Multibeam electronic scanning array antennas may one day replace costly mechanically steered parabolic antennas for satellite telemetry, tracking, and command.

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 multibeam 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 highselectivity 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 heterojunction 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 singlepole 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, absorptivetype 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 highrejection, low-pass ceramic filter. The transmit channels’ overall gain is 20 dB.