Patent Application
20210367329
The present invention relates to a method for controlling an antenna array, and an antenna array so controlled, to produce directional and omnidirectional antenna patterns as desired in both a receiving and transmitting operational mode.
An aircraft-mounted directional surveillance antenna provides a preferable (directional) radiation pattern for communications with other aircraft. The directional surveillance antenna on a host aircraft transmits interrogations that are received by a transponder onboard a threat aircraft. Said interrogations elicit replies from the transponder of the threat aircraft. The directionality of the transmitting directional surveillance antenna minimizes the number of threat aircraft receiving (and therefore responding) to individual interrogations from the host aircraft.
To ensure reception by the host aircraft, the threat aircraft responds to received interrogation signals through an omnidirectional antenna. The receiving host aircraft uses the directional pattern of its directional surveillance antenna to determine a direction of arrival of responses from the threat aircraft, thus providing a bearing to the threat aircraft.
The directional surveillance antenna comprises multiple antenna elements to provide the directional pattern. For example, a four-element array may be mounted with an element situated to the forward, aft, left, and right sides of the aircraft in order to provide the directional capability.
As described above, the host aircraft transmits the interrogation signal in a directional pattern and uses that directional antenna pattern to receive the response, from which the bearing to the threat aircraft can be determined. As also described above, the threat aircraft transmits its response according to an omnidirectional pattern. But, any aircraft may, at different times, operate as a host aircraft and as a threat aircraft. Typically, different antenna arrays are employed to generate the directional and the omnidirectional antenna patterns.
The foregoing and other features of the invention will be apparent from the following more particular description of the invention, as illustrated in the accompanying drawings, in which like reference characters refer to the same elements throughout the different figures. The figures are not necessarily to scale, emphasis instead being placed upon illustrating the principles of the invention.
It is desirable to combine the functions of the directional surveillance antenna with the omnidirectional transponder antenna to minimize the number of antennas mounted on an aircraft. Since the transponder replies must be transmitted according to an omnidirectional antenna pattern, it is necessary to provide a means by which a multi-element directional surveillance antenna, which transmits and receives in a directional pattern, can be switched to an omnidirectional mode for transmitting transponder replies to interrogations. Thus, the present invention teaches a technique for generating an omnidirectional radiation pattern from the directional surveillance antenna.
Before describing the methods and systems for generating an omnidirectional antenna pattern and a directional pattern from an antenna array, it should be observed that the specification describes and the drawings illustrate only those details that are pertinent to understanding the present invention without obscuring the disclosure with structural and functional details that will be apparent to those skilled in the art having the benefit of the description herein.
The following embodiments are not intended to define limits as to the structure, function or method of the invention embodiments, but only to provide exemplary constructions. The embodiments are permissive rather than mandatory and illustrative rather than exhaustive.
One prior art embodiment of a two-element directional antenna array 92 (referred to as a beacon antenna system in certain applications) is shown in
Specifically, the phase relationship of the signals at the ports 94 and 95 (also referred to as terminals and functioning as input ports when the antenna array 92 operates in a receiving mode), when processed through the feed network 101 generates signals at the ports 102 and 103 that follow the illustrated cardioid patterns 114 and 115, respectively. For example, a signal arriving at the antenna elements 108 and 109 (and input to respective ports 94 and 95) from a direction 97 produces a maximum signal magnitude at the port 102 in the direction 97. While that same signal generates a minimal signal magnitude at the port 103 in the direction 97.
The phase relationship between the signals at the ports 94 and 95 varies with the angle of arrival of the received signal. The signals at the ports 94 and 95 are operated upon by the feed network 101 to produce signal amplitudes at ports 102 and 103 that vary with the angle of arrival according to cardioid patterns 114 and 115. Analysis of the signals at the ports 102 and 103 (specifically a ratio of the two signals) determines the angle of arrival of the received signal, and from that information the bearing to the threat aircraft is easily determined.
The spacing between the antenna elements 108 and 109 is less than a wavelength at the operating frequency. Therefore, the same signal cycle is received at each antenna element 108 and 109, but the elements receive different phase angles of the signal during a single signal cycle.
The transmit and receive patterns for the antenna array 92 are the same due to the antenna reciprocity theorem.
In the transmit mode the antenna array can generate one of two different directional patterns, either the cardioid pattern 114 or 115, when an input signal is applied to only one of the ports 102 and 103. The radiation pattern will be the cardioid pattern 114 if the input signal is applied to the RF port 102 and no signal is supplied to the RF port 103. The antenna pattern 115 is created if the signal is applied to the RF port 103 and no signal is supplied to the port 102.
The feed network 101 processes the input signal and generates an output signal at each port 94 and 95 (when the antenna array operates in the transmit mode) for driving the antenna elements 108 and 109 to produce either the radiation pattern 114 or the radiation pattern 115, depending on which input port is driven. As in the receive mode, each pattern is created by phase and amplitude relationships of the signals at the ports 94 and 95 as imparted by components within the feed network 101, for driving the antenna elements 108 and 109.
