Experimental Validation of a Dual-Band TTD-Based RF Front-End for a Phased Array

Article information

J. Electromagn. Eng. Sci. 2026;26(2):163-169

Publication date (electronic) : 2026 February 25

doi : https://doi.org/10.26866/jees.2026.2.r.345

Yoonseon Choi, Joon Yong Park, Youngseok Bae

Agency for Defense Development, Daejeon, Republic of Korea

^*Corresponding Author: Youngseok Bae (e-mail: youngseok.bae@add.re.kr)

Received 2025 May 19; Revised 2025 September 12; Accepted 2025 October 17.

Abstract

I. Introduction

Recently, there has been a growing demand for the high-precision detection and tracking of low-altitude, small aerial targets, such as drones and small unmanned aerial vehicles (UAVs). Due to their small size and high maneuverability, these targets are difficult to detect and track using conventional radar systems. Accordingly, modern radar systems are rapidly evolving with a core focus on multifunctionality, high precision, and wideband capability, thus aiming to simultaneously provide broad frequency coverage and beam-steering functionality. In particular, the advancement of active electronically scanned array (AESA) technology heavily relies on the performance of transmit/receive modules (TRMs), thus leading to a sharp increase in demand for TRMs that are more compact, support more channels, and provide wider bandwidth [1]. Early TRMs were designed to operate over relatively narrow frequency bands; however, modern multifunction radar and communication systems must be capable of flexibly operating across multiple frequency bands. They are also required to simultaneously perform precision tracking and engage in electronic warfare responses [2–9].

In high-frequency bands, in particular, challenges such as increased insertion loss, mutual interference, and thermal concentration become more pronounced. To achieve the desired system performance under these conditions, it is essential to adopt multilayer radio frequency (RF) printed circuit board (PCB) designs, implement stripline-based RF transmission structures [12], ensure independent channel control, and develop highly reliable thermal management strategies [11, 13]. The key technical challenges in achieving wideband operation include managing insertion loss and gain flatness at high frequencies, minimizing noise figure in wideband receiver chains, optimizing transmit power and efficiency, suppressing inter-channel interference, and enabling high-speed switching control [12]. To meet these requirements, high integration and precisely engineered circuit optimization are essential.

Further, to realize system-wide capabilities, such as high-resolution detection and simultaneous multifunctional operation, the number of array elements must be increased. Consequently, multichannel integration at the TRM level becomes essential. A quad transmit/receive module (QTRM) architecture can be adopted to meet these requirements, thus offering superior integration and system-level optimization compared to conventional single- or dual-channel TRM designs. In particular, when combined with wideband characteristics, the QTRM is well-suited for multifunction radar and communication systems, as it ensures consistent transmit/receive performance across a wide instantaneous frequency range. In particular, a QTRM combined with wideband characteristics must adopt a true time delay (TTD)-based transmit/receive control [3] scheme—rather than a conventional phase-shifter-based architecture—to ensure consistent transmission and reception performance across a wide instantaneous frequency range and to eliminate beam squint, where the beam direction varies with frequency. This approach enables the simultaneous detection of multiple targets and coverage of a wide search area [12], supports integrated electronic warfare and communication capabilities [7], and satisfies the requirements for system miniaturization and weight reduction through multichannel integration [11, 13].

However, although the QTRM architecture has generally been introduced to achieve high integration, compactness, and system simplification, it has primarily been designed for narrowband operation. In addition, research on QTRM structures that incorporate TTD for dual-band applications remains extremely limited.

Accordingly, this paper presents the design and experimental results of a QTRM-based RF front-end that incorporates dual-band transmit/receive characteristics and a TTD control scheme for the detection of small sized targets. By adopting the TTD approach, the system aims to maintain consistent beam-steering performance across the entire frequency range and effectively eliminate the beam squint issues inherent in conventional phase-shifter-based designs. In addition, the proposed multichannel integrated architecture aims to achieve system miniaturization, suppress inter-channel interference, ensure transmit power stability, and improve receive sensitivity. The designed QTRM is integrated with an actual antenna system to evaluate the overall RF system performance. Through this validation, the study demonstrates that the TTD-based dual-band QTRM can serve as a viable architecture for future high-performance AESA radar and multifunctional communication systems.

