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SSPAs Used to Generate High-Powered Signals as Part of Active Array Antenna.

Satellite communications systems that transmit signals directly to homes, called direct broadcast satellites (DBS), are currently in operation with more in development. This "first-generation" DBS uses conventional communication system architecture, the same architecture used by most fixed-service satellite communication systems. The output portion of this architecture consists of a high-power travelling-wave tube amplifier (TWTA) for each channel, and an output multiplexer to combined channels of different frequencies into a single antenna port. The antenna is a multiple feed-horn reflector antenna that produces a transmit beam shaped to closely match the intended geographic coverage area as seen from geostationary altitude.

For the First-generation DBS, the high-power TWTA produces up to 250-watts, the antenna produces beams that cover United States time zones and operates in the Ku-band (approximately 12 GHz).

Recently, at the lower frequencies--C-Band--The high-power TWTA has been replaced with a solid-state power amplifier (SSPA) on a direct replacement basis. However, it will be sometime before a 250-watt Ku-band SSPA is feasible since the individual Ku-band devices are currently only at the 1 to 3-watt power level.

In order to take advantage of solid-state technology at an earlier date, an alternate communication system architecture has been developed. This architecture uses a large number of relatively low-power solid-state amplifiers each feeding a small subarray of patch-radiating elements. By establishing the appropriate amplitude and phase of the signal at each subarray, this active array antenna (A.sup.3.) can produce the transmit patterns desired. This architecture also lends itself to an implementation as a relatively fat, integrated assembly using miniature microwave-circuit techniques.

The A.supp.3. features both reliability and performance advantages when compared to the conventional implementation. The A.supp.3. uses solid-state devices that experience no knwn wearout mechanisms, such as cathode depletion, that ultimately limit TWTA lifetime. The solid-state amplifiers use low-voltage power that eliminate problems associated with high-voltage breakdown.

There are lower RF losses between the output of the final active amplifier and the antenna-radiating element due to the small separation between the amplifier and the radiator and the absence of the traditional output multiplexer. Each amplifier module will contain a phase shifter that can be adjusted in orbit to maintain the proper phase relationships between elements. This also provides a capability to change the antenna radiation pattern in order to maximize the systems' revenue-generating ability. Such flexibility to produce radiation patterns not even conceived of before launch is a great benefit.

A method that has been developed to determine the phase and amplitude of the excitation needed at each antenna element to produce a desired radiation pattern. Solutions can be found that permit only the phase of the excitation to vary between elements (uniformly illuninated) or that permit both the phase and amplitude of the excitation to vary from element-to-element.

The first step in using the excitation synthesis program (ESP), is to define the desired coverage area. The description of a coverage area (for example, the Eastern Time Zone) starts with a list of locations (latitude and longitude) outlining the area on the earth's surface. These are converted to angular deviations from the antenna axis, computed for a specific geostationary satellite longitude. The antenna axis can be pointed at any point on earth, but most frequently is pointed near the center of the coverage area. The converted outline is expanded slightly to allow for a satellite pointing error. This boundary is transformed into a set of constraint points that are approximately equally spaced along the boundary and within it. The density of the points must be adequate to assure that no undesirable performance occurs between them.

When a synthesis problem starts, the drive function is unknown. The two parts of the drive function, phase and amplitude, must be described so that the parameters are the unknowns.

The amplitude of the excitations on the antenna surface is described by a two-dimensional finite power series. Real coefficients of this finite series are the unknown parameters in amplitude. Initial length of the series can be specified as can initial values of the coefficients. For amplitude drive function that vary over the surface, all terms are modified to satisfy the constraint that the total RF power into the A.supp.3. remains constant while ratios of the amplitude coefficients vary. To examine phase-variable, constant amplitude solutions the amplitude series is one term, set to unity.

When an optimum is obtained, the coefficients are used to compute the gain at each point of a rectangular grid, several degrees on each side, generating a gain-array table. A graphics processing program converts this array into antenna pattern contour plots upon which an overlay can be shown. The set of constraint points is one of several overlays available.

Because the program allows for discrete elements at about one inch spacing on an atenna size greater than 60 by 120 wavelengths, computation of gain at each point can require the addition of more than 7200 complex quantities, prior to the conversion of dB. A lower precision method uses gaussian quadrature integration, and involves summing only about 200 complex quantities per constraint point. (Sixty constraint points are used for the Eastern Time Zone.) Similar savings exists for the other data must be computed while optimizing.

Future designs of direct broadcast satellites will have their SSPA's of high efficiency, each set into, and driving part of, an active array antenna can provide the needed RF power. A synthesis method has been developed to determine the drive parameters, phase and amplitude, for each element of the active array antenna that results in a radiation pattern with specified coverage characteristics.

The realization of a practical active array at 12 GHz requires the inexpensive batch fabrication of large numbers of efficient multi-stage SSPA's. Amplifiers of this type fabricated using MBC technology were combined with a circularly-polarized antenna array in a lightweight active subarray structure. Full arrays of this type are expected to find use in the DBS as well as other communications and radar systems applications that require mobility, efficiency, and light weight.
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Copyright 1985 Gale, Cengage Learning. All rights reserved.

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Author:Balcewicz, J.; Coloday, S.; Johnson, H.
Publication:Communications News
Article Type:evaluation
Date:Jun 1, 1985
Words:1001
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