Avionics systems designers constantly seek savings in size and weight through integration. The approach also lowers the number of cables needed and the power requirements. The use of integrated antenna modules is one approach that helps to achieve reductions in size, weight, and power usage in modern integrated avionics systems.1 Part 1 of this two-part article will review active antenna modules for avionics systems; next month, Part 2 will conclude with a specific antenna module solution.
As an example, the Traffic Collision and Avoidance System (TCAS) and Transponder system used on many aircraft employs four separate antennas, ten cables, and separate receivers and transmitters. With such a low level of integration, the system is costly, large, and heavy. In contrast, an integrated TCAS/Transponder system uses two combined antenna modules electrically connected to the single TCAS/Transponder transmit/receive unit.1-3 These modules must provide directional and omnidirectional operation. Directional operation supports the TCAS 1090 MHz, TCAS 1030 MHz transmit, and Transponder 1030 MHz receive functions. Omnidirectional operation is used for Transponder 1090 MHz transmit and sometimes TCAS 1030 MHz transmit functions.
For the integrated TCAS/Transponder system, L-band antenna monopole should satisfy the following performance requirements for the 1030-to-1090-MHz frequency band:
1. During directional operation, the antenna gain should be greater than 1.0 dB, the beamwidth greater than 90 deg., the sidelobe/backlobe level less than -8.0 dB, the gain variance from sector to sector less than 0.4 dB, the impedance matching a VSWR of less than 1.40:1, and the switching from directional to omnidirectional modes less than 1 s.
2. During omnidirectional operation, the antenna gain should be greater than -2.0 dB, gain ripples should be less than 2.0 dB, and impedance matching equivalent to a VSWR of less than 1.40:1.
For integrated systems that include TCAS functionality, such as TCAS/ Transponder systems, TCAS/Transponder/ Universal Access Transceiver (UAT) systems, TCAS/Transponder/ Distance-Measurement-Equipment (DME) systems, TCAS/Transponder/ Automatic Dependent Surveillance- Broadcast (ADS-B) systems,1 the combined antenna module must provide directional and omnidirectional modes, transmit and receive modes, and directionfinding (DF) functionality with acceptable bearing accuracy. The three main techniques used for airborne DF functionality are the amplitude- comparison method, the phase-comparison method, and the amplitude-phase method.4-6 The amplitude-comparison approach is relatively simple and cost effective but relatively inaccurate. The phase-comparison method, implemented by means of a phase interferometer, is more precise. The amplitude-phase approach6 is a compromise between the simplicity of the first approach and the accuracy of the second approach. Characteristics of airborne amplitude/phase monopulse systems are shown in Table 1.
This report will review different directional/omnidirectional antenna modules for the avionics integrated systems,1 including the TCAS amplitude monopulse system, which requires a directional antenna mode, and other systems such as Transponder, UAT, ADS-B, and DME systems that require an omnidirectional antenna mode. Figure 1 shows a functional block diagram of an antenna module for an amplitude monopulse system.
The switched beam-forming network (SBFN) can be constructed with a hybrid network, phase shifters, switches, dividers, etc. The directivity of the antenna is divided into N sectors horizontally. These sectors are sequentially scanned by the monopulse method. Usually, the antenna interface includes M (where M = N) coaxial connectors (Fig. 1) coupling the antenna module to the transmit/receive module through coaxial cables.
The amplitude monopulse architecture can include a four-monopole antenna with SBFN.1,3,7-10 A conventional TCAS directional antenna is a four-monopole, vertically polarized, monopole array that can transmit in four selectable directions at 1030 MHz. The antenna receives replies with bearing information from all directions simultaneously at 1090 MHz, using amplitude-ratio monopulse techniques. This antenna and interface require a complicated amplitude and/or phase calibration network.
