1. Introduction
Modern wireless communication applications require an antenna to have multiple functionalities (e.g., beam steering, direction finding, radar, control, and command) within a limited space [
1,
2]. Reconfigurable antennas became a popular approach since they can switch between functions within a single structure without resorting to multiple antennas. Antenna reconfiguration is achieved through a deliberate change in the antenna’s geometry and/or electrical behaviour which results in a change in the antenna’s functionalities [
3]. Such antennas typically have two or more discretely or continuously switchable states. These different states are normally obtained by changing current paths of the antenna through either rearranging the antenna itself or altering its surrounding medium. Reconfigurable antennas have been widely applied in many modern radiofrequency (RF) systems used for wireless and satellite communication, imaging and sensing [
4].
Since the first patent on reconfigurable antennas appeared in 1983 by Schaubert [
5], the topic has gained a lot of attention. In the literature, different designs have been proposed to achieve reconfigurability in terms of either frequency, radiation pattern, polarization, or a combination of two or three of the former ones. The desired reconfigurability can be obtained by various reconfiguration techniques that can be incorporated into an antenna design in order to redistribute its surface current through the change in the feeding network, the physical structure of the antenna, or the radiating edges [
4].
The reconfiguration techniques can be divided into several categories as shown in
Figure 1. Four major types of reconfiguration techniques are used, namely the electrical, optical, physical, and material reconfigurations. Electrically reconfigurable antennas use RF microelectromechanical systems (MEMS), PIN diodes, or varactors to redirect the surface currents. Those that rely on a photoconductive switching elements belong to optically reconfigurable antennas. For physical reconfiguration technique, the geometry of the antenna can be altered with the help of mechanical deformation. Finally, with the help of smart materials such as ferrites, ferroelectrics, liquid metal, liquid crystal, and liquid dielectrics, the reconfiguration of an antenna can be achieved [
4,
6,
7,
8,
9].
The most widely used reconfiguration technique is the electrical one. RF-MEMS rely on mechanical movement of microscaled switches, offer a good isolation and require minimal power consumption. PIN diodes operate in two modes (ON and OFF) and offer a faster switching speed (1–100 nsec) compared to the MEMS (1–200
sec). Other designs use varactor, as they can offer a variable capacitance and thus a continuous tuning ability (RF-MEMS and PIN diodes provide only discrete tuning). Even though these switches can be easily integrated into antenna structure, they require high voltage (RF-MEMS, varactors) and biasing lines that may add losses and distort the antenna radiation pattern [
4]. An example of an electrical reconfiguration technique applied for WLAN and Bluetooth can be found in Reference [
10].
Optical switches use light of a laser diode to activate photoconductive material. These switches do not require any biasing lines and thus eliminate the problem of introducing unwanted interference [
4]. However, optical switches are bulky and the integration of the these components into a compact antenna might become a challenge [
1].
Physical switches do not require biasing lines, laser diodes, or optical fibers. Moreover, they can potentially offer high power handling capability, lower losses, and allow for continuous tuning [
6]. However, their response time is relatively slow and the integration of the reconfiguration element into the antenna structure can be complicated [
4].
As for the reconfiguration techniques based on smart materials, liquid crystals, ferrites, and ferroelectrics can be used to reconfigure the substrate of antenna. The change in a material is achieved by changing its permittivity or permiability under different voltage levels (liquid crystals) or a static applied electric/magnetic field (ferroelctrics/ferrites). Liquid crystals are widely used at optical frequencies, however their property of changing the dielectric constant can be used for reconfigurable antennas at microwave frequencies as well [
6]. Their power consumption is relatively low, however, they have to be kept at the temperature range between 20 °C to 35 °C in order to stay in the liquid crystal state [
6]. The major advantage of the ferrite-based reconfigurable antennas is their high permittivity (
) and permeability (
) values that allow for antenna miniaturization and wide tuning range. However, the main disadvantage is the relatively complex biasing network required to achieve the maximum tuning range in bulk material and higher DC power consumption [
6]. Compared to ferrites, ferroelectric thin films require less bias voltage and have smaller losses [
6].