II. System Requirements

The performance specifications and design requirements of the proposed RF front-end are summarized in Table 1. The key transmission requirement is a final transmit (Tx) output power that exceeds 0.77 kW, while the receive (Rx) chain is designed to achieve a gain of at least 60 dB.

Table 1

Specifications and design requirements

As depicted in Fig. 1, the proposed architecture forms one QTRM by integrating four single-channel TRMs. Four of these QTRMs are then grouped to form a TRB, thus resulting in 16 TRBs. The system comprises 256 Tx/Rx channels by arranging 16 TRBs. The TTD module utilized in the system provides a 7-bit (0–508 ps) time delay resolution. In the Tx and Rx paths, the signal passes through the TTD assembly, thus ensuring phase linearity across the design frequency bands. The processed signal is then routed through the Tx/Rx driving amplifier (TDA) to the RF distribution network, amplified by the common TRM assembly and dual-band TRM assembly, and finally radiated via the antenna elements.

Fig. 1

RF front-end system configuration.

III. QTRM Characterization and Measurement

The transmit output power of each channel was measured using the experimental setup depicted in Fig. 2(b). Power was sequentially supplied to the QTRM using a power supply unit, while RF pulses were applied via a signal generator. The output signals were then measured using a signal analyzer. Next, as depicted in Fig. 2(c) and 2(d), the experimental setup was configured to measure the receive gain using a network analyzer.

Fig. 2

QTRM structure and experimental setup: (a) QTRM structure, (b) TX power, (c) Rx gain, and (d) noise figure.

Then, the noise figure was evaluated using a signal analyzer in conjunction with a noise source. All 64 units were tested, and the results met the required performance specifications. However, this paper presents selected results for a specific unit at 10 GHz. The maximum Tx output power per channel of a QTRM was 38.1 dBm and the minimum was 37.7 dBm. The maximum RF receive gain was 13.8 dB, and the minimum was 12.5 dB. The noise figure ranged from 3.9 dB (min) to 4.1 dB (max).

IV. RF Front-End Integration and Beam Steering

Fig. 3 presents the structures of the single and array antennas along with their measurement results. The measured realized gains for the single antenna met the required specification of at least 5 dBi across the design frequency band. The gain measurement for the 16×16 radiating element assembly, which includes the 256-channel power distribution network, reflects the results corrected for system-level RF losses along the feed and signal path. The array antenna achieved a maximum gain of 25.5 dBi at 10 GHz, and 27.8 dBi at 15 GHz was obtained. In addition, the measured gain satisfied the required specification of at least 24 dBi acrㅍoss the entire design frequency band.

Fig. 3

Antenna structures and measured realized gain: (a) single antenna, (b) array antenna, (c) realized gain at 10 GHz, and (d) realized gain at 15 GHz.

Fig. 4 presents the measurement setup for Tx output power and receive gain. RF pulses (pulse width 8 μs; duty 10%) generated by the signal generator were applied to the input port of the RF front-end; the received signal level was measured using a signal analyzer via a horn antenna. To measure the receive gain, RF pulses were transmitted from the horn antenna, and the signal level received by the RF front-end was measured accordingly. The Tx output was measured across all four quadrants of the system. For each quadrant, the measured values were calibrated based on input power, cable loss, input/output characteristics of the QTRM and TTD assemblies, antenna gain, and the characteristics of a 40 dB attenuator. Consequently, the total Tx output at 10.7 GHz exceeded 0.93 kW, and the Rx gain was 61.5 dB.

Fig. 4

Measurement set-up for (a) Tx output power and (b) Rx gain.

To validate the fundamental performance of the integrated RF front-end, the measured boresight radiation patterns were compared with ideal array-factor simulations. As depicted in Fig. 5, the measured patterns at both 10 GHz and 15 GHz show agreement with the simulation results. At 10 GHz, the measured gain was 25.8 dBi. This value is a deviation of −0.6 dB from the simulated gain of 26.4 dBi. The measured half-power beamwidth (HPBW) was 6.89°, almost identical to the simulation value of 6.9°.

Fig. 5

Comparison of simulated and measured beam patterns at (a) 10 GHz and (b) 15 GHz.

