Frequency-Adjustable Small Zeroth-Order Resonant Antenna with Capacitor-Loaded Rectangular Slot on Ground Plane

A capacitor-loaded electrical small zeroth-order resonant (ZOR) antenna is proposed. The proposed antenna is designed on the basis of a mushroom structure for zeroth-order resonance. To obtain a compact size, the proposed antenna has a rectangular slot on the ground plane, and the chip capacitor is mounted on the slot. The resonant frequency is easily controlled from 2.82GHz to 2.29GHz by changing the capacitance from 1 pF to 7 pF, respectively.Therefore, the proposed antenna has the advantages of a small antenna size as well as easy frequency adjustment.


Introduction
Since portable wireless devices such as laptops, mobile phones, personal digital assistants (PDAs), and global positioning systems (GPSs) require increasing functions, the space to integrate the necessary components has become smaller.Antenna size is especially critical because it is dependent on the operating frequency.A number of techniques have been proposed to reduce the antenna size, among which the most straightforward approach is to use a substrate with a high dielectric constant.However, this leads to poor efficiency and narrow bandwidth.Much effort has been given to further reducing the antenna size, such as creating a meandering edge of the patch [1] or using stacked patches [2].A defected ground structure antenna in one study was able to achieve miniaturization by about 68% compared to a conventional antenna [3].In another study, fractal-shaped defects were presented to reduce the antenna size [4].
It has also been presented that additional loads can be used to miniaturize an antenna.Shorting posts have also been used in different arrangements to reduce the overall size of a patch antenna [5].Capacitive loads are an alternative way to reduce antenna size.It has been reported that a monopole or planar inverted F antenna (PIFA) with a capacitive load can provide both much lower profile and miniaturization [6,7].
Recently, the metamaterial concept was applied to several resonant antennas.The zeroth-order resonant (ZOR) antenna has been presented to reduce antenna size [8].Its zero propagation constant allows the antenna size to become independent of the resonant frequency because of the infinite wavelength.It has been reported that the frequency can be further decreased by distributing inductive and capacitive loads [9].
In this study, we propose a simple technique to further reduce antenna size by introducing capacitor-loaded slots on the ground and controlling the resonant frequency by the way of the capacitances of lumped elements.The S-parameter and radiation patterns will be shown at different capacitances.The zeroth-order resonance phenomenon will be demonstrated with electric-field distributions along the aperture from fullwave simulation.

Antenna Design
ZOR can be achieved by the way of a composite right-/lefthanded transmission line (CRLH TL).A mushroom-like structure is well known to realize ZOR [8].The unit cell of a general CRLH consists of a series inductance and capacitance, as well as a shunt inductance and capacitance, as shown in Figure 1(a).The conventional CRLH TL consists of a series Therefore, the frequency is determined by the shunt parameter (CR and LL) [8].In order to design a smallsize antenna, the CR should be increased, but this is difficult.In order to do so, larger patch size, lower substrate thickness, or higher permittivity is necessary.These cause the antenna size to increase the patch size, while degrading the antenna performance to increase the permittivity.With decreased substrate thickness, the bandwidth of the antenna is decreased.Also, it is difficult to realize a higher shunt inductance (LL) without changing the dimensions.In order to overcome the limitations associated with decreasing the resonant frequency, we propose a slot and additional capacitor on the ground plane.A more compact and frequencyadjustable ZOR antenna is proposed by loading slots and chip capacitors on the ground plane of the mushroom structure, as shown in Figure 2. Its equivalent circuit model is modified as in Figure 1(b).
The slot is placed around the via which is connected to the rectangular slot in series.The chip capacitor on the slot increases the capacitance from the original slot.When the chip capacitor is loaded on the rectangular slot on the ground plane, the shunt parameter is changed.The ZOR frequency of the modified structure becomes Frequency (GHz)  Therefore, when the capacitance of the chip capacitors ( V ) is changed, the ZOR frequency can be easily adjusted.

