The antenna module Fig. 2(a)> provides directional and omnidirectional operations by using the switched-line 0/180-deg. phase shifter10,11 between antenna terminal 7 of the 4 x 4 matrix and the antenna monopole A3. The simple two-PIN-diode phase shifter was implemented in the antenna SBFN. The switched-line phase shifter Fig. 2(a)> uses two lines of different lengths: one is a "reference" line (L1), the other is a "delay" line (L2). Since only one of the two arms is selected, switches are required. PIN diode switches were chosen because high-power capability is the top priority. Unfortunately, a four-monopole antenna with a switched phase shifter suffers from limited bandwidth. Figure 2(b) shows a block diagram of the broadband antenna module3 without the narrowband switched phase shifter. During omnidirectional transmit mode (for Transponder, DME), the transmit signal passes from the integrated unit through the single-pole, double-throw (SPDT) switch and four-way divider to the four monopoles. The broadband divider can be implemented using conventional Wilkinson dividers or directional couplers. The four-way divider and the BFN are located close to the antenna monopoles to minimize phase and amplitude imbalance between the four terminals of the antenna module. During the TCAS directional transmit mode, the transmit signal passes from the integrated unit through the SPDT switch and the BFN to the four antenna monopoles. During the directional receive mode, which provides a bearing measurement, all four of the antenna terminals are monitored, and receive signals pass through the four SPDT switches and the BFN to the integrated transmit/receive unit.

Figure 3 shows an antenna module with four-folded monopoles.

The capacitive hat gives a uniform current distribution of the vertical feeding post and allows the antenna to be resonant with a reduced height and a wider bandwidth. The geometry of the top capacitive element allows a tradeoff between antenna size and bandwidth. This element stores charge so that more vertical current can flow than if the top disk were not present. Each of the four antenna module inputs corresponds to a beam in one of four directions: F, R, A, or L. Directional and omnidirectional antenna gains are related to monopole dimensions. The directional transmit mode is implemented by alternate activation of one of the SBFN's input ports 1, 2, 3, or 4. The position of the antenna pattern depends on which input is activated.

The five-monopole antenna module (Fig. 4) overcomes the limitation of the four-monopole antenna module.

It adopts both directional and omnidirectional modes without the switched 0/180-deg. phase shifter. During omnidirectional mode, the active center monopole (A5) is surrounded by four equidistant, symmetric parasitic elements (A1, A2, A3, and A4) on a circumference of radius r1, with r1 chosen to optimize directional and omnidirectional characteristics. The parasitic elements are shorted to ground by single- pole single-throw (SPST) switches SW1, SW2, SW3, and SW4 Fig. 4(a)>. Parasitic posts lead to higher gain and better omnidirectional patterns.11

Antenna elements and antenna inputs are switched using PIN diodes. By controlling PIN diode states (on or off), different sets of short-circuited monopole function as directors or reflectors. The active center element is electrically connected to transmitter/receiver through SPDT switch SW5. During the omnidirectional mode, the antenna matching is mainly controlled by the diameter of center monopole A5. Under directional operation, the array is electronically switched to each of the four positions sequentially by the alternating activation of the four antenna module terminals 1, 2, 3, or 4 Fig. 4(a)>. During the directional mode, all four outside active monopoles (A1, A2, A3, and A4) are electrically connected to the BFN antenna terminals while the center parasitic monopole (A5) is shorted by SPST switch SW6.

Figure 4(b) illustrates the fivemonopole directional/omnidirectional parasitic antenna without the hybrid BFN. This antenna consists of one active center monopole (A5) connected to the transmitter and four parasitic elements (A1, A2, A3, and A4) in a circle. The parasitic elements are switched using PIN diodes connected to the ground plane. By switching, different sets of parasitic elements function as directors and reflectors around the active center monopole.13,14

Usually, the parasitic elements become reflectors when shorted to ground plane. The parasitic elements become directors when opened. During the omnidirectional transmit mode the active center monopole (A5) is surrounded by four equidistant, symmetric parasitic elements (A1, A2, A3, and A4). The switch circuits of all parasitic elements are switched off. Since the parasitic elements act as directors, the omnidirectional mode is realized. During the directional mode, different combinations of switched elements (SW1, SW2, SW3, and SW4) support different radiation patterns.