In a similar manner, at 15 GHz, the measured gain of 28.0 dBi showed a difference of −0.8 dB from the simulated 28.8 dBi. The measured HPBW of 4.66° was also in agreement with the simulated 4.6°. These results confirm that the 256-channel array antenna and RF front-end were accurately manufactured and are operating as designed.

Finally, to evaluate the azimuth and elevation beam-steering performances as well as the dual-band operation characteristics of the designed RF front-end, near-field radiation pattern measurements were conducted as depicted in Fig. 6(a). The measurement results are presented in Fig. 6(b). By applying TTD in dual-band operation, stable beam-steering characteristics were effectively achieved, suppressing the beam squint phenomenon. Beam-steering performance within a ±30° azimuth range was experimentally confirmed—that in both the X- and Ku-bands, the beam direction remained consistent regardless of frequency, with steering errors within 1°.

Fig. 6

(a) Measurement setup and (b) normalized radiation patterns in azimuth and elevation beam-steering tests.

In addition, similar results were observed in the elevation beam-steering tests at ±15°, further confirming that the TTD-based design ensures stable beam-steering performance even under dual-band operating conditions.

V. Analysis of Nonideal TTD Characteristics on Performance

To investigate the system’s robustness, the impact of the nonideal characteristics of the TTD on beamforming performance was analyzed. The performance under thermal stress was examined, along with the effects of its finite control range and insertion-loss variation. The thermal stability of the TTD component is demonstrated in Fig. 7, which reveals the measured phase-delay characteristics at a high temperature of 80°C. The results confirm highly stable operation, exhibiting the same linear trend observed at room temperature and in calculations. The maximum deviation from the ideal calculated phase was merely 7.5°, which indicates high component-level reliability for operational environments.

Fig. 7

Comparison of (a) calculated and (b) measured TTD phase delay at 80°C.

The response of the system to the TTD’s finite control range was also simulated. In a practical array, the TTD must compensate for both the beam-steering delay and the inherent random phase errors in each channel. Saturation occurs if the required total delay exceeds the TTD’s range of 0–508 ps. Fig. 8 presents a simulation case where large initial phase errors cause 8 out of 256 channels to saturate. The analysis revealed that the system exhibits degradation, thus preserving negligible pointing error with only a minor gain reduction of 0.272 dB.

Fig. 8

Simulated beam pattern in the TTD saturation condition: (a) array factor, (b) phase distribution, and (c, d) normalized radiation pattern in azimuth and elevation directions.

Finally, the influence of insertion-loss variation dependent on the TTD delay setting was investigated, as depicted in Fig. 9. The insertion loss fluctuated by approximately 2.1 dB over the entire delay range at 10 GHz.

Fig. 9

Measured insertion loss of the TTD with delay at (a) 10 GHz and (b) 15 GHz.

A simulation based on this characteristic predicted a reduction of 1.05 dB in radiation efficiency. The results include a quantitative analysis of the performance margins associated with the nonideal properties of the TTD components.

VI. Conclusion

In this study, a 256-channel RF front-end that incorporated TTD-based QTRMs was designed and implemented as part of a dual-band active phased array antenna system for the detection of small-sized targets. The system was integrated with a radiating element assembly, and its performance was experimentally evaluated. By applying the TTD assembly at the front end of the RF signal path, the beam squint in the dual-band environment was effectively suppressed. The beam-steering tests conducted in both azimuth and elevation directions confirmed that the main beam remained stable within a 1° error margin across both frequency bands, thereby verifying the effectiveness of the TTD-based architecture in supporting reliable beam-steering under dual-band operation. Further, an analysis of the system’s limitations confirmed its robust design. The TTD demonstrated high stability under the thermal condition. Simulations also revealed that TTD’s nonideal characteristics, including finite control range and insertion loss, causes a predictable loss in system performance. Therefore, the TTD-based dual-band RF front-end architecture proposed in this study demonstrates scalability and modularity for application in multiband or wideband AESA with precise beam steering performance. In particular, this architecture is expected to serve as a practical hardware foundation for the development of high-resolution phased array systems for which channel-level beam-steering accuracy and inter-band consistency are essential.

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