A multidisciplinary team led by AFRL scientists is developing a geodesic dome phased-array antenna (GDPAA) for a proposed future Air Force (AF) technology demonstration.1 AFRL is also developing a second-generation S-band electronic scanning array (ESA) proof-of-concept (POC) panel to support the demonstration efforts.
In August 2004, AFRL engineers, the Air Force Space Battlelab, and industry partners successfully demonstrated a prototype six-panel multi-beam ESA (see Figure 1). Two previous AFRL Technology Horizons® articles, "Space Ground Link Subsystem"2 and "Geodesic Dome Phased-Array Antenna,"3 described (1) an AFRL-developed, low-cost transmit/receive (T/R) module; (2) the test results of a single-panel ESA; (3) a conceptual GDPAA; and (4) the successful demonstration of a six-panel S-band ESA. This follow-up article focuses on the design and manufacture of the low-cost T/R modules used in the prototype six-panel array. Low-cost component design and implementation issues are critical factors in developing a practical phased-array antenna. The AFRL-designed T/R module consists of four channels comprising two receive (Rx) channels and two transmit (Tx) channels. Each module contains high-selectivity ceramic bandpass filters, polarization switches, four 4-bit digital phase shifters, four 5-bit digital attenuators, dual low-noise amplifiers (LNA), and dual power amplifiers capable of transmitting 1 W minimum output per channel. The radio frequency (RF) portion of the module utilizes a ground coplanar waveguide structure. The team designed the T/R module with high-efficiency hetero-junction bipolar transistor (HBT) power amplifiers and also for component replacement under "power on" conditions. To meet cost-related objectives, researchers designed the RF connectors for insertion into the beam forming structure using a novel format to make the power, digital logic, and RF output connections available on the same side of the module. A single field-programmable gate array controls the T/R module through a general-purpose computer interface bus.4
Figure 2 shows the block diagram of the four-channel T/R module. Dotted lines distinguish the transmitter paths (Tx1 and Tx2) from the receiver paths (Rx1 and Rx2). The transmit operating frequency is 1.75- 2.1 GHz, and the receiver operating frequency is 2.2-2.3 GHz. The transmit path consists of input (at Tx1 and Tx2) and output from one of the two antenna ports (A1 or A2). As illustrated, the transmitter signal passes through a 4-bit phase shifter (φ shift of 22.5°, 45°, 90°, and 180°); a single-pole double-throw (SPDT) switch to open and close the RF path; a 5-bit attenuator (with progressive attenuation levels of 1, 2, 4, 8, and 16 dB); a preamplifier; and another, absorptive-type SPDT switch before reaching the embedded power combiner. The absorptive SPDT switch induces left- or right-hand circular polarization in the signal. A 90° hybrid (polarizer) provides quadrature phase to the input signal, and monolithic microwave integrated circuit (MMIC) amplifiers boost the polarizer's quadrature output to a power level exceeding 30 dBm before its transmission through a high-rejection, low-pass ceramic filter. The transmit channels' overall gain is 20 dB.
For the two receive paths (the downlink), the input signal feeds to a high-rejection bandpass ceramic filter (diplexer) via the A1 and A2 ports. The input signal passes through a series of amplifiers, phase shifters, attenuators, and SPDT switches before reaching the receiver ports (Rx1 and Rx2). The total gain across the receiver band is 30 dB.
Two diplexers are required for maintaining optimum performance. The transmit side of the diplexer filter, inserted after the transmit amplifier, prevents wide-band noise from entering the receiver and degrading performance. The diplexer's receive section prevents the coupled transmit signal from degrading the linearity of the receive LNA. This ceramic filter consists of four resonators in a coaxial structure and produces a 1 dB insertion loss in the receive band. A six-section (six resonator cavities) transmit filter has a low-pass structure with 0.5 dB of loss and a rejection of 65 dB at the crossover frequency of 2.15 GHz. The T/R module, with associated control circuitry, is 6.75 in. long and 3 in. wide; its thickness, including housing, is 0.75 in. (see Figure 3).
To minimize costs, the AFRL/ industry team evaluated several commercial off-the-shelf (COTS) main components for the transmit and receive channels. For a receive frequency range of 2.2-2.3 GHz, the team evaluated several silicon germanium LNA devices and finally selected one based on its low cost and ease of use. They also selected COTS gallium arsenide HBT MMICs for the transmit channels' power amplifier to ensure power output capability of 32 dBm. The amplifier provides an overall gain of 20 dB, with >40% added power efficiency across the 1.7-2.1 GHz band.
Researchers based the phase shifter design on a single MMIC chip with two back-to-back SPDT switches in a surface-mounted package.4 By consolidating two separate SPDT switches, the device design reduces the component count from 8 devices per phase shifter (32 per T/R module) to 4 devices per phase shifter (16 per T/R module). The team measured the insertion loss of the phase shifter at 4.5 dB, observing a total change in insertion loss of 0.4 dB in all phase states.
To optimize the T/R module board design, researchers placed the beam former's input/output to the antenna's input/output on the same side of the module, enabling component replacement under "power on" conditions. They fabricated the control board on a multilayered FR-4 substrate, placing the T/R filters on the backside of the module to ensure isolation and using grounded coplanar technology to reduce coupling and grounding effects. Using a 4-bit phase shifter, a 5-bit attenuator, and polarization switching, the team attained transmit channel power output of 30 dBm per channel and an overall 30 dB gain. Researchers used a 4-bit phase shifter and 5-bit attenuators for the receive channel as well, demonstrating a noise figure of 2 dB and an overall gain exceeding 20 dB.
Following the August 2004 demonstration, AFRL researchers implemented several design changes to improve the performance and control of the T/R modules.5 The first of the new-generation T/R modules have been fabricated, and researchers completed testing in February 2006. Once analyzed, the test results—along with a report on the updated module design—will appear in a future issue of AFRL Technology Horizons. If the AFRL-developed S-band ESA POC panel meets the research team's requirements, engineers plan to fabricate a 54-panel array to demonstrate the possibility of using a single GDPAA to replace multiple parabolic reflector antennas for telemetry, tracking, and command at AF satellite tracking sites around the globe.
Mr. Paul J. Oleski and Lt Robert Patton, of the Air Force Research Laboratory's Information Directorate, Dr. Boris Tomasic and Mr. John Turtle, of the Air Force Research Laboratory's Sensors Directorate, Mr. Sarjit S. Bharj, of Princeton Microwave Technology, Inc., and Dr. Shiang Liu, of The Aerospace Corporation, wrote this article. For more information, contact TECH CONNECT at (800) 203-6451, or place a request at http://www.afrl.af.mil/techconn_index.asp . Reference document IF-H-05-22.
- Tomasic, B., et al. "The Geodesic Sphere Phased-Array Antenna for Satellite Communication and Air/Space Surveillance — Part 1" AFRL in-house technical report AFRLSN- HS-TR-2004-031, Jan 04.
- Oleski, P. "Space Ground Link Subsystem." AFRL Technology Horizons, vol 5, no 2 (Apr 04): 20-12. http://www.afrlhorizons.com/Briefs/ Apr04/IF0311.htm.
- Oleski, P., et al. "Geodesic Dome Phased-Array Antenna." AFRL Technology Horizons, vol 6, no 4 (Aug 05): 16-18. http://www.afrlhorizons.com/Briefs/Aug05/ IFH0504.htm.
- Bharj, S., et al. "Affordable Antenna Array for Multiple Satellite Links." Antenna Application Symposium, 2000.
- Bharj, S., et al. "Daisy-Chain-Controlled Multibeam T/R Module for AFSCN." Antenna Application Symposium, 2004.