . By the time the received frequency component f 1  reaches radiation element  2  ( 504 ) on the receiver side  550 , the frequency component f 2  will also be picked up by radiation element  2  ( 504 ) on the receiver side  550  at substantially the same moment. Therefore, at the output receiver  506  of the receiver  550 , both frequency components f 1  and f 2  are collected by the signal output receiver  506  at substantially the same time, and the received signal can be recovered with linear phase (same group delay). 
         [0029]    As can be seen from above, neither the transmitter  100  nor the receiver  150  has linear phase, since one frequency will be radiated (or received) earlier than the other frequency. However, the non-linear phase characteristics of the transmitter  100  is corrected and compensated for by the receiver  150  through opposite arrangements of the radiation elements with respect to the inter-element transmission lines and signal inputs/outputs. In other words, the frequency which is radiated into free space first (or last) will be picked up by the receiver first (or last), respectively. Both frequencies would experience the same delay in the inter-element transmission lines  108 ,  508 , since the lengths  524 ,  534  of inter-element transmission lines  108 ,  508  in the transmitter  100  and the receiver  550 , respectively, are substantially the same. Therefore, at the output  506  of the receiver  150 , the signal can be recovered with linear phase (same group delay). 
         [0030]    Upon reading this disclosure, those of skill in the art will appreciate still additional alternative designs for LPDA system with linear phase characteristics. For example, while the present invention is illustrated with two radiation elements on each of the transmitter and the receiver, a different number (two or more) of radiation elements may be present on each of the transmitter and the receiver, positioned with respect to their corresponding transmission lines according to the present invention. Thus, while particular embodiments and applications of the present invention have been illustrated and described, it is to be understood that the invention is not limited to the precise construction and components disclosed herein and that various modifications, changes and variations which will be apparent to those skilled in the art may be made in the arrangement, operation and details of the method and apparatus of the present invention disclosed herein without departing from the spirit and scope of the invention as defined in the appended claims.