ARRAY ANTENNA FOR RECEIVING COMMUNICATION SIGNAL

Abstract: An array antenna for receiving microwave-band signals. The array antenna comprises radiating elements (1), power feed circuits A (2) in subarrays, LNAs (3) (low noise amplifiers), a frequency converter (4), and a power feed circuit B (5) for combining the outputs of the subarrays. In the case of this configure, in the equation expressing the noise temperature of the whole of the array antenna, which is the noise temperature observed from the side of the elements (1), the gain of the LNAs (3) is the denominator of the term representing the loss of the power feed circuit (B) (5) for combining the outputs of the subarrays. Therefore, when the gain of the LNAs (3) is large enough, the effects given to the noise temperature by the losses of the power feed circuits (A, B (2, 5) for combining the outputs of the subarrays, can be ignored. Also, when each subarray comprises the plural elements (1) and the power feed circuit A (2), the higher the number of the subarrays is, the less the losses of the power feed circuits A (2) can be. As a result, the G/T is improved. Also, since an optimum number of the subarrays can be selected, the improvement of the G/T can be realized with an irreducible minimum increase in cost.

Description: Technical Field The present invention relates to an array antenna for receiving a signal in a microwave band. Background Art Fig. 2 shows a conventional array antenna for receiving radio communication. In Fig. 2, the reference numeral 1 represents a radiating element, 2 a feeder for combining the radio waves which are received by the radiating elements 1, 3 a low noise amplifier (LNA) and 4 a frequency converter. The operation of the conventional array antenna will now be explained. The radio waves received by a plurality of radiating elements 1 are combined by the feeder 2 and amplified by the LNA 3. Thereafter, the frequency of the combined radio wave signal is converted into a signal having predetermined frequency by the frequency converter 4, and the signal is supplied to a receiver (not shown). The gain over temperature G/T, which is the most important index in a receiving antenna, is a value obtained by dividing the gain G of the antenna by the noise temperature T of the whole array antenna for receiving radio communication. In the structure shown in Fig. 2, the noise temperature T of the whole array antenna for receiving radio communication observed at the radiating elements 1 is obtained from the following formula (1) : T = T a + (L - 1)T 0 + LT e + LT c /G 1 wherein T a : the noise temperature of the antenna L : the loss of the feeder 2 T 0 : environmental temperature T e : the equivalent input noise temperature of the LNA 3 T c : the equivalent input noise temperature of the frequency converter 4 G 1 : the gain of the LNA 3. In the conventional array antenna for receiving radio communication having the above-described structure, it is possible to enhance the G/T by increasing the antenna directive gain G. In order to increase the antenna directive gain G, however, it is necessary to increase the number of radiating elements 1, which leads to the increase in the length of the wiring in the feeder 2 and, hence, to the increase in the loss L of the feeder 2. Since the increase in the loss L of the feeder 2 gradually deteriorates the G/T, there is a limit to the realizable G/T. In addition, BBC RESEARCH DEPARTMENT REPORT, n° 6, July 1988, TADWORTH GB pages 1-14, MADDOCKS 'A FLAT-PLATE ANTENNA FOR DBS RECEPTION' discloses an array antenna for receiving radio communication as defined in the preambule of claim 1. US-A-3,803,625 discloses also prior art. Disclosure of the Invention Accordingly, it is an object of the present invention to eliminate the above-described problems in the prior art and to provide an array antenna for receiving radio communication which has an enhanced G/T and which can be realized with the minimum necessary increase in cost. To achieve this aim, in an array antenna for receiving communication according to the present invention, the antenna is divided into the minimum necessary number of sub arrays and an LNA is added to each sub array. By adding the LNA to each sub array, it is possible to reduce the influence of the loss of the feeder for combining the outputs of the sub arrays, thereby enhancing the G/T. Since it is possible to realize the desired G/T by the minimum necessary number of LNAs, the array antenna is realized at a low cost. Brief Description of the Drawings Fig. 1 is a block diagram of the structure of an embodiment of an array antenna for receiving radio communication according to the present invention; and Fig. 2 is a block diagram of the structure of a conventional array antenna for receiving radio communication. In Fig. 1, the reference numeral 1 represents a radiating element, 2 a feeder A in a sub array, 3 an LNA, 4 a frequency converter, 5 a feeder B provided outside of the sub array, and 6 the sub array. Best Mode for Carrying Out the Invention An embodiment of the present invention will be explained hereinunder with reference to Fig. 1. First Embodiment In Fig. 1, the reference numeral 1 represents a radiating element, 2 a feeder A in a sub array, 3 an LNA, 4 a frequency converter, and 5 a feeder B for combining the outputs of the sub arrays. This embodiment is characterized by the fact that a single array antenna is composed by N (n: plural number) sub arrays (sub array 1 to sub array N) and that the sub array is composed of a predetermined number of radiating elements 1, and the feeder A 2 and the LNA 3, which are connected in cascade to the radiating elements 1. In the antenna having the structure shown in Fig. 1, the noise temperature T of the whole antenna observed at the radiating elements 1 is obtained from the following formula (2): T = T a + (L a - 1)T 0 + L a T e + L a ((L b - 1)T 0 + L b T c )/G 1 wherein T a : the noise temperature of the antenna L a : the loss of the feeder A 2 T 0 : environmental temperature T e : the equivalent input noise temperature of the LNA 3 L b : the loss of the feeder B 5 T c : the equivalent input noise temperature of the frequency converter 4 G 1 : the gain of the LNA 3 In other words, the noise temperature T is represented by the formula which is obtained by substituting L a into L and (L b - 1)T 0 + L b T c into T c , respectively, in the formula (1). The loss L a of the feeder A 2 and the loss L b of the feeder B 5 are represented by the following formulas (3): L a = kd(n/N 1/2 - 1) L b = kdn(1 - 1/N 1/2 ) wherein k : the loss of the feeder A 2 or the feeder B 5 per unit length d: interval between the elements 1 n: the square root of the number of elements 1 Consequently, if the gain G 1 of the LNA 3 is sufficiently large, it is possible to approximately disregard the loss L b of the feeder B 5, so that the influence of the loss L b of the feeder B 5 on the G/T is reduced. With the increase in the number N of sub arrays, the length of the wiring in the feeder A 2 can be shortened, so that the loss L a of the feeder A 2 is reduced and, hence, the G/T is enhanced. However, since the increase in the number of LNAs 3 leads to an increase in the cost, it is desirable that N is limited to the minimum necessary. It is therefore favorable to set the desired value C of G/T and design the antenna so that N is the minimum while the G/T satisfies the following relationship (4): G/T ≥ C As described above, according to the present invention, since the antenna is divided into sub arrays and an LNA is added to each sub array, the loss of the feeder at the subsequent stage of the LNA is unlikely to increase the G/T. In addition, with the increase in the number of sub arrays, the loss of the feeder within the sub array is reduced, so that the G/T is enhanced. By appropriately selecting the number of LNAs, it is possible to reduce the number of LNAs while obtaining the necessary G/T, which leads to a reduction in cost.

