1. Introduction
With the continuous development of wireless communication systems, in order to meet the strict requirements of antennas in the fields of remote communication, radar detection and imaging, electronic jamming countermeasures, and so on, it is necessary to develop high-gain antennas with flexible beam scanning capability to compensate for propagation loss and meet the communication needs of serving multiple distributed users simultaneously as well as improving the communication capacity. Traditional phased array antennas [
1,
2] could realize the beam scanning function and provide excellent radiation performance through massive transmit/receive (TR) modules.
However, a large number of phase shifters and RF front-ends would result in high design complexity and large power consumption. In comparison, parabolic antennas [
3] do not need the feed network and are able to realize beam scanning through mechanical rotation, but they have the disadvantage of being large in size and high in profile. Recently, owing to their low-cost and concise structure, reconfigurable reflectarray antennas (RRAs) [
4,
5,
6] and transmitarray antennas (RTAs) [
7,
8,
9,
10,
11,
12,
13,
14,
15,
16] have become good candidates in the design of large-aperture beam steering antenna systems.
The reconfigurability can be realized by integrating solid-state control devices in elements such as PIN diodes [
7,
8,
9,
10], a microelectromechanical system (MEMS) [
11], or varactor diodes [
12,
13,
14,
15,
16,
17] to obtain different phase compensation by changing the current distribution of the electromagnetic radiator and to realize the beam scanning function. RRAs mimic parabolic antennas and make use of the integrated elements to manipulate the reflected phase. Owing to this, the steering beam and the feeding antenna are on the same side of the RRAs, and thus, the obvious blockage effects exist, which prevents its further application to some extent. On the contrary, RTAs could achieve the isolation beam and feeding. RTAs combine the advantages of both lens antenna [
18] and microstrip array antenna [
19], which utilize spatial feeding and avoid complex feed network.
At present, most of the work focuses on the design of 1-bit RTAs, among which the typical RTA element is the receiver–transmitter structure [
7], and the RF power transmission is realized by coupling microstrip lines in between. Furthermore, the improved elements are also used to achieve the dual-polarization RTA [
8], and the rotated elements are utilized to improve the operating bandwidth [
9]. However, due to the limited discrete-phase resolution of 1-bit elements, the freedom of the electromagnetic manipulation of the transmission phase is restricted, and the aperture efficiency of the RTAs is reduced. Accordingly, 2-bit RTAs by the use of two PIN diodes and MEMS switches could provide a better phase manipulation capability, but the corresponding complexity of the bias circuits and design costs would increase significantly [
10,
11]. On the other hand, the improvement of the phase resolution is also restricted by only the 2-bit elements.
To improve phase resolution, several RTAs with continuous phase tuning capability have been developed in [
12,
13,
14,
15,
16]. In [
12,
13], five layers of active frequency selective surfaces (AFSSs) with integrated varactors are designed, but the capability of the beam scanning is limited. To alleviate the problems, the corresponding equivalent circuit model is used for the design of the RTA element. In [
14,
15], a band-pass filter model is utilized as the prototype to design RTAs operating at different frequencies, but the aperture efficiency and radiation performance (i.e., scanning range, SLLs) are not satisfactory.
In [
16,
17], a varactor-based phase shifter using a coupled line structure could provide a continuous phase shift from
to
. Moreover, two PIN diodes are used to alternatively turn on to generate an additional
or
phase shift. Although a
phase compensation can supply more freedom, the addition of more active devices into the elements leads to a high design complexity and an increased insertion loss, which then become the main hindrance for the continuous phase tuning of RTA elements to offer a better radiation performance. Overall, RTAs exploiting phase shifters loaded by varactors are verified to be an effective design solution to flexibly tune phase. However, several challenges, such as a relatively narrow operating bandwidth, the freedom of phase manipulation and radiation performances (i.e., low aperture efficiency and scanning range), are still needed to be addressed.
Herein, an improved RTA with continuous phase tuning in virtue of the varactors incorporated in the FSS is investigated using theoretical analysis, full-wave simulation and experimental measurements. Firstly, an equivalent circuit model of a second-order bandpass filter is established for the transmission characteristics of the three-layered FSS, and the corresponding RTA element could realize a continuous phase tunability by loading two active varactor diodes. Secondly, the RTAs consisting of elements are fabricated for the measurement. The experimental results show that the proposed RTA could achieve two-dimensional (2D) continuous scanning from to at an operating band of 11.8–12.6 GHz, with a maximum gain of 22.76 dBi and a scanning gain loss of 3.3 dB. Finally, the sidelobe level is lower than −17.8 dB, which is far more than most lower-level RTAs.
The rest of this paper is arranged as follows. The
Section 2 introduces the working principle of RTAs. In the
Section 3, the design and simulation RAT are described and the realization result of RTA is given. Finally, the conclusion is drawn in the
Section 4.
3. Design and Measurement
Figure 9a shows the fabricated DC controlling board, composed of 32-DAC7718s, 4-STM32F103RCT6 microcontroller unit (MCU) and electronic buttons. In order to ensure the compensation precision of continuous phase and reduce the phase errors, 12-bit eight-channel TI DAC7718 chips are used to realize the high-precision bias voltage supply. The output DC bias voltage is controlled in real-time by the integrated button modules via the MCU.
The RTA is constructed using a four-layer metallic hybrid plate as shown in
Figure 9b.