Liquid metal-based reconfigurable antennas are widely used for flexible, stretchable, and wearable electronics and sensors. They offer intrinsic reconfigurability and conformability, as well as high electrical conductivity and low loss. However, they have a relatively low switching speed, limited power handling, and repeatability issues [
9]. The research in liquid metal application for reconfigurable antenna is relatively new, however several review papers and books have been already written [
9,
11,
12,
13].
Liquid dielectric antennas have drawn a significant amount of attention for a number of reasons, (1) conformability: any antenna shape can be achieved due to the nature of liquid; (2) reconfigurability (both physical and chemical): it is easy to change the resonance frequency and bandwidth by changing the height/width of the liquid stream and the chemical composition; (3) low cost: liquid dielectrics are cheap and easily available compared to more costly liquid metals (e.g., mercury (Hg) or eutectic gallium indium allow (EGaIn)), (4) transparent and biocompatible, (5) high permittivity which helps to miniaturize antennas. Besides antennas, liquid dielectric materials have been applied in the field of metamaterials and metasurfaces [
14,
15,
16,
17], for the design of reconfigurable frequency selective surfaces (FSS) [
18], absorbers [
19,
20,
21,
22,
23], sensors [
24], reflect-arrays and array lenses [
25], and polarization converters [
18]. However, these topics fall beyond the scope of the present review and the details can be found in the review papers [
15,
26].
There are a few books [
1,
27] and several review papers published [
4,
6,
14,
28,
29,
30] up to date. They provide comprehensive overview and comparison of modern antenna reconfiguration techniques. However, a systematic review on liquid dielectric reconfigurable antennas is still missing although it is an important promising category. To the best of our knowledge, the only review covering liquid dielectric reconfigurable antennas was done in Reference [
31], however it was not comprehensive enough. In the present review we summarize the recent progress on this liquid dielectric reconfigurable antennas. The specific focus of this review paper is to systematise and analyse the research on active antennas that utilize liquid dielectric for radiation reconfiguration (both frequency, pattern, and polarization). It should be noted that even though this review is focused specifically on reconfiguration of active antennas by means of a liquid dielectric(s), the concepts discussed here can be applied to other electromagnetic applications such as sensors, filters, arrays, frequency selective surfaces, and meta- surfaces. The purpose of this paper is to thoroughly review state-of-the-art reconfiguration techniques for antennas in order to establish a general classification for the reconfiguration techniques which helps to systematically analyse this research field and draw conclusions on the research perspectives, gaps, and limitations.
For the review, several parameters were used to evaluate and compare the performance of a frequency reconfigurable antennas. The most obvious one is the frequency tuning range, which can be defined as follows [
6]
where
and
correspond to the highest and lowest resonant frequencies of a given antenna, where the resonant frequency is defined as the location of the
minima.
On the other hand, the total spectrum defined in Equation (
2) can be used to asses the total antenna working range [
6]
where
and
correspond to the maximum and minimum usable frequencies defined as location of the
dB.
Another parameter that can be used to quantify the spread of frequencies is the tuning ratio, as given in Equation (
3) [
6].
In terms of the tuning type, frequency reconfigurable antennas can provide discrete or continuous tuning mechanism. A discrete tuning mechanism will only provide a fixed number of resonant states, while in the case of a continuous tuning mechanism, the resonant frequency can be of any value within a tuning range .
Radiation efficiency is another important parameter to be considered. The radiation efficiency of frequency reconfigurable antennas may vary through the tuning range , that is why it is important to provide the radiation efficiency values for the whole . Moreover, it is best if the radiation efficiency of a frequency reconfigurable antennas can be compared to that of an equivalent antenna with a fixed operational frequency (not reconfigurable). The radiation efficiency is normally compromised by the reconfigurability technique used.
