Abstract:
A dipole array is provided for use as an Ultra Short Pulse (USP) transmitter or receiver in UWB communications systems, which reduces the output pulse dispersion. Instead of having all the dipole elements serially fed by a transmission line, the feeding in the array is made independently through a central point and the radiation is emitted and received broadsided with respect to the array plane. This configuration minimizes the relative time delay between radiating resonance frequencies.

Description:
RELATED APPLICATIONS  
       [0001]     This application claims priority from U.S. Provisional patent Application Ser. No. 60/572,355 filed May 19, 2004. 
     
    
     FEDERALLY SPONSORED RESEARCH  
       [0002]     Partial support for the present invention was provided by the National Science Foundation, and accordingly the U.S. Government may have certain license or other rights in the invention. 
     
    
     FIELD OF INVENTION  
       [0003]     This invention relates to transmission and reception of ultra short pulses (USP) commonly used in ultra-wideband (UWB) communication systems, and more specifically relates to antenna arrays for use in such systems.  
       BACKGROUND OF INVENTION  
       [0004]     The Ultra Wide-Band (UWB) technique, wherein the signal is defined as having greater than 25% relative bandwidth as determined by: BW/f c , has been the subject of intense research efforts during the last several years because it presents a large bandwidth at short distance communication, which is desirable for many indoor wireless systems. See W. Stutzman and G. Thield, “Antenna theory and design,” 2nd ed., John Wiley &amp; Sons. New York, 1998. In order to implement a UWB technique, it is necessary to develop a relatively dispersionless antenna which maintains a good phase and amplitude linearity over a wide bandwidth transmitting and receiving ultra short pulses (USP). Among all the wide-band antennas, the log-periodic dipole array (LPDA) could provide the widest bandwidth. It is known that on the log-periodic antennas, each specific frequency has an active region which has a strong current excitation. As the frequency changes, such current excitation remains the same, but it moves locally toward the direction of the active region. Such a radiation mechanism would introduce a large time delay between the frequency constituent of the temporal pulse thus resulting in a severe dispersion to the short-pulsed UWB signal.  
       SUMMARY OF INVENTION  
       [0005]     Now in accordance with the present invention a dipole array is provided which reduces the dispersion. Instead of having all the dipole elements serially fed by a transmission line, and instead of tuning each other element with an out-of-phase signal, the feeding in this array is made in parallel, through a central point such as a power divider. A transmission line is connected to the power divider for feeding the broadband signal to the power divider to ensure feeding with appropriate amplitude and phase correction into the dipole elements.  
         [0006]     The configuration of the invention minimizes the relative time delay between radiating resonance frequencies since all frequency components of the pulse are transmitted or, received at the same time. This array also provides for a wide bandwidth since it enables placing of a sequence of parallel dipole elements of successively varied lengths with each additional dipole providing for an additional frequency band. The overall bandwidth of the array is constituted by the sum of the individual bandwidths of each dipole. Typically a broadband signal is split up by the power divider, and then fed into all the dipole elements in parallel. Thus, all frequency components of the signal will be simultaneously fed into and radiated out by the corresponding active elements. The radiation is emitted and received broadsided with respect to the array plane. 
     
    
     BRIEF DESCRIPTION OF DRAWINGS  
       [0007]     In the drawings appended hereto:  
         [0008]      FIG. 1  ( a ) is a schematic diagram of a ICDA array in accordance with the invention of two elements;  FIG. 1 ( b ) is the extension to 12 elements; and  
         [0009]      FIG. 1 ( c ) shows the power divider for these elements;  
         [0010]      FIG. 2  contains graphs depicting variation of SWR of each element using Method of Moment (MoM) and Finite Difference Time Domain (FDTD);  
         [0011]      FIG. 3  is a graph showing calculated and measured SWR for the ICDA array of  FIG. 1 ;  
         [0012]     FIGS.  4 ( a ) and  4 ( b ) are graphs depicting transmission coefficients for the ICDA array; and  
         [0013]      FIG. 5  presents the calculated transmission coefficient (amplitude and phase) for twelve elements as in  FIG. 1 ( b ).  
     