The common TCAS/Transponder antenna module1 provides directional or omnidirectional (if necessary) TCAS radiation antenna patterns, omnidirectional Transponder radiation antenna patterns, and directional TCAS and Transponder receive antenna patterns. The TCAS electronics uses signals from the directional antenna to determine the bearing from the host system to another aircraft. The novel antenna module consists of the four-monopole antenna, SBFN, matching network, and interface.8,9
Each SBFN port is coupled through a cable to a corresponding receiver. An airborne amplitude monopulse system estimates an intruder aircraft's bearing by comparing magnitudes of signals received by the four-monopole directional antenna. The antenna aperture is divided into four sectors (or quadrants). The incoming signals are processed inside the antenna module to produce four electrical connector signals, such that each electrical signal represents a unique quadrant of the polar coordinate system. A significant bearing accuracy improvement can be achieved with the bearing algorithm (or index) using all four received signals in four sectors. In this case, the bearing may be detected not only by an antenna pattern main lobe(s) but by the sidelobe(s)/backlobe(s) of another beam(s). This additional information provides greater bearing accuracy and eliminates strong requirements for the sidelobe/backlobe level suppression.
The L-band antenna module provides the directional antenna patterns by using the special SBFN including the 4 x 4 hybrid matrix and switched 0/180-deg. phase shifter Fig. 2(a)>.8-10
The four 90-deg. hybrids are serially interconnected to form a 4 x 4 hybrid matrix. The four ports, 5, 6, 7, and 8, of the hybrid matrix are connected to the four antenna monopoles, A1, A2, A3, and A4, respectively, and the other four ports, 1, 2, 3, and 4, are connected to a transmit/receive network through cables. The eight-port hybrid matrix in the SBFN provides equal amplitudes and specific relative phases for the four antenna monopoles. The directional transmit mode is implemented by the alternative activation of one of the input ports, 1, 2, 3, or 4, of the SBFN while the switched phase shifter provides a 0-deg. phase shift. Each of the four antenna module inputs corresponds to a beam in one of four directions: front (F), right (R), aft (A), or left (L). During the receive directional mode, which provides a bearing measurement, all four of the antenna connectors are monitored.
The relative signal intensity from four SBFN ports 1, 2, 3, and 4 shows the azimuth direction of a selected object according to the special bearing algorithm (index) and antenna lookup tables (LUTs).11 Also, the TCAS/ Transponder antenna module should provide an omnidirectional transmit mode when the signal passes through only one SBFN input (2) while the switched phase shifter is in its 180- deg. phase shift state. In this case, the four antenna monopoles are activated with equal magnitudes and progressive 90-deg. phase shifts (0, 90, 180, and 270 deg.).8-10
The performance of different quadrature hybrids for possible antenna beam-forming-network (BFN) applications was considered in ref. 9. For broadband coupled-line hybrids Fig. 2(b)>, coupling tolerances are difficult to realize because of the narrow gap between the strips. The threebranch hybrid has a larger bandwidth than the two-branch hybrid but greater insertion loss and larger dimensions. Two-branch hybrids were used in the L-band BFN Fig. 2(a)>. These hybrids have low insertion loss, low phase error, and adjacent output ports that permit combining them in the planar BFN design.12 The disadvantage of the two-branch hybrid is its narrow frequency coverage, but for the required 10-percent bandwidth of a TCAS/Transponder system, its bandwidth is acceptable.
The SBFN includes the switched 0/180-deg phase shifter connected to one output of the hybrid matrix. To provide the amplitude balance (compensate phase shifter loss) at all four output ports, the BFN hybrids H1, H2, and H4 should have unequal power division10-12 and hybrid H3 should have equal power division. Therefore, the 4 x 4 matrix should be asymmetrical, taking into account the switched 0/180-deg. phase shifter loss, and providing equal magnitudes and specified phases at the four antenna monopoles. To realize the unequal power division, the hybrid impedances (or admittances) should be different from divider impedances (or admittances) with equal power division.
The straight connection of the four two-branch hybrids (without additional connection lines) makes the bandwidth of the 4 x 4 matrix slightly narrower than a single two-branch hybrid due to the undesirable interaction between the four hybrids. When the four hybrids are connected using quarter-wavelength transmission lines (inverters) the property of the circuit is improved. The four inverters, I1, I2, I3, and I4 Fig. 2(a)>, between the hybrids provide considerably widened bandwidth for improved insertion loss, isolation, and return loss of the 4 x 4 matrix. The hybrid matrix of the SBFN handles receive and transmit signals through the antenna monopoles and is configured to selectively switch between directional and omnidirectional operation.