A Single-Point-Fed Wideband Circularly Polarized Rectangular Dielectric Resonator Antenna

A single-point-fed circularly polarized (CP) rectangular dielectric resonator antenna (DRA) with wide CP bandwidth is presented. By using TE 111 and TE 113 modes of the rectangular DRA, a wideband CP performance is achieved.The coupling slot of the antenna contains a resistor loaded monofilar-spiral-slot and four linear slots. Design concept of the proposed antenna is demonstrated by simulations, and parameter studies are carried out. Prototype of the proposed antenna was fabricated and measured. Good agreement between the simulation and measurement is obtained. The measured impedance bandwidth (|S 11 | < −10 dB) and 3 dB axial-ratio (AR) bandwidth are 51.4% (1.91–3.23GHz) and 33.0% (2.15–3.00GHz), respectively.


Introduction
Dielectric resonator antenna (DRA) [1][2][3][4] is a good candidate for wireless service equipment due to its attractive characteristics such as small size, wide bandwidth, and high radiation efficiency.
DRAs with circular polarization (CP) performance are more applicable for the wireless systems than the linearly polarized (LP) ones [5][6][7][8][9], because the CP wireless systems can support flexible orientation between the receiving and transmitting antennas.The published CP DRAs can be divided into two classes: single-point-fed [5][6][7] and dualpoint-fed [8,9] ones.Single-point-fed CP DRA is easy to implement, but its axial-ratio (AR) bandwidth is narrow.Dual-point-fed CP DRA has a wider AR bandwidth, but it needs a complicated power divider.Recently, several singlepoint-fed CP DRAs with wide 3 dB AR bandwidths have been reported.For example, a CP hollow rectangular DRA with wide AR bandwidth of 12.4% has been reported in [10].In [11], simple inclined slot fed CP trapezoidal DRA with an AR bandwidth of as wide as 21.5% has been obtained.Single-point-fed wideband CP DRAs can also be realized by using novel feeding structures [12][13][14].And in [15], a 3 dB AR bandwidth of 24.6% has been achieved by merging resonances of the DRA and coupling slot.
In recent years, some CP DRAs with wideband [16,17] and dual-band [18,19] performances have been realized by taking advantages of the higher order modes of the DRAs.In this letter, a single-point-fed wideband CP rectangular DRA using higher order mode is investigated.The TE 111 and TE 113 modes of rectangular DRA are excited by microstrip line through a slot etched on ground plane to achieve wideband CP operation.Ansoft HFSS [20] was used to simulate the performances of the proposed DRA.Parametric studies have also been carried out to investigate the DRA.To verify the simulation, a prototype antenna was fabricated and measured.Reasonable agreement between the simulated and measured results was attained.The measured results show a wide 3 dB AR bandwidth of 33.0% and a wide impedance bandwidth (| 11 | < −10 dB) of 51.4%.

Antenna Structure
Geometry of the single-point-fed wideband CP rectangular DRA is shown in Figure 1.The rectangular DRA has dimensions of  =  = 22 mm and  = 40 mm, and it is manufactured from a ceramic material with permittivity of   = 10.The substrate (  = 2.2) has a side length of   = 50 mm and a thickness of  = 1 mm.The coupling slot etched on the ground plane consists of a monofilar-spiral-slot and four linear slots.Resonant frequencies of TE 111 and TE 113 modes of the rectangular dielectric resonator (DR) can be calculated by using dielectric waveguide model (DWM) [21] or Ansoft HFSS.The calculated results of the DR in proposed antenna are shown in Table 1.Discrepancies between the two results are caused by error of the DWM.

Results
To verify the design, the proposed single-point-fed wideband CP rectangular DRA was fabricated and measured.Photographs of the fabricated antenna are shown in Figure 2. To avoid air gap between the DR and ground plane, the resistor is soldered at the bottom of the substrate by two via holes.Simulated and measured reflection coefficients of the proposed antenna are shown in Figure 3   Figure 4 shows the simulated and measured broadside ARs of the proposed antenna.The simulated and measured 3 dB AR bandwidth are 31.2%(2.19-3.00GHz) and 33.0%(2.15-3.00GHz), respectively.From Figure 4, two simulated minimum AR values of 1.7 dB and 0.9 dB are obtained at 2.42 GHz and 2.90 GHz, respectively.Simulated H-fields in the proposed DRA at 2.42 GHz and 2.90 GHz are shown in Figures 5(a) and 5(b).H-field distributions demonstrate that the TE 111 mode is excited at 2.42 GHz and the TE 113 mode is excited at 2.90 GHz. Figure 6 shows the simulated and measured patterns of the proposed single-point-fed wideband CP rectangular DRA.The radiation patterns of proposed antenna are broadside as expected, because both of the TE 111 and TE 113 are broadside modes.And the antenna is RHCP in broadside direction across the CP bandwidth.Figure 7 shows the simulated and measured RHCP gains of the proposed DRA.The measured peak RHCP gain is 6.8 dBi at 2.70 GHz.And the measured result shows an average RHCP gain of 5.4 dBi across the 3 dB AR bandwidth.