The last parameter is the physical size of the antenna. In this review, the total size in millimeters (mm) and the longest dimension with respect to the shortest wavelength is calculated where possible for comparison between different designs.
In terms of the classification, generally there are two types of frequency reconfigurable antennas that use liquid dielectrics for reconfiguration. The first type of antennas, called liquid-based antennas in this review, have a liquid dielectric as the main radiating part, for example, liquid-based monopole antennas. Such type of antenna design originates from dielectric resonators [
32], where liquid dielectrics are used instead of solid ones. Liquid-based antennas are discussed in
Section 2. The second type of liquid dielectric antennas uses dielectric liquid to locally modify the currents of a metal-based antenna, such as for example, patch and slot antenna. This type of antennas is called liquid-assisted antennas in this review. When most of the liquid-based antennas are three-dimensional antennas that typically require large quantities of liquid dielectric material, the liquid-assisted antennas typically have a planar and compact design and require less liquid to switch between different frequency states. Liquid-assisted antennas are discussed in
Section 3. Following
Section 3, discussions are presented in
Section 4. Lastly, conclusions are drawn in
Section 5.
4. Discussion
The main advantage of liquid dielectric materials is their fluidity and conformability. These two properties are widely used in the design of reconfigurable antennas. In the case of liquid-based antennas, where a dielectric liquid is used as the main radiating structure, the antenna geometry can take any shape due to its conformability. Complex shapes are hard to realize with conventional solid DRAs, so liquid dielectric antennas can offer a competing advantage and also provide reconfigurability. Liquid dielectrics offer an easy reconfiguration technique, where by changing the height, volume, or shape of the liquid, radiation characteristics of an antenna can be modified. High permittivity dielectric liquids, such as water (), can help to reduce the size of an antenna. However, water is lossy at higher frequencies (>1 GHz) which reduces the radiation efficiency. Adding salts (NaCl, KCl) to water will increase the conductivity and widen the bandwidth. The permittivity of water depends on the ambient temperature, which can be used as another tuning mechanism or can become a limitation (narrow temperature range of operation). Other organic liquids were proposed that have a wider temperature range of operation. Oils are less lossy and thus can have a better radiation efficiency, however, their permittivity is much smaller than that of water, which will affect the tuning range. Many liquid dielectric materials discussed in this review are easily available and low cost. Moreover, most of these liquids are optically transparent which means that liquid antennas based on them can be seen as optically transparent as well and could be integrated with other components, such as solar cells, on a single platform.
In the case of the liquid-assisted antennas, liquid dielectric materials are used to modify the local currents to reconfigure a metal-based antenna. The most crucial thing for a liquid-assisted reconfigurable antenna is the placement of the liquid holders, for example, microfluidic channels or cavities. The surface current and electrical field distribution of an antenna under consideration have to be modeled and analysed. Then the channels/cavities need to be placed near the location(s) of the current/electric field maximum to maximise the tuning range. It is even possible to independently control two resonant modes by strategically placing the channels. Radiation pattern can be reconfigured by placing the channels near the radiating edges of an antenna.The higher the permittivity of the dielectric liquid, the wider frequency tuning range can be achieved. If microfluidic channels are used, only discrete reconfiguration states can be achieved. On the other hand, if a cavity is used, it is possible to have a continuous reconfiguration (tuning). All liquid-assisted antennas reported in the literature up to date are planar unlike the 3-D liquid-based ones. This is because liquid dielectric is used only as a reconfiguration mechanism while there is a main radiating part. This also means that the amount of liquid dielectric material required for liquid-assisted antennas is smaller compared to the liquid-based ones.
Based on the analysis of the papers considered in this review, the designs with frequency, pattern, and polarization reconfiguration constitute 74%, 18%, and 8%, respectively, of the total number of papers under review. Most of the reconfiguration designs are for frequency reconfiguration. It demonstrates how liquid dielectric materials are particularly useful for frequency reconfiguration techniques due to their fluidity. The major part of the discussed designs fall into the category of liquid-based antennas (65% of the total number of papers under review). This demonstrates how the conformability and fluidity of liquid dielectric materials help to realize many different antenna shapes and designs.