The radiation patterns 114 and 115 are, in effect, caused by a combination of the radiation pattern and coupling of the antenna elements 108 and 109. Whichever one of the RF ports 102 and 103 does not receive an input signal (when in the transmitting mode) remains connected to the system during operation. Thus, this non-driven port influences the net radiation pattern from and the coupling between the antenna elements 108 and 109.
Other patterns may be produced by applying some signal to both ports 102 and port 103, but this detail is not relevant to the present embodiment.
In one application, the antenna elements 108 and 109 are collocated (spaced apart with less that a wavelength at an operating frequency between the elements). Thus, the antenna pattern 114 produces a maximum signal in the forward direction and a null in the rearward direction (in both the transmit and receive operational modes). Conversely, the antenna pattern 115 has a maximum in the rearward direction and a null in the forward direction (in both the transmit and receive operational modes).
The cardioid patterns 114 and 115 are illustrated in greater detail, including polar coordinates and relative magnitude numerical values in dB, in
One preferred embodiment of the current invention is shown in
With the switches 104, 105, and 106 configured as in
When the RF switches 104, 105, and 106 are switched to the positions shown in
In the
In sum, when connected to the detuning network, the antenna element 109 imposes minimal effect on operation of the antenna element 108, such that the element 108 operates essentially as in isolation, with a radiation pattern determined according to the physical and electrical characteristics of the antenna. When operating in isolation the radiation pattern may also be referred to as an independent radiation pattern.
Thus, when the antenna element 108 is a monopole antenna operating in isolation, the antenna element 108 exhibits an omnidirectional radiation pattern. Other antenna element types may be utilized in lieu of a monopole antenna, as required by the requirements of a specific application.
Note that feeding the RF signal to the antenna element 108, with the element 109 open-circuited or short-circuited, as done according to the prior art, generates an antenna pattern at the antenna element 108 that exhibits significant asymmetry due to coupling between the two antenna elements 108 and 109. This coupling is minimizes according to the present invention.
Connecting the unused or inactive antenna element 109 to the de-tuning network 110, as taught by the present invention, and appropriately configuring the detuning network 110, minimizes coupling between the two antenna elements. The result is a significant reduction in azimuth pattern asymmetry for the radiation pattern of the active antenna element 108. By optimizing the de-tuning network (i.e., selecting appropriate inductive and capacitive component values, which is most easily accomplished by simulating operation of the antenna elements 108 and 109) it is possible to reduce the pattern asymmetry to almost zero, thus enabling the antenna 108 to transmit according to an omnidirectional pattern when the element 108 comprises a monopole antenna.
Thus, the present invention adds a third pattern (the omnidirectional pattern in addition to the two directional patterns 114 and 115 of the antennas 108 and 109) to the existing antenna array without requiring additional antenna elements or, in an aircraft surveillance system application, without requiring additional antenna mounting space on the aircraft.
Returning to
The embodiment described above utilizes a two-element array of antenna elements 108 and 109 (which in one embodiment and when operating in isolation are both individually omnidirectional) and controls the signals input to each antenna element to generate a desired pattern from the antenna array. The invention, however, is not limited to two-element antenna arrays.
Multi-element antenna arrays can utilize the same technique of detuning one or more unexcited elements to electrically remove this element(s) from influencing the radiation pattern of the active element(s). In such an embodiment, the antenna pattern of the active element(s) can be modified to reduce pattern sidelobes, improve gain, improve radiation pattern characteristics, and/or achieve a radiation pattern that is nearly identical to the radiation pattern of the excited element(s) when operating in isolation. Thus, the concepts of the present invention can be extended beyond only generating omnidirectional antenna patterns from directional antenna arrays (as described in conjunction with
In addition to achieving the desired radiation pattern for the array 200, the amplitude and phase of one or more of the signals 220, 222, 224, and 226 can be controlled to reduce antenna sidelobes in a desired direction or to increase the antenna gain in a desired direction.
In the
Although described above and illustrated in the context of transmitting operation, the array 200 functions similarly for operation in a receiving mode, with the feed network and transmitter replaced by suitable receiving components.
Additionally, although the antenna elements 202, 204, 206, and 208 are illustrated in
In the alternative configuration of
Certain of the embodiments have been described herein as operating in a receiving mode and other embodiments have been described as operating in a transmitting mode. According to the antenna reciprocity theorem the receive and transmit properties (gain, radiation pattern, etc.) for any antenna are identical and therefore the described embodiments can operate in either mode.
While the invention has been described with reference to preferred embodiments, it will be understood by those skilled in the art that various changes may be made and equivalent elements may be substituted for elements thereof without departing from the scope of the present invention. In addition, modifications may be made to adapt a particular situation more material to the teachings of the invention without departing from the essential scope thereof. Therefore, it is intended that the invention not be limited to the particular embodiment disclosed as the best mode contemplated for carrying out this invention, but that the invention will include all embodiments falling within the scope of the appended claims.