Fabrication and Measurement
The proposed antenna is fabricated on Rogers RT/Duroid 5880 substrate with a dielectric material constant of 2.2 and a thickness of 1.6 mm, as shown in Figure 3.The coppercovered planes have a thickness of 35 m.The shorting is conducted via silver, which is a material with good conductivity.The conductive epoxy is used to mount the chip capacitors on the slot.
In Figure 4, the measured return losses with different capacitances (1, 3, 4, and 7 pF) are compared with the simulated results.For instance, when 1 pF of the capacitor is used, the experimental return loss is 25.02 dB at 2.82 GHz.The return loss at 7 pF capacitance is 18.46 dB at 2.29 GHz.Thus, the resonant frequency can be tuned from 2.82 GHz to 2.29 GHz by varying the capacitance from 1 pF to 7 pF, respectively.The relationship between the resonant frequency and the loaded capacitance is plotted in Figure 5. Since the proposed antenna is designed by using two unit cells of the CRLH TL, it is observed that the additional −1st order resonance occurred at the lower frequencies [10].In this work, only zeroth-order resonant modes are used for antenna applications because the negative resonance shows very low antenna gain and efficiency.For EM simulation, the parasitic resistance and inductance are extracted by a through-reflectionload (TRL) calibration and taken into account.Thus, the simulation and measurement results show excellent agreement.
The radiation patterns of the proposed antenna with 1, 3, 4, and 7 pF chip capacitors are measured in the anechoic chamber.The simulated and measured patterns on the XZ plane are compared in Figure 6.As expected from a typical ZOR antenna, monopole patterns are observed.
The measured peak gain of the proposed antenna is −0.1 dBi with 1 pF capacitance, which decreases with a higher capacitance.The measured efficiency is 78% when the capacitance is 1 pF, and it is decreased to 75% with 7 pF.
In order to demonstrate the zeroth-order resonance phenomenon of the proposed antenna, the vectors of the electric field are plotted in Figure 7.It is observed that the phases of the electric field are all in the same direction in each cell because of a zero phase constant and infinite wavelength.Figure 7 shows the snapshot of the E-field when the phase is varied.The phase of the E-field is constant along the antenna aperture.Therefore, it is successfully demonstrated International Journal of Antennas and Propagation that the proposed antenna operates in the zeroth-order resonant mode.

Conclusion
A frequency-adjustable ZOR antenna has been presented by integrating an additional rectangular slot and capacitors.The ZOR characteristic is used to effectively reduce the antenna size.The rectangular slot on the ground plane is introduced in order to further reduce the antenna size.In addition, the chip capacitor is used to easily adjust the resonant frequency.The addition capacitance from the chip capacitor results in further size reduction.From simulation and measurement results, the proposed ZOR antenna's frequency can be adjusted from 2.82 to 2.29 GHz by changing the capacitance from 1 pF to 7 pF.79.9% size reduction with a 7 pF capacitor is achieved compared with a conventional half-wavelength patch antenna at 2.29 GHz.Therefore, the possibility of a frequency tuning capability and miniaturization is successfully demonstrated in this work.The chip capacitors of the proposed antenna are potentially replaced by varactor diodes.

Figure 1 :
Figure 1: Equivalent circuit model.(a) General CRLH TL unit cell.(b) CRLH TL unit cell with slot and additional capacitor.

Figure 2 :
Figure 2: Illustration of the proposed antenna geometry with dimensions (units: mm).

Figure 3 :
Figure 3: Picture of the fabricated antenna prototype: top and bottom views.

Figure 4 :
Figure 4: Results of S parameters at different capacitance values.(a) Simulated return losses and (b) measured return losses.

Figure 5 :
Figure 5: Relationship between the resonant frequency and the loaded capacitance.

Figure 6 :Figure 7 :
Figure 6: Radiation patterns on the XZ-plane: (a) simulated radiation patterns and (b) measured radiation patterns with different capacitances.