Active phased array antenna systems are used in a wide variety of applications. As one example, a satellite may include an antenna system of this type in order to facilitate communication between the satellite and one or more ground stations on earth. In a phased array antenna system, especially for a satellite, it is desirable that the antenna system be relatively small in volume and relatively light in weight. On the other hand, antenna systems of this type typically use circuits such as monolithic microwave integrated circuit (MMICs). Circuits such as MMICs generate a substantial amount of heat during operation. As the frequency of antenna operation increases, there is an increase in the amount of heat which is emitted by these circuits, which in turn can affect temperature gradients across the array.
In particular, in a phased array antenna system, the existence of temperature gradients across the array can produce phase errors, which affect the accuracy of the antenna system. The higher the frequency of antenna operation, the smaller the permissible temperature gradients across the array. For example, where the phased array is operating at a frequency of about 5 GHz, the maximum allowable temperature gradient across the array is about 20.degree. C. In contrast, when the array is operating at a frequency of about 80 GHz, the maximum allowable temperature gradient across the array is only about 1.3.degree. C. If the maximum temperature gradient across the array cannot be kept below the maximum allowable gradient, then it is necessary to provide additional circuitry in the antenna system to effect dynamic phase error control compensation, which increases the complexity, cost and weight of the antenna system. Thus, it is important to have an efficient technique for cooling the circuitry of the antenna system, so that a substantially uniform temperature is maintained across the array.
One traditional phased array antenna system has a configuration commonly known as an array slat arrangement, and uses forced flow of a liquid coolant. However, the thickness, volume and weight of this arrangement are greater than desirable, and the forced flow of the liquid coolant requires hardware for handling the coolant, which increases the effective volume and weight of the overall antenna system.
A different approach, which is more recent, is commonly known as a tile array, and uses a multi-layer circuit board. The circuit board has the antenna elements and the circuit components of the antenna system mounted thereon, and cooperates with a relatively thin cooling arrangement. This has the advantage of being ultra thin and low in weight, and also provides shorter conductors for radio frequency signals than the traditional array slat approach. However, while this known approach has been generally adequate for its intended purpose, it has not been satisfactory in all respects.
More specifically, the ultra thin configuration makes it difficult or impossible to use radio frequency circulators and/or isolators, as a result of which a given antenna system is typically configured to either send or receive signals, but not both. Further, only a limited amount of circuitry can be provided directly on the multi-layer circuit board within the size limits of the antenna element array, even where some of the circuit components are mounted in a stacked or "piggy-back" arrangement. As a result, it is difficult to provide multi-beam capability in an antenna system. A further consideration in such ultra thin antenna configurations is that it is typically difficult to separately optimize the cooling system and the packaging of the radio frequency circuitry, because the compactness of the system causes various design aspects to become interdependent. A further consideration in these ultra thin antenna systems is that, since various circuit components are provided directly on the multi-layer board, problems can occur as a result of different coefficients of thermal expansion.