Antenna arrays are widely used in communication and radar systems because of their high directivity and ability to control beam direction. Some examples of these systems are military radars, vehicles collision avoidance systems, cellular base stations, satellite communication systems, broadcasting, naval communication, weather research, radio-frequency identification (RFID) and synthetic aperture radars. Antenna arrays are excited using either a serial or a corporate feed network. Serially-fed antenna arrays are more compact than their corporate-fed counterparts (e.g., serially-fed antenna arrays have a substantially shorter feeding or transmission line than corporate-fed arrays). Furthermore, the ohmic and feed line radiation losses are smaller in serially-fed arrays than in corporate-fed arrays. Hence, the efficiency of serially-fed arrays can be higher than that of corporate-fed arrays.
Serially-fed antenna arrays are not without their drawbacks, however. For example, serially-fed antenna arrays have a narrow bandwidth due to the non-zero group delay of the feed network causing variation of the phase shift with frequency between the antennas of adjacent antenna units. Therefore, beam direction varies (beam squint) as the frequency changes, thereby reducing the array boresight gain and causing performance degradation, especially in narrow beam width systems.
More particularly, the main beam angle of an antenna array is determined by phase shifts between adjacent antennas of the array. In serially-fed antenna arrays, the phase shift is adjusted using a frequency dependent phase shifter. Therefore, the antenna array beam angle changes as the frequency changes resulting in beam squinting given by equation (1):
                              θ          beam                =                              sin                          -              1                                ⁡                      (                                                            θ                  f                                -                                  θ                                      f                    o                                                                                                K                  o                                ⁢                                  d                  E                                                      )                                              (        1        )            where: θbeam is the main beam angle, θfo and θf are the phase shifts between any two of the adjacent antennas at the center frequency and at an offset frequency, respectively, and dE is the inter-element spacing (i.e., the space between adjacent antennas in the antenna array). According to equation (1), the beam squint occurs because the phase shift between the adjacent antennas varies with frequency. In order to eliminate the beam squint, the phase shift between the antennas must be frequency independent. In other words, the group delay, which is calculated from equation (2) below, between adjacent antennas must be zero.
                              Group          ⁢                                          ⁢          Delay                =                              -                          1                              2                ⁢                                                                  ⁢                π                                              ⁢                                    d              ⁢                                                          ⁢                              θ                f                                                    d              ⁢                                                          ⁢              f                                                          (        2        )            
To obtain a zero group delay between the adjacent antennas (and thereby eliminating, or at least substantially reducing, beam squint), one or more NGD circuit(s) may be integrated between the adjacent antennas. In such an instance, the NGD value must be equal to the value of the positive group delay of the interconnecting transmission lines. FIGS. 1A and 1B depict conventional serially-fed antenna array arrangements wherein NGD circuits are integrated between adjacent antennas to have an overall group delay of approximately zero. In FIG. 1A, and for each set of adjacent antennas, an NGD circuit comprising a lossy parallel resonance circuit is serially-integrated into the transmission line between the two antennas. In FIG. 1B, an NGD circuit comprising a lossy series resonator circuit is integrated into the transmission line in a shunt arrangement. In each of these arrangements, in order to have a uniformly excited antenna array, an amplifier and corresponding matching circuits can be used as illustrated in FIGS. 1A and 1B.
The use of conventional NGD circuits in this manner is not without its shortcomings, however. The conventional NGD circuits employ lossy elements (e.g., a lossy resonator) to generate a desirable amount of NGD. As such, these circuits suffer from a large amount of loss in order to generate NGD (e.g., certain conventional NGD circuits may have a typical loss of 6 dB or more, meaning that more than 70-75% of the power is dissipated in the NGD circuit), which significantly limits their application.
Accordingly, there is a need for NGD circuits that minimize and/or eliminate one or more of the above-identified deficiencies.