Precise antenna pattern control continues to be a major consideration in communication system design efforts, particularly where the application may require narrow beam focussing or operation in the presence of jamming radiation. For large reflector-type antenna systems, (e.g. Cassegrain antenna systems) the configuration and relative displacement of the antenna components gives rise to a significant sidelobe problem. A conventional proposal to solve this problem has involved the placement of a plurality of low gain auxiliary feed elements around the periphery of the main reflector, with the auxiliary feed elements being coupled through an analog multitap time delay weighting and combining network to be coupled with the signal path for the main feed element. Locating the auxiliary feed elements around the periphery of the main reflector is for the intended purpose of intercepting a variety of diverse signal paths that impinge upon the main horn which contain noise signal components that contribute to the degradation of the main signal of interest lying with the main lobe. Ideally a sufficient number of these auxiliary elements can be readily installed in the edge of the main reflector, so that adequate coverage can be achieved. In addition, the low gain auxiliary elements, individually, do not receive enough of the desired signal to significantly affect tracking circuits or to cause unacceptable dispersion.
Unfortunately, the use of such an auxiliary cancellation array entails two serious drawbacks. The first is a severe differential dispersion effect which itself must be compensated, typically through the use of a multitap weighting and combining network. The second is a substantial grating effect which will not satisfy most large aperture requirements for present day antennas, since it is not possible to null more than a single jammer at any time for certain angles, and at other angles simultaneous jammer nulling cannot be accomplished without excessive degradation of the system signal-to-thermal noise ratio.
For the purpose of signal processing, the RF signals from each of the auxiliary feed elements are fed to a multitap IF weighting and combining network which, by definition, employs attendant down conversion circuitry. Customarily, the down-conversion components for each element include one or more cascaded stages containing local oscillator, preselection filter, low noise amplifier and mixer. Each of the down-converted and filtered auxiliary element outputs is coupled to its own associated multitapped weighting and combining network, with successive taps being coupled through respective I/Q weighting circuits to plural inputs of a dispersive summation device, the output of which represents the "modified" signal for that particular auxiliary feed path. The weights are adjustable by way of respective analog correlation loops for each tap. All of the dispersively weighted and summed auxiliary feed (sidelobe cancellation) waveforms are then combined to provide a "best estimate" of the jamming waveform which is to be cancelled from the main antenna. This estimate is then up-converted to the original RF frequency and coupled to the main antenna feed by way of a directional coupler.
Now, although the signal processing aspects of the auxiliary array serve to compensate for the dispersion effects, they not only do not eliminate the severe grating effects, but they suffer from a number of unfavorable aspects in and of themselves. Because of the number of components involved for each auxiliary feed, the signal processing network is both complex and difficult to maintain. Also, components such as low noise amplifiers, which are extremely sensitive RF components, add considerable cost to this approach. In addition, the preselection filter for the main feed channel cannot be too narrow because of differential phase variations to which the adaptive circuit is sensitive. As a result full jamming is coupled to the preselection filter which creates the possibility of intermodulation problems.