A proposed Ka-band transmitting/ receiving system would embody a unique combination of established and semi-proven design features. Although this system is intended primarily for telecommunication use aboard a spacecraft, its design could be adapted to terrestrial military and commercial radar systems. Systems like this one could be especially suitable as replacements for prior systems in which traveling-wave-tube amplifiers (TWTAs) are used in the final transmitter stages.
The proposed system (see figure) would include a monopulse receiving feedback loop and a mirror that could be moved by piezoelectric actuators in the feedback loop to adjust the aim of the transmitted and received radio beams. Unlike in a phased-array tracking system, phase shifters (which can be complex and expensive) would not be needed in this monopulse tracking system. Moreover, the monopulse-tracking loop could be combined with other subsystems used in established subreflector and antenna designs.
Instead of a TWTA, the final transmitter power amplifier in the proposed system would be a quasi-optical power amplifier (QOPA) — a combination of a planar array of 25 amplifiers and corresponding planar arrays of antenna elements, such that freespace power combining would take place at the output. The goal of this QOPA would be to operate at a power of 20 W and produce a minimum gain of 13 dB in the frequency range from 31.8 to 32.3 GHz.
The amplifiers would be identical to commercially available GaAs monolithic microwave integrated circuits (MMICs). Accompanying the QOPA, on the same circuit board, there would be two arrays of antenna elements: a drive array (a planar array of identical input antenna elements) and a transmitting array (a planar array of identical output antenna elements). The drive array would be fed via a hard horn, providing uniform illumination to each array element. By use of microstrip transmission lines, all of equal length, the input and output terminals of the MMIC amplifiers would be connected to the corresponding drive and transmitting antenna elements, respectively. This QOPA design would offer the following advantages (among others):
- The separation of the input and output drive arrays helps eliminate the problem, encountered in prior QOPA systems, of oscillation and allows the use of high-gain amplifiers.
- Unlike a TWTA, the MMIC amplifiers would not necessitate a high-voltage power supply.
- The array of MMIC amplifiers could be actively cooled from its back side; unlike in prior QOPA arrays, it would not be necessary to rely on edge cooling, which is less effective and thus limits the achievable power to a lower level. This is important for future inclusion of wide band-gap devices such as GaN.
- The failure of a single amplifier would not be catastrophic: as long as the other amplifiers continued to operate, the loss in performance would be relatively small. For maximum efficiency, the independent bias lines allow individual modules to be turned off as output power demands change.
The system would include a frequency- selective surface (essentially, a radio frequency dichroic reflector) intended to reflect the transmitted beam while passing the received monopulse beam. The FSS would provide between 40 and 60 dB of isolation between the transmitted and received beams.
This work was done by Abdur Khan, Dan Hoppe, Larry Epp, and Raul Perez of Caltech for NASA’s Jet Propulsion Laboratory. For further information, access the Technical Support Package (TSP) free on-line at www.techbriefs.com/tsp under the Electronics/ Computers category. NPO-30559
This Brief includes a Technical Support Package (TSP).
Advanced Ka-Band Transceiver With Monopulse Tracking
(reference NPO-30559) is currently available for download from the TSP library.
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Overview
The document is a proposal abstract for the development of a High Power Ka-Band Transmit/Receive Module System with Monopulse Tracking, submitted to NASA by Principal Investigator Larry W. Epp on February 6, 2002. The primary objective is to create a robust communication system for spacecraft that enhances Ka-band telecommunications capabilities, enabling higher data rates essential for future NASA missions.
The proposed system addresses significant challenges in high-power Ka-band communications, particularly in pointing accuracy compared to X-band systems. It incorporates a monopulse feedback loop and an actuated mirror for precise tracking, eliminating the need for phase shifters, which simplifies integration with existing antenna designs. This innovative architecture aims to replace Traveling Wave Tube Amplifiers (TWTAs) for spacecraft applications, particularly for power levels below 80 W, thereby reducing costs and complexity.
The initial goals include developing a >20 W amplifier operating in the frequency range of 31.8 – 32.3 GHz, with a minimum gain of 13 dB and efficiency of less than 26%. The design anticipates a total mass of approximately 300 grams and a volume of around 125 cc. The system employs off-the-shelf solid-state High Power Amplifier (HPA) MMICs, with plans to transition to higher efficiency Gallium Nitride (GaN) devices in the future.
The system's architecture utilizes power combining through a spatial or hard horn feed, which allows for quasi-optical combining without alignment issues. Each array can combine up to 25 elements with independent bias control, enabling dynamic switching of amplifiers to optimize efficiency based on spacecraft power needs. The design also includes a Ka-Ka Frequency Selective Surface (FSS) for isolation between transmit and receive functions, and a piezoelectric steered mirror for tracking.
Overall, the proposed Ka-band system is designed to minimize technical risks while enhancing adaptability for various missions. It aims to support scientific endeavors that require high-power Ka-band communication without the prohibitive costs associated with traditional TWTAs, thus enabling new opportunities for lower-cost spacecraft missions. The document emphasizes the importance of developing efficient and economical power amplifiers to meet the growing demands of space communications.