Claims: An array antenna for receiving radio communication comprising : a plurality of sub arrays (6) ; and a first feeder (5) for combining the outputs of said sub arrays (6) and outputting the combined output ; each of said sub arrays including a plurality of radiating elements (1) ; a second feeder (2) for combining the outputs of said plurality of radiating elements (1) ; and a low noise amplifier (3) for amplifying the output of said second feeder (2) and outputting the result to said first feeder (5), characterised in that the number of said sub arrays (6) is set so that the G/T, wherein G denotes the antenna directive gain and T denotes the noise temperature of the whole array antenna, is not less than a desired value C and the number of said sub arrays (6) is the minimum. An array antenna for receiving radio communication according to claim 1, further comprising a frequency converter (4) for converting the frequency of the output of said first feeder (5).

Inventors: CHATANI YOSHIYUKI, HIGA MORIO, ORIME NOBUTAKE, CHATANI, YOSHIYUKI, HIGA, MORIO, ORIME, NOBUTAKE, CHATANI, YOSHIYUKI MITSUBISHI DENKI, HIGA, MORIO MITSUBISHI DENKI, ORIME, NOBUTAKE MITSUBISHI DENKI
Applicants: MITSUBISHI ELECTRIC CORP, MITSUBISHI DENKI KABUSHIKI KAISHA
Application year: 1992
Main classification: H01Q21
IPC codes: H01Q21, H01Q23