Retrodirective Array Fits Compact Designs

Phase conjugation using a heterodyne mixer is an effective technique for achieving retrodirectivity. The approach may provide conversion gain, allowing a nonplanar structure to be used in the retrodirective array. This technique also allows full-duplex communication to be achieved.^5-7 A number of different retrodirective arrays have been proposed, but most designs are broadside radiators.^8-11

In contrast, an end-fire retrodirective array was designed which can be surface mounted on a carrier and can attain highly directive patterns while scanning in the end-fire direction rather than in the broadside direction. To demonstrate the approach, an antenna array was designed and fabricated that includes four end-fire elements for both receiving and retransmitting, in addition to a passive FET mixer which has shared input/output port for the phase conjugation process. The isolation between receiving and retransmitting signals is realized by a 90-deg. hybrid coupler.

The implemented array shows good retrodirectivity over an incident range of ±32.5 deg. in a compact size for use in a variety of electronic products. Surface mounting is simplified since the antenna array radiated in a direction normal to the backplane on which it is mounted. It is an excellent candidate for RFID applications.

A schematic diagram of the proposed retrodirective array is shown in Fig. 1. In operation, an incident signal is divided into two parts by couplers and then conjugated by the FET mixer. The radio-frequency (RF) and immediate-frequency (IF) signals—i.e., phase-conjugated signals—share the drain of the FET device while the local-oscillator (LO) signals are applied to the FET gate. The phase-conjugated signals are then combined in-phase by couplers and retransmitted by the antenna. This system could be used for data transmission by adding a switch to each branch or by the use of a modulated LO signal.

Figure 2 presents a phasor diagram of the retrodirective antenna array. An incident signal is divided into two parts with equal power and phase difference of 90 deg. Following mixing, a phase-conjugated signal appears at the RF/IF port and is canceled at the isolated port of the coupler. At the same time, RF leakage is canceled at the RF/IF port, appearing at the isolated port which is connected to a 50-Ω matched load to minimize RF leakage signals. This straightforward design ensures good isolation between receiving and retransmitting signals and provides strong suppression of RF leakage signals.

A prototype circuit of a four-element retrodirective antenna array was fabricated on printed-circuit-board (PCB) substrate with relative dielectric constant, ε_r, of 2.65 and thickness, h, of 1 mm. The antenna array was arranged with element spacing of 0.68λ at 5.8 GHz and was driven by an LO signal at 11.6 GHz. The total size of the antenna array is 180 × 165 × 1 mm (Fig. 3).

A Yagi-Uda antenna fed by means of coplanar-waveguide (CPW) transmission line, which can be integrated with a phase conjugation circuit and operate with end-fire beam retrodirectivity, was designed for use with the proposed retrodirective antenna array. The antenna offers the benefits of low profile and compact size and can also achieve wide bandwidth and high gain in a conformal configuration.

The passive FET mixer was designed using a model NE76038 GaAs MESFET from NEC/CEL. The transistor has a gate length of 0.3 μm and is supplied in a plastic package. Fabricated with ion-implantation technology, the MESFET features 1.8 dB noise figure with 7.5 dB associated gain at 12 GHz. Only one matching network is used for the RF/IF port since these two signals are close together in frequency.

Also, no couplers are needed because the LO and RF signals, with frequencies which are far apart, are applied to different sides of the MESFET. This significantly reduces circuit size and complexity. The hybrid coupler is employed for increased isolation, and a one-to-eight Wilkinson power divider is used for the LOs.

The performance of the antenna array was characterized using monostatic and bistatic radar-cross-section (RCS) measurements as shown in Fig. 4. During monostatic measurements, the interrogating and receiving antennas were moved together. During bistatic measurements, the position of the interrogating antenna was fixed while the receiving antenna was moved into a range of -90 deg. ≤ θ ≤ +90 deg.

Figure 5 presents measured monostatic and bistatic RCS patterns for the proposed retrodirective antenna array. The LO and RF frequencies were set to 11.6 GHz and 5.81 GHz, respectively, and the retransmitting frequency was 5.79 GHz. This small frequency offset provided good isolation for the measurement process. The measured 3-dB beamwidth shown in Fig. 5(a) is 65 deg. Figure 5(b) shows the measured bistatic scattering patterns with the sources at 0, 15, and 30 deg. Retrodirectivity can clearly seen at each illuminated angle.

In summary, a novel end-fire retrodirective array has been presented. A Yagi-Uda antenna was used with the retrodirective array to provide a very low profile. The system uses an antenna array including four end-fire elements for both receiving and retransmitting and a passive FET mixer which has shared input/output port for the phase conjugation process.

A hybrid coupler was introduced for high isolation at the shared port and suppression of undesired signals. A four-element prototype was implemented to verify the concept. The fabricated array exhibits good end-fire retrodirectivity with high isolation and compact size—all traits suitable for RFID applications.

Lei Chen, Doctor

Chao Wang, Engineer

Jian-Feng Yu, Doctor

Xiao-Wei Shi, Professor

Science and Technology on Antenna and Microwave Laboratory, Xidian University, Xi’an, Shaanxi, 710071, People’s Republic of China