The topic of liquid dielectric reconfigurable antennas is relatively new and some of the practical and implementation issues have not been resolved yet. One of the main limitations of this kind of reconfigurable antennas is their low switching speed. Indeed, compared to other commonly used techniques, such as RF-MEMs and PIN diodes with their switching speeds of about 1–200
sec and 1–100 nsec, respectively, the switching speed of most of the discussed liquid-based techniques is on the order of magnitude of one second. Moreover, switching speed of liquid dielectric reconfigurable antennas is rarely discussed in the papers, even though this is a very important parameter when evaluating antenna reconfiguration. For a liquid dielectric reconfiguable antenna, switching speed primarily depends on the method used to obtain different liquid dielectric volume (height, type) and in most of the works it is done manually with a measuring tool like a syringe (when the method is not mentioned it is assumed to be manual). Some authors used micro-pumps [
46,
47,
51,
52,
80,
82,
87,
90,
93,
94] and only one group demonstrated a closed loop system with a microcontroller and a mirco-pump [
52]. Only these few papers mentioned the switching speed of their proposed reconfigurable antennas [
80,
87,
92].
Other important parameters for reconfigurable antennas are the reliability and repeatability of the results. It is a measure of the accuracy and consistency of the measurements when the amount of liquid is repeatedly changed inside a container, microfluidic channel or a cavity. A good reliable antenna needs to be able to handle many cycles of liquid dielectric refills and still give consistent predicted results within a certain minimal error. However, only two papers mentioned this important parameter when evaluating their proposed designs [
64,
87,
92,
94].
DC power consumption is another important practical issue. All the additional mechanisms (e.g., a micro-pump), that are used to reconfigure an antenna, consume additional power besides the amount of power consumed by the antennas itself. A high power consumption will limit the maximum operation time of the antenna if the power supply is a portable battery. However, DC power consumption has only been stated in one work considered in this review [
80]. It should be noted that the DC power needed for PIN-diode-based switches has to be kept on during the whole time. On the other hand, the pump switching system has to be kept on only when the antenna needs to be switched to another configuration. These differences need to be taken into consideration when estimating the total power consumption of such antennas, which can be especially important for mobile applications.
Another important parameter related to power is the RF power-handling capability. One of the advantages of liquid reconfigurable antennas is their highly linear behaviour which means they are expected to remain linear under high-peak-power excitations [
9]. However, power-handling capability of liquid dielectric reconfigurable antennas was only discussed in one paper [
87].
The total size and weight of a resulting liquid dielectric reconfigurable antenna can be another practical issue. Once all the necessary additional components (such as micro-pumps, tubing, etc.) are integrated, the total size and weight of the antenna will increase. However, the implementation of the micro-pumps and tuning for liquid dielectric material delivery is easier than implementing a DC biasing network for a PIN diode-based switches.
Generally, it is not easy to compare conventional (RF-MEMs and PIN diodes) and liquid dielectric approaches and there is very little work published directly comparing the two. In Reference [
86], the authors compared two polarization reconfigurable patch antennas of similar designs. In one of them, the reconfiguration mechanism was realized with microfluidics and in the other one—with PIN diodes. These two antennas were compared side by side in terms of electrical size, radiation efficiency, radiation pattern, and switching speed. It was found that even though the switching speed of the liquid reconfigurable antennas is much slower (∼sec) than that of the conventional PIN diode-based ones (∼
sec), liquid reconfiguration approach offers advantages in terms of the higher radiation efficiency (22% compared to 10% in the PIN diode case), longer electrical length, and zero DC interference [
86]. It means that liquid dielectric reconfigurable antennas can be useful in application where the switching speed is of less importance than the antenna performance.