In some instances, it is desirable to configure a system to receive all of the electromagnetic signals within the receiver's capability dictated by its sensitivity and bandwidth limitations. Usually the signals of Interest are incident from widely diverse directions. Thus, in one principal method, an antenna having a wide azimuth beamwidth, such as an omnidirectional antenna, is chosen as the system's receptor element.
The low-directive gain of such antennas is a severe limitation on the system sensitivity. In addition, the wide beamwidth does not permit directional resolution of multiple signals. Such resolution is usually desired to prevent garbling of signals that cannot otherwise be resolved in frequency or time-of-occurrence. Directional resolution is also useful in cases where the direction of incidence of the signals is to be estimated.
To overcome these disadvantages, alternative systems have been configured using high-gain narrow-beam antennas. In one such system, a narrow-beam is scanned over the azimuth coverage sector (either by mechanical motion of the antenna or by use of electronically controllable phase shifters). The disadvantage of this system is that the beam cannot look everywhere at once. This is especially a problem for multiple signals from diverse directions if they have rapidly changing waveforms (high information rate or short-pulse signals). These signals may hoe be sampled at a sufficiently high rate by the scanning beam to prevent information loss.
In another system, the antenna is configured to form multiple, narrow beams, that are contiguous and fixed in direction. Each beam port of the antenna is connected to a separate receiver. If the number of beams is sufficient to cover the entire azimuth sector of interest, then the system can exhibit the advantages of high directive gain (high sensitivity), good directional resolution (direction estimate and suppression of garble), and complete, simultaneous directional coverage (no information loss). However, the disadvantage of this system is the high cost of the multiple receivers.
Still another group of alternative systems have been configured to overcome these disadvantages by scanning a narrow beam at scanning rates so high that the beam will intercept each signal at least twice as often as its information rate. Thus, in the case of pulsed signals, the beam is caused to scan through its complete coverage sector within the time period of the shortest pulse expected. Such rapid scan is obtained by heterodyne techniques. This results in bandwidth spreading of the received signals, which, in the case of a pulsed signal, yields a predictably compressed pulse whose time of occurrence is directly related to the emitter azimuth location.
Early embodiments of array antennas using this rapid-scan heterodyne technique suffered a sampling loss which degraded sensitivity. This loss of signal (relative to the maximum signal energy available) is caused by the fact that the signal received by each radiator is summed coherently for only the portion of the scanning period during which the beam is pointing nearly in the direction of signal incidence. During the rest of the scan period, the signal received by each radiator is coherently summed at the resistor-terminated ports of the combiner (if it is a resistively isolated type of combiner) or is reflected back from the combiner to the radiators (in the case of a non-isolated type of combiner). The sampling loss in dB for an N element array is given by 10 log N; this is approximately the same numerical value as the gain of the array (relative to the gain of a single element). Although this antenna system can resolve emitters in azimuth as well as a conventional N element array, it provides only the sensitivity that can be attained with only a single element of that array.
A recent embodiment of an array antenna using this rapid-scan heterodyne technique (patent application Ser. No. 519,161, filed August 1983 by R. M. Rudish) overcame the sampling loss deficiency of the earlier embodiments by providing a multibeam capability. The earlier embodiments scan a single beam, whereas the recent embodiment uses a multiple beam-forming device such as a Butler matrix to produce and scan a comb of multiple contiguous beams in unison. At any instant, the beams are spread over the entire coverage sector of the antenna. At all times, at least one of the beams will be directed at any emitter within the antenna's coverage. The beams are displaced from each other in direction by approximately their beamwidths. They are also displaced from each other in time; that is, the time at which a particular beam coincides with a given direction differs from that at which each of the other beams coincide with that same direction. The time differences are known, fixed amounts related to the scan rate. Thus, by using a tapped delay line summing network, the recent embodiment can impart just the right differential delays to the individual beams to make their outputs from any one emitter occur in unison so that they can be summed coherently. In this manner, the recent embodiment recovers the signal loss which occurred in the arrays of earlier systems due to sampling the signal during only a portion of its time of presence. Thus, full array gain can be realized for an increase in system sensitivity. Yet, because there is only a single, summed output, far fewer components are needed to complete the processing of that output than is the case for other types of multiple-beam-antenna/multiple-receiver systems.
The recent embodiment of this rapid-scan heterodyne technique will be described in greater detail for the case of an N-element array since it forms the foundation for the improvements of the present invention.