Figure 9b shows the fabricated RTA; the aperture size of the RTA is about 196 × 196 mm
with 256 elements and 512 varactors. The varactor diodes are soldered in the elements on the top and bottom with two vias in the center of the patch. The 16 DC bias sockets are evenly distributed on the left and right sides of the fabricated board, in which each socket consists of 16 bias voltage lines.
Under the free-space measurement, the transmission characteristics of the fabricated RTA is experimentally tested. The RTA is fabricated with the low-cost printed circuit board (PCB) technology. Furthermore, this RTA’s measurements are conducted in a microwave anechoic chamber, utilizing a standard horn antenna as the feeding source, which is positioned at a distance of 135 mm from the transmission array surface, with a focal length-to-diameter ratio (F/D) of 0.7. Simultaneously, an ultra-wideband horn antenna is employed as the receiving antenna for far-field radiation pattern testing. During the testing process, a control circuit is employed to apply varying voltages to the varactor diodes, effectively mimicking different
values used in simulation. The measured results are illustrated in
Figure 10. It can be observed that when the DC voltage is increased from 0 V to 8 V, the operating frequency of the passband moves from 11.4 to 13 GHz, and the corresponding range of phase tunability is about
at 12.2 GHz. When comparing the simulated results in
Figure 8b, as
is tuned from 0.2 to 0.5 pf, the operational center frequency of the passband shifts from 11.5 to 12.8 GHz, and the corresponding phase compensation range is up to
at 12.2 GHz. It is seen that the measured results coincide with the simulated ones, but there are still some deviations of the operating passband frequencies, which is probably caused by the fabricated errors, soldering problem of the large amount of varactor diodes, and so on. Furthermore, according to the literature [
14], this type of varactor diodes is reported to work in C-band, but a not in
band. Thus, the parameters of lumped RLC in equivalent circuit are not fully accurate, which is the main reason for the deviation of the operating frequencies. Also, due to the insertion loss of the active devices, the transmission loss is slightly larger than the simulated one.
According to the transmission characteristics in
Figure 10, the distribution of the voltages of DC bias for the RTAs operating at 12.2 GHz is illustrated in
Figure 11. The reverse in bias voltage from 0 V to 7.2 V ensures that the transmission magnitude is maintained at about 3 dB and the corresponding phase tunability is about
. By directly controlling the DAC via the MCU, the fast beam-forming process could be flexibly manipulated.
Figure 12 shows the measured steering beams at 11.8–12.6 GHz. It can be seen that the RTA could well achieve 2D beam steering from
to
, and the radiation pattern could remain stable at the whole operating bandwidth. As illustrated in
Figure 12b, at 12.2 GHz, the scanning gain loss is within the 3.3 dBi value range, which is close to the simulated ones. Furthermore, the proposed RTA exhibits a low sidelobe level of −17.8 dB, as demonstrated in
Table 1. Furthermore,
Figure 13 shows the measured beams steered in both horizontal and vertical planes at 11.8, 12.2, and 12.6 GHz, respectively.
Figure 14a depicts the radiation patterns of main beam co-polarization and cross-polarization at 12.2 GHz. It can be seen that the cross-polarization performance of the proposed RTA is satisfied with a difference of about 28 dB, between main beam co- and cross-polarization patterns. The maximum gain of the measured main pattern is about 22.76 dBi in
Figure 14b, and the corresponding aperture efficiency is 24.65%. The calculation formula of aperture efficiency is as follows:
where
is the measured gain of the RTA,
is the wavelength at the operating frequency and
is the physical area of the RTA.
The simulated and measured radiation gain versus different frequencies is depicted in
Figure 14b. Due to some fabrication issues such as dielectric loss, manufacturing tolerances, and an insertion loss of coaxial cable, the measured gain exhibits slightly lower values than those of the simulated one. Nevertheless, both the simulated and measured gains remain almost stable in the whole operating frequency band. Compared with the recently reported RTAs, the proposed RTA uses only two active devices to achieve high radiation characteristics at a low cost. It shows that two-dimensional beam scanning is performed in 11.8 to 12.6 GHz, and the scanning beam range is
. Compared to most RTAs, the lowest sidelobe levels could be achieved with about −15 dB. All the measured results validate that the proposed RTA exhibits an excellent radiation performance.
4. Conclusions
A novel RTA with continuous phase tuning for 2D wide beam scanning application at
band is investigated. Based on the design theory of tunable FSS, the RTA element with a three-layer FSS is constructed according to the equivalent structure of the second-order parallel bandpass filter. Two varactor diodes are loaded across the narrow rectangular slots on the two top and bottom patches, and direct coupling using the vias from the top patches to the bottom ones is applied to reduce transmission loss. By applying different reverse bias voltages to the varactors, an almost
continuous transmission phase compensation is realized. The RTA composed of
elements is fabricated for validation. The measurement of the proposed antenna shows that the continuous 2D beam scanning from
to
, from 11.8 to 12.6 GHz, could be flexibly carried out using the digital MCU. The beam scanning gain loss could also be less than 3.3 dB. At an operating frequency of 12.2 GHz, the maximum gain of RTA is 22.76 dBi with a sidelobe level of −17.8 dB, and the corresponding aperture efficiency is 24.65%. Compared to the sate-of-the-art work, as shown in
Table 2, the proposed RTA exhibits several merits of the radiation performance, such as a wide scanning range, small scanning gain loss and low sidelobe levels. The proposed RTA can be a good candidate for future wireless communication, sensing and imaging applications.