    
     DESCRIPTION OF PREFERRED EMBODIMENT  
       [0014]     In the present invention, the new dipole array concept used is called an independently center-fed dipole array (ICDA). The feeding is made independently through a central point as seen in the schematic diagram of FIGS.  1 ( a ) and  1 ( c ). Simulations, using Method of Moment (MoM) and Finite-Difference Time Domain (FDTD) and experiments with a two-element array exhibited the usefulness of this approach. Experimentally the impact of mutual coupling on the SWR of each dipole was evaluated and the transmission coefficient, S 21  as well was measured.  
         [0015]     Simulations:  FIG. 1 ( a ) shows the ICDA array in schematic form. The MoM and FDTD methods were used to calculate the SWR of each element, when the other is present or, absent. The codes used for the simulations were based on equations introduced in Stutzman, et al, Berenger, et al and Umashankar, et al, op. cit. In both simulations, we assume L 1 =0.25, L 2 =L 1 ·0.8=0.2, d=L 1 ·0.6=0.15, a=2·L 1 /150 (see  FIG. 1 ). The other FDTD parameters were: cell size, Δ=(2·L 1 )/21 and region of calculation (in terms of number of cells), 56×63×56.  FIG. 2  shows the variation of the standing wave ratio, SWR, of each element. The terms SWR 1  and SWRN 1  are the SWR of element  1  when element  2  is present or, absent, respectively. Similarly, SWR 2  and SWRN 2  are the SWR of element  2  when element  1  is present or, absent, respectively. Thus, coupled elements exhibit similar SWR values as the isolated ones. FIGS.  1 ( b ) and  1 ( c ) also show the extension of this concept to twelve elements which cover the necessary 3.1-10.6 GHz bandwidth of UWB communication systems. The dipole array of the invention may comprise any linear set with a functional relationship between the separation of elements and their related lengths and thickness, such as occurs in but not limited to a log periodic array. The array may include as many elements as are needed in order to provide the required bandwidth.  
         [0016]     Experimental results Commercial tunable dipole antennas SNA600 were used, with center frequencies ranging from 550 MHz to 800 MHz and a bandwidth of 100 MHz each. Using the ratio values from the simulations, the center-frequencies of element  1  and element  2  were 610 MHz and 750 MHz, respectively. The lateral distance between the elements was 7.5 cm. Each element was connected to a Hewlett Packard 8510 network analyzer through a 3-dB power divider. Two pairs of such elements were placed in an anechoic chamber 5 m apart, one serving as a transmitter and the other as a receiver. The two arrays were facing each other, parallel to the radiation phase front. The power divider has a 50/3 ohm resistor on each port. The input impedance of the ICDA could be calculated as follows:  
           Z   in     =           (       Z     in   ,   610       +     50   /   3       )     ·     (       Z     in   ,   750       +     50   /   3       )           Z     in   ,   610       +       50   /   3     ·     Z     in   ,   750         +     50   /   3         +     50   3         ,       
 
 where, Z in,610  was the input impedance of 610-MHz element, Z in,750  was the input impedance of 750-MHz element.  FIG. 3  shows the calculated and measured SWR of the ICDA. It can be seen that the measured SWR and the calculated SWR are within the estimated error. This result confirms the conclusion that mutual couplings do not have a critical impact on the SWR. 
 
         [0017]      FIG. 4 ( a ) shows the S 21  amplitude characteristic of isolated elements  1  and  2 . Element  1  had a 3-dB range between 560-MHz to 660-MHz. Element  2  had a 3-dB range between 700-MHz to 800-MHz with the exception of a few points where the amplitude fluctuated at 4-dB level. The S 21  amplitude characteristic and phase characteristic of the ICDA are shown in  FIG. 4 ( a ) and  FIG. 4 ( b ), respectively. It can be seen from  FIG. 4 ( a ) that in the range between 560-MHz to 800-MHz the amplitude characteristics do not fluctuate beyond the fluctuation of an individual element. Also, as shown in  FIG. 4 ( b ) the phase characteristics are linear in the entire range of 560-MHz to 800-MHz.  FIG. 5  shows theoretical calculations for a twelve element antenna using FDTD method. These calculations demonstrate that such antenna meets the FCC bandwidth allocation for UWB systems in the range of 3.1-10.6 GHz.  
         [0018]     In the foregoing the characteristics of the ICDA array are thus analyzed numerically and demonstrated experimentally. The simulations show that the mutual coupling does not significantly impact the SWR of each dipole. This is confirmed by the experimental data. The S 21  amplitude characteristic of the ICDA doesn&#39;t fluctuate beyond the individual element&#39;s fluctuation. Also, the phase characteristic is linear in the whole range of individual elements. The data indicates that this concept may be expanded to a larger number of dipolar elements to enable realization of a linear-phase antenna for UWB communication systems.  
         [0019]     While the present invention has been described in terms of specific embodiments thereof, it will be understood in view of the present disclosure, that numerous variations upon the invention are now enabled to those skilled in the art, which variations yet reside within the scope of the present teaching. Accordingly, the invention is to be broadly construed, and limited only by the scope and spirit of the claims now appended hereto.