| 摘要 |
<p>There is increasing need for modern radar systems to provide target recognition and classification and effective operation in a hostile environment. These requirements demand wide frequency bandwidth operation which must be addressed by special techniques in the antenna. So-called frequency independent antennas have been around since the late 1950's and various well-known forms of very wide bandwidth antennas have been successfully employed in both civil and military equipment. However, these are all low to medium gain devices, up to about 18dB with respect to isotropic radiation. When attempts are made to use these as elements in large arrays to provide high gain, low side-lobe levels, beam steering, adaptive beamforcing, etc, over frequency bandwidths of an octave or more, the array performance specification cannot be met. This is because the elements in the array cannot be ideally positioned with respect to each other at all frequencies with the result that grating lobes, which are replicas of the main lobe of the radiation pattern, are produced at the higher frequencies. Target detection ambiguities result. The invention is a skewed Log Periodic Dipole Array (LPDA) which, when used as the element in a linear or planar array, serves to remove the element spacing restriction and hence eliminates the grating lobe problem. LPDA's are well-known in the form of a periodically spaced linear array of dipoles, used for example in wide-band communications links and domestic TV reception. They can be designed to operate efficiently over several octaves or even a decade in frequency. The skewed form of the device has all dipoles greater in length than the shortest one, i.e. the highest frequency one, skewed such that they are "Z" shaped. Of particular interest is the case when the angle between the end and centre segments of the Z dipole is 90 degrees. The skewed LPDA now has a constant width equal to the length of the highest frequency dipole. Mathematical modelling has established that planar and linear arrays of skewed LPDA's can provide grating lobe free performance over at least an octave. The upper frequency limit of operation is governed largely by the accuracy with which the feed point can be constructed. <IMAGE></p> |
