# A Review on Reconfigurable Liquid Dielectric Antennas
## 3 Liquid-Assisted Antennas
### Microfluidic Channels
The location of the E-field minimum and maximum of a slot antenna resonator depends on its mode. In particular, the location of the E-field maximum of the second mode is the same as the E-field minimum of the first mode. It means that by strategically placing the channels, it is possible to separately tune the two modes, as illustrated in Figure 5d. The radiation efficiency is relatively low, however, if acetone is used instead of water, the radiation efficiency can be improved [85]. In Reference [87], a liquid dielectric FC-40 (\(\epsilon_{r}\) = 1.9, \(\tan\delta\) = 0.0005) was used to move a metallized plate inside a microfluidic channel, thus changing the resonant frequency of a planar monopole antenna. A continuous tuning form 1.7 to 3.5 GHz can be achieved with the tuning speed of 1565 MHz/s. The power handling of such antenna was improved compared to a previous work [88], where a liquid metal was used instead of the metallized plate. Specifically, it was found that this antenna can operate with 15 W of continuous power. The further power increase is limited by the maximum temperature handling of the micro-pumps used to move dielectric liquid. In Reference [89], the resonance frequency of a patch antenna was modified by pushing two metallic rods inside a microfluidic channel with the help of a mineral oil. Two microfluidic channels were made in a photopolymer resin (\(\epsilon_{r}\) = 3.2, \(\tan\delta\) = 0.05) that was 3D printed. Two bras cylinders were inserted into the channels and could be moved along the non-radiating patch edges by injecting oil through a syringe. Frequency tuning in two closely spaced bands was demonstrated. In Reference [90], a liquid-assisted switch was proposed for a Vivaldi antenna that function in two operating bands: from 2.5 GHz to 3.25 GHz and from 3.25 GHz to 4.50 GHz. The switch was introduced at the back slot such that when the liquid is introduced in the switch, it shortens the current path and thus results in a higher operating frequency. The liquid of choice was a 2 mol KCl solution that was found to provide the highest efficiency of up to 87%. In Reference [86], a microfluidic water-based switch was implemented for a polarization reconfigurable patch antenna. The patch consists of two orthogonally overlapped strips that are electrically isolated at their overlap by four gaps, as illustrated in Figure 5e. A microfluidic channel is implemented around the overlap. By capacitively loading the gaps with water and air in an alternating manner, two antenna states can be activated independently. With a center frequency around 2.4 GHz, fractional bandwidth of 2.19-2.39%, and efficiency of 22% this antenna can operate in two perpendicular polarization states. Table 5 summarises liquid-assisted antennas that utilize microfluidic channels to control the current distribution. Unlike the liquid-based antennas, liquid-assisted antennas with microfluidic channels mostly provide discrete tuning with only several states. Figure 5: Planar antennas with microfluidic channels. (**a**) [81], (**b**) [82; 83], (**c**) [84], (**d**) [85], (**e**) [86].