Many types of antennas are in wide use today throughout the communications industry. Array antennas generally have a distribution network for electrically coupling electromagnetic signals to and from a radiating element to support transmitting and receiving operations. In particular, many of the antenna applications of today utilize dual polarized antenna designs. In dual polarized antenna designs, electrical isolation is generally defined as the isolation from a first port to a second port in the antenna system (i.e., the port-to-port isolation at the connectors). In contrast, dual polarized antennas also have radiation isolations defined in the far-field of the antenna which differ from port-to-port isolations defined at the antenna connectors. It is the problems associated with port-to-port isolations in the dual polarized antennas that we now direct our attention.
In describing port-to-port isolations in a dual polarized antenna system, it is typically best described in terms of Scattering Parameters (s-parameters). In applying a Scattering Parameter analysis, the dual polarized antenna system is generally treated as a two-port system. The first port (port 1) includes a signal going into port 1 (represented by "a.sub.1 ") and a signal coming out of port 1 (represented by "b.sub.1 "). The second port (port 2) similarly includes a signal going into port 2 (represented by "a.sub.2 ") and a signal coming out of port 2 (represented by "b.sub.2 "). With these representative signals, the Scattering Parameters can be determined so to completely characterize the two-port network. The set of Scattering Parameters for a two-port network includes the parameters S.sub.11, S.sub.12, S.sub.21 and S.sub.22. S.sub.11 is determined from the ratio of "b.sub.1 /a.sub.1 ", S.sub.12 is determined from the ratio of "b.sub.1 /a.sub.2 ", S.sub.21 is determined from the ratio of "b.sub.2 /a.sub.1 " and S.sub.22 is determined from the ratio of "b.sub.2 /a.sub.2 ". Of these four parameters, the S.sub.12 and S.sub.21 parameters are considered when determining the port-to-port isolation in a dual polarized antenna. These two parameters characterize the signals passing from one port to another where S.sub.12 represents a signal going from port two to port one and S.sub.21 represents a signal going from port one to port two. Accordingly, in dual polarized antenna systems, the S.sub.12 and S.sub.21 parameters represent the leakage signals between ports one and two that may be present at the ports' connectors.
Poor sensitivity in dual polarized antennas can therefore result when part of an input (i.e., transmit) signal at the input port (port one) leaks or is otherwise coupled as a leakage signal to the output port (port two) and combines with a desired received signal at port two. When isolation is minimal, the antenna system will perform poorly in that the reception of incoming signals will be limited only to the strongest incoming signals due to the presence of leakage signals interfering with the weaker desired signals. Consequently, dual polarized antenna system performance can often be dictated by the isolation characteristic of the system.
One known technique for designing dual polarized antennas having a favorable isolation characteristic is by incorporating proper impedance matching within the distribution network. Impedance matching has been used to minimize the amount of impedance mismatch that a signal may experience when passing through the distribution network. In general, when impedance mismatches are present in an antenna system, part of an incoming signal will be reflected back and not passed through the area of impedance mismatch. When a signal is reflected from an area of impedance mismatch in a dual polarized antenna system that is designed for both transmitting and receiving electromagnetic signals, the reflected signal can result in a leakage signal that accesses the output port (port 2) where received signals are present. The presence of this leakage signal at the output port causes a significant degradation in the overall isolation characteristic and performance of the dual polarized antenna system. Impedance mismatch can cause these leakage signals to occur, and degrade the port-to-port isolation, if (1) a cross-coupling mechanism is present within the distribution network or radiating elements (2) reflecting features are present beyond the radiating elements. Proper impedance matching can result in an increased isolation characteristic for a dual polarized antenna, but impedance matching still falls short of achieving the necessary degree of isolation that is now being required in the wireless communications industry.
Another technique for designing an antenna having an increased isolation characteristic is spacing the individual radiating elements sufficiently apart in an antenna array. However, the area and dimensional constraints placed on the antenna designs of today generally renders the physical separation technique impractical in all but a few instances for wireless communications applications.
Other techniques for improving the isolation characteristic of an antenna, particularly a dual polarized antenna, are to place a physical wall between each of the radiating elements or to use coaxial cable (i.e., shielded cable) to feed signals to and from the antenna system. Alternatively, the ground plane of the dual polarized antenna system can be modified so that the input and output ports (ports 1 and 2 respectively) do not share the same ground plane. That is, the ground plane associated with each of the input and output ports is separated by either a physical space or a non-conductive obstruction which serves to alleviate possible leakage of an input signal by coupling via the ground plane to the output port. However, none of these techniques lead to a significant improvement in the isolation characteristics typically exhibited in the antenna designs of today, and particularly dual polarized antenna designs.
Notwithstanding the above discussed techniques, none are capable of providing the high degree of isolation that is specified in certain wireless communications applications that require high reception sensitivities in dual polarized antennas. Consequently, there is a need for a technique that facilitates the design of a dual polarized antenna system having a high degree of isolation between the respective input and output ports. This high degree of isolation is particularly required for antennas used in the wireless communications industry, such as Personal Communications Services (PCS) and Cellular Mobile Radiotelephone (CMR) service.