# A Review on Reconfigurable Liquid Dielectric Antennas
## 3 Liquid-Assisted Antennas
### Microfluidic Channels  
A planar monopole antenna is a variation of a monopole antenna that is printed on a dielectric substrate. It consists of a quarter wavelength \(\lambda_{g}\) strip line and a ground plane printed on the top and bottom of the substrate, respectively, where \(\lambda_{g}\) is the guided wavelength in microstrip environment. Such antenna produces an omnidirectional radiation pattern. The guided wavelength \(\lambda_{g}\) depends on the permittivity of the substrate. In Reference [81], the permittivity of the substrate was modified locally by introducing channels that are perpendicular to the monopole stripline. The channels are integrated into the substrate closer to the monopole end where the electric field is strongest. By gradually filling four channels with water, a number of discrete frequency states can be achieved. A schematic of such frequency reconfigurable antennas is shown in Figure 5a. When all the channels are vacant, the center frequency is around \(5.5\,\mathrm{GHz}\). By gradually filling the channels, starting from the one on the far end, the center frequency shifts to \(4.98\,\mathrm{GHz}\), to \(4.76\,\mathrm{GHz}\), and finally to \(4.48\,\mathrm{GHz}\). The corresponding impedance bandwidth is gradually changing from \(36.9\%\) to \(20.7\%\). Patch antenna is a widely used type of a planar antenna that consists of a square (rectangular) patch and a ground plane placed on top and bottom of a dielectric substrate, respectively. The patch can be fed through a via at the center or through a microstrip line. The fringing electric fields at the edges of the patch are responsible for the radiation of the antenna. Moreover, the radiation characteristics can be modified by changing the height and permittivity of the dielectric substrate. If these two properties of the substrate can be modified in real time, instant frequency tuning can be achieved. In Reference [82], several frequency states are introduced to a square patch antenna with the help of four microfluidic channels introduced between the patch and the ground. The channels were strategically placed near the radiating edges of the patch, where the electric field amplitude is maximum in the \(\mathrm{TM}_{100}^{2}\) mode, as illustrated in Figure 5b. Each channel has two states: (1) air and (2) water. By mixing these two sates between the four channels, several frequency states can be achieved. The switch between frequency states is not continuous but discrete that is why the authors called it \"frequency hopping\". The achieved frequency hopping range is from \(1.4\,\mathrm{GHz}\) to \(1.9\,\mathrm{GHz}\). In general, if the number of channels is \(N\), then the number of different frequency states is \(S\!=\!2^{N/2}\)[82]. Moreover, [PERSON] et al. showed that the same principle can be applied to an array of patch antennas, for example, a \(4\times 1\) patch antenna array. In Reference [83], a similar idea was realized on a patch antenna with two microfluidic channels placed near the patch edges. By changing the type of liquid (among water, acetone, methanol, and ethanol) inside the channels, frequency tuning was realized from \(16.8\,\mathrm{GHz}\) to \(28.4\,\mathrm{GHz}\). In Reference [84], the performance of a patch antenna was altered with the help of four small cylindrical containers placed like pillars between the ground and the patch, as shown schematically in Figure 5c. Because the electrical field between the patch and the ground plane is stronger than the one above the patch, it is more efficient to put the pillars there, as the authors demonstrated. The containers were strategically placed at the four corners of the patch to be able to control the type of polarization produced by patch. By changing the water level inside the cylinders, frequency and polarization tuning can be achieved. In Reference [85], a slot antenna was used with two channels filled with air, water or acetone to independently control the first and second resonant modes.