A Novel Phase Conjugator for Active Retrodirective Array Applications

A novel phase conjugator for active retrodirective array applications is presented. The circuit provides conversion gain as well as phase conjugating operation by using active devices, and results in a simple and compact design by using a common port for RF and IF signals. Measurement results indicate 3.2 dB conversion gain with RF-IF isolation of 20 dB. This phase conjugator can be used as a component in a retrodirective array when combined with an antenna. Introduction: A retrodirective antenna has unique characteristics that it reradiates a signal towards the source with no apriori knowledge of the arrival direction. Heterodyne mixing is a simple and effective technique to achieve retrodirectivity using a LO twice that of the received RF frequency. In this case, the lower sideband product has the same frequency as the RF, but the phase is conjugated. When combined with an antenna and placed in an array, the phase-conjugated signal will be reradiated towards the source direction [1,2]. Therefore, this type of mixer has many useful applications in wireless communications and radar applications. In this paper, a novel and compact mixer with conversion gain is demonstrated for this application. One important issue for this scheme is undesired RF leakage, which can be reradiated. Hybrid structures are typically used to eliminate the RF leakage (nonphase-conjugated signals) from the circuit [1]. The isolation between incoming and outgoing signals can also be enhanced by introducing a small offset between the RF and IF frequencies [3]. Using orthogonal polarizations for transmitting and receiving is another way to enhance the RF-IF isolation. However, this technique limits performance by making it impossible to handle arbitrary polarizations. Active devices such as MESFETs are attractive since they can provide conversion gain as well as mixing operation. This allows the array to send amplified signals towards the source location without including additional amplifiers, which will increase circuit size and cost [4]. This paper presents a new phase conjugator with active devices for retrodirective antenna arrays, which also makes it possible to use the same polarization for both transmitting and receiving when placed in a retrodirective array. This allows the size of each element to be made very small, which is very important since the scanning range of a retrodirective antenna array depends on the array spacing. Phase Conjugator Concept: The schematic of the new dual-channel phase conjugator is shown in Fig. 1. The circuit contains two ports, one for the LO which is applied in phase to the two channels and a second combined RF/IF port. The channels are identical except for a 90° phase delay line at the RF frequency. This delay line causes RF cancellation at the RF/IF port for isolation. Since the LO frequency is twice that of the RF frequency, the LO from the two channels will experience a 180° delay when combined at the RF/IF port, providing good LO isolation. This novel architecture allows an extremely compact design. When placed at the feed of an antenna, the retransmitted signal will have the same polarization as that of the incoming signal. Circuit Performance: A prototype circuit is fabricated on RT/Duroid (25 mil thickness, εr=10.2) and is shown in Fig. 2. Circuit performance is tested by using an SMA connector at the RF/IF port instead of an antenna. The measurement setup is shown in Fig. 3. Two synthesizers are used to provide RF (5.99 GHz, -30 dBm) and LO (12 GHz, 0 dBm) signals. The signals going toward the output port are tapped off through a directional coupler with 10 dB attenuation to a spectrum analyzer. As shown in Fig. 4, the circuit achieved RF/IF isolation of 20 dB and conversion gain of 3.2 dB at the output port. Conclusion: A new compact phase conjugator was proposed. This new structure allows high RF/IF isolation due to its hybrid structure, and uses FET mixers for conversion gain. The circuit uses one port for both input and output, and shows high RF/IF isolation as well as conversion gain. This compact architecture can be used as the building block for wide range retrodirective antenna array due to its compact size, which can be easily incorporated into an array, as well as its excellent performance. Current efforts are being directed towards developing a retrodirective array based on this architecture. Acknowledgement: This work is supported by the US ARO MURI program undercontract DAAHO4-96-1-0005. The authors also thank Mr. William Deal for hishelpful advice. References1. POBANZ, C. W, and ITOH, T.: ‘A Conformal Retrodirective Array forRadar Applications Using a Heterodyne Phased Scattering Element,’ IEEEMTT-S International Microwave Symposium Digest, 1995, pp. 905-908.2. CHANG Y., FETTERMAN H. R., NEWBERG I. L., and PANARETOS S.K.: ‘Microwave Phase Conjugation Using Antenna Arrays,’ IEEE Trans.Microwave Theory and Tech., 1998, 46, pp. 1910-1919.3. KARODE S. L., and FUSCO V. F.: ‘Frequency Offset RetrodirectiveAntenna Array,’ Electronics Letters, vol.33, (no.16), July 1997, pp. 1350-1351.4. MIYAMOTO R. Y., QIAN Y., and ITOH, T.: ‘A Retrodirective Array UsingBalanced Quasi-Optical FET Mixers with Conversion Gain,’ IEEE MTT-SInternational Microwave Symposium Digest, 1999, pp. 655-658. List of Figure Captions Figure 1 Schematic of the proposed phase conjugating element.Figure 2 Photo of the phase conjugating mixer.Figure 3 Measurement setup.Figure 4 Measurement results. antenna90 deg. at RF freq.180 deg. at LO freq.LO 12 GHzIF (6.01 GHz) RF (5.99 GHz)FET mixers Figure 1 Schematic of the proposed phase conjugating element Figure 2. Photo of the new phase conjugating mixer 90 deg. at RF freq.180 deg. at LO freq.