For a directional beam, three of the four parasitic elements are shorted to ground with the forth floating. The directional pattern can be steered by alternating open elements. Via electronic control, the main beam can be switched to one of four directions.

The directional information can be obtained to sample the received signal with several different radiation patterns, since the switching time of a PIN diode is only of the order of a few nanoseconds.13 The DF performance of this antenna is sufficient and the cost reduction of a single receiver compared to traditional antenna arrays outweighs the loss in performance for broad-side angles.

Figure 4(c) shows a sketch of a five-folded-monopole antenna with a single metal base ground plate (base plate, 1), four feeding posts (2), four shorting posts (3) (which can also be implemented as part of the ground plate), four capacitive hats (11), one center feeding post (9) with capacitive hat (10), an SBFN multilayer card assembly (6), and four electrical connectors (7). The shorting post provides a reduction in physical size. The electrical characteristics of the folded monopole antenna depend on the height and size of the capacitive hat, the groundplane dimensions, the diameters of the feeding and shorting posts, and the spacing between them. The vertically polarized antenna is designed for the 1-GHz band. The antenna dimensions have been optimized to achieve a low profile and for good gain and matching for both directional and omnidirectional modes. The heights of the feeding posts and shorting posts are equal to 0.043?. Each capacitive hat area is equal to 0.027?. The four feeding posts (2) and the center post (9) are electrically coupled to the four capacitive hats (11) and the center capacitive hat (10), respectively, on one side, and to the antenna terminals of the SBFN on the other side. The space between the diagonal feeding posts (2) is 0.25?. Each shorting post (3) is electrically coupled to one of the capacitive hats (11) on the one side and to ground plate (1) on the other side.

Next month, this two-part article will conclude with an exploration of an active antenna module for TCAS applications.

REFERENCES

1. L. G. Maloratsky, "RF Design of Avionics L-Band Integrated Systems," Microwave Journal, 2009.

2. D. Kutman et al., "Multifunctional Aircraft Transponder," US Patent No. 6,222,480, April 2001.

3. L. G. Maloratsky et al., "Combined Aircraft TCAS/Transponder with Common Antenna System," US Patent No. 7,436,350, October 2008.

4. "Signal Sorting Methods and Directional Finding," www.microwaves101.com/enciclopedia/.

5. "An Introduction to Dipole Adcock Fixed-Site DF Antennas," Application Note AN-005, PDF Products, December 1999.

6. M. F. Gard, M. F., et al., "Electronic Compass," US Patent No. 5,850,624, December 1998.

7. B. E. Dinsmore et al., "Apparatus and Method for an Amplitude Monopulse Directional Antenna," US Patent No. 5,191,349, March 1993.

8. L. G. Maloratsky et al., "Aircraft Directional/Omnidirectional Antenna Arrangement," US Patent No. 7,385,560, June 2008.

9. L. G. Maloratsky, "Switched Directional/Omnidirectional Antenna Module for Amplitude Monopulse Systems," Antenna Magazine IEEE, October 2009.

10. L. G. Maloratsky et al., "Switched Beam Forming Network for an Amplitude Monopulse Directional and Omnidirectional Antenna," US Patent No. 7,508,343, March 2009.

11. L. G. Maloratsky, "Analyze Bearing Accuracy of a Monopulse System,"Microwaves & RF, Part 1, March 2009, Part 2, April 2009.

12. L. G. Maloratsky, Passive RF & Microwave Integrated Circuits, Elsevier, 2004.

13. T. Seki, "Low-loss and Compact Sector Antenna that Adopts Omni-directional Characteristics," 2000 IEEE Antenna and Propagation Society Int. Symposium, Vol. 2.