Parametric Studies
Effects of the resistor on antenna performances are investigated by HFSS simulations.Simulated AR of the proposed DRA without resistor loading is shown in Figure 4.The DRA without resistor has a 3 dB AR bandwidth of 6.1% (2.72-2.89GHz).And the 3 dB AR bandwidth of the loaded one is about 5.0 times of the unloaded one.Simulated reflection coefficient of the unloaded one is shown in Figure 3.The resistor has a small effect on the reflection coefficient of the proposed antenna.Simulated total efficiencies of the proposed DRA with and without resistor loading are shown in Figure 8.The efficiency of the loaded one is smaller than the unloaded one because of the loss introduced by the resistor.The minimum efficiency and average efficiency of the loaded one across the CP bandwidth are 0.62 and 0.89, respectively.Figure 9 shows the simulated RHCP and LHCP gains of the proposed DRA with and without resistor loading.From Figure 9, the LHCP component drops a lot when the antenna is loaded, and the resistor has a small effect on the RHCP component of the proposed antenna.
To explain the effect of resistor on antenna performance, the electric field  →   in the slot is studied.Figure 10 shows the simulated  →   at  = 2.4 GHz and  = 2.9 GHz in different time steps for the antenna without resistor loading. →   is strong at the position of the resistor for  = 2.4 GHz, while it is weak at the position of the resistor for  = 2.9 GHz.This means that some power will be dissipated in the resistor at 2.4 GHz and very little power will be lost in the resistor at 2.9 GHz when the antenna is loaded, and that is why the resistor has effect on only lower band of the antenna.Moreover, it can be seen that the equivalent magnetic current (   →   =  →   × Ẑ) in the monofilar-spiral-slot at 2.4 GHz is in a travelling waveform.The resistor, which acts as a resistive termination for the monofilar-spiral-slot, can improve travelling wave performance of the equivalent magnetic current and thus the CP performance at the lower band.Effects of the location of resistor on antenna performances have also been studied.The 3 dB AR bandwidth as function of   is shown in Figure 12, and it can be observed that CP bandwidth of the proposed antenna increases with the increase of   .Minimum total efficiency and average total efficiency across the 3 dB AR bandwidth versus   are also exhibited in Figure 12.From Figure 12, both of the minimum and average total efficiency decrease with the increase of   .Proper value of   can be decided based on Figure 12 and the special design objectives.
Comparison between the proposed antenna and some of the recently published single-point-fed CP DRAs is listed in Table 2.It is seen that the proposed antenna has a wide bandwidth and reasonable antenna efficiency.

Conclusion
A single-point-fed wideband CP rectangular DRA has been proposed and discussed in this letter.The design concept is based on utilizing the fundamental TE 111 and high order TE 113 modes of rectangular DRA.Proposed design concept has been certified by HFSS simulation.And parametric studies have been carried out to investigate effects of the resistor on antenna performances.A prototype antenna was designed and fabricated.The measured 3 dB AR bandwidth and impedance bandwidth (| 11 | < −10 dB) are as wide as 33.0% and 51.4%, respectively.

Figure 9 :
Figure 9: Simulated gains of the proposed DRA with and without resistor loading.(a) RHCP gains.(b) LHCP gains.

Figure 12 :
Figure 12: Effects of   on AR and total efficiency of the proposed antenna.
. Measured impedance bandwidth (| 11 | < −10dB) is 51.4% (1.91-3.23 GHz), and it agrees well with the simulated one.With reference to Figure 3, there are four resonant frequencies found at 2.24 GHz, 2.44 GHz, 2.84 GHz, and 3.16 GHz across the measured impedance bandwidth.The first couple of resonant frequencies are due to the TE 111 modes, and the second couple of resonant frequencies are caused by the TE 113 modes.

Table 1 :
Calculated resonant frequencies of TE 111 and TE 113 modes of the proposed rectangular DR.
Figure 7: Simulated and measured RHCP gains of the proposed single-point-fed wideband CP rectangular DRA.Figure 8: Simulated total efficiencies of the proposed DRA with and without resistor loading.

Table 2 :
Comparison between the proposed antenna and other published single-point-fed CP DRAs.