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REPLACE THIS LINE WITH YOUR PAPER IDENTIFICATION NUMBER (DOUBLE-CLICK HERE TO EDIT) < II. DESIGN OF MS ANT ENNAS The MS antenna proposed here is composed of a MS and a source antenna, both designed using planar technology. The source antenna generates a LP signal which is converted into a CP signal through the use of the MS. Fig. 1 (a) shows the geometry of the unit cell used in our MSs. Each unit cell is a rectangular loop with a diagonal microstrip. The MSs consist of 16 unit cells in a 4×4 arrangement as shown in Figs. 1(b) and 1(c). The right-handed metasurface (RH-MS) shown in Fig. 1(b) is used to convert a LP signal into a RHCP signal, while the left-handed metasurface (LH-MS) shown in Fig. 1(c), having a mirror image of the RH-MS, is used to convert a LP signal into a LHCP signal.
(a) (b) Fig. 1 (a) Unit cell, (b) RH-MS, and (c) LH-MS
(c)
Two source antennas, a simple patch antenna and a simple slot antenna, are used in our studies to design our MS antennas. Figs. 2 show one of our proposed MS antennas using a simple patch antenna as the source antenna to generate LP signal for conversion. The patch antenna with microstrip-fed, as shown in Fig. 2(a), has a rectangular patch with a size of Wp×Lp=53×32 mm2 printed at the centre of the substrate on one side and a ground on the other side. The microstrip feed line has a two-stage /4-impedance convertors for better impedance matching in the presence of the MS. The MS and patch antenna, as shown in Fig. 2, are designed on two Rogers substrates, RO4350B, having the thicknesses of t1=0.762 mm and t2=1.524 mm, respectively, with a dielectric constant of r=3.48 and a lost tangent of =0.0031. They are separated by a cavity height of h 1=6 mm and, for convenience in implementation, are designed to have the same area of Ts×Ts=120×120 mm2. Results have shown that, when the LH-MS shown in Fig. 1(c) is used as the MS in Fig. 2(b), the MS antenna will convert the LP signal generated from the patch antenna into a LHCP signal. This MS antenna is denoted here as MS Ant 1. However, if the LH-MS shown in Fig. 1(b) is used instead, the MS antenna will convert the LP signal into a RHCP signal and the MS antenna is denoted here as MS Ant 2. Since MS Ant 1 is identical to MS Ant 2, except that the MSs used are mirror images of each other, it is expected that they will have the same performance in terms of RLBW, ARBW and realized gain, but orthogonal polarizations. These will be verified by simulation and measurement described later. Another source antenna used in our studies is a slot antenna having a slot of Ls×Ws etched at the centre on one side (the ground plane) of the substrate as shown in Fig. 3(a). A 50-Ω feed line is printed on the other side of the substrate. The length Lf
2
and width Wf of the feed line are used for impedance matching. The MS and slot antenna are separated by a cavity height of h 2=7 mm and, again, for convenience, both the MS and slot antenna are designed to have the same area of 120×120 mm2 and on the Rogers substrates, RO4350B, having the electrical parameters described previously. Simulation results have shown that using the RH-MS and LH-MS shown in Figs. 1(b) and 1(c) as the MS in Fig. 3(b) will generate a RHCP and a LHCP signals, respectively, and the corresponding MS antennas are denoted here as MS Ants 3 and 4.
(a) (b) Fig. 2 (a) LP patch antenna (source antenna) and (b) side view of MS antenna
(a) (b) Fig. 3 (a) LP slot antenna (source antenna) and (b) side view of MS antenna
Simulation studies have showed that the four MS antennas can generate CP signals. More studies have showed that, with the use of the MSs, the four MS antennas always have two lowest dips in axial ratio (AR) at around 2.4 and 2.6 GHz, regardless of which source antenna is used. Moreover, the return-loss bandwidths (RLBWs) for RL > 10 dB are much wider than the axial-ratio bandwidths (ARBWs) for AR < 3 dB which therefore determine the operating bandwidths of the antennas . Since the main function of our MS antennas is to convert LP signals into CP signals, the performances of these antennas have been optimized in terms maximizing the ARBW using various parameters of the MS through computer simulation. In the optimization process, we maximized the spacing between the two dips, yet keeping the AR below 3 dB between them. The optimized dimensions of the MS, and the dimensions of the patch antenna and slot antenna are listed in Tables I, II and III, respectively, which are used to fabricate the MSs, patch antenna and slot antenna as shown in Figs. 4 and 5. T ABLE I DIMENSIONS OF MS (UNIT:MM) Px
Py
a
B
w1
w2
Ts
26.25
24.15
23.1
22.05
1.05
1.26
120
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> REPLACE THIS LINE WITH YOUR PAPER IDENTIFICATION NUMBER (DOUBLE-CLICK HERE TO EDIT) < T ABLE II DIMENSIONS OF P ATCH ANTENNA (UNIT: MM) Wp Lp wf1 wf2 wf3 lf1 lf2 lf3 53
32
2
1
4.8
17
17
10
T ABLE III DIMENSIONS OF SLOT ANTENNA (UNIT: MM) Ws Ls Lf Wf Hs 5.8
34
2.5
3
57.1
(a) (b) Fig. 4 (a) LHCP metasurface (LH-MS) and (b) RHCP metasurface (RH-MS)
3
AR=0 dB, and the peak shoots up to 7 dB at 2.5 GHz. When a is decreased to 22.5 mm, the two dips merge together with a minimum AR of 1 dB at around 2.6 GHz. Thus a can also be used to adjust the ARBW by using the lower-frequency dip. Fig. 6(d) shows that the parameter b affects the higher-frequency dip. When b is decreased from 21.05 to 20.5 mm, the AR of the lower-frequency dip decreases to 0.1 dB with frequency remaining at around 2.42 GHz and the higher-frequency dip shifts up to 2.7 GHz with AR=1 dB. The peak goes slightly higher to 5 dB at 2.6 GHz. When b is increased from 21.05 to 21.5 mm, the higher-frequency dip shifts down to 2.55 GHz, but the peak shoots up to 14 dB at 2.5 GHz. So b can be used to adjust the ARBW by moving the higher-frequency dip. Fig. 6(e) shows that w1 affects both the higher- and lower-frequency dips. When w1 is decreased from 1.05 to 0.85 mm, both higher- and lower-frequency dips shift down to about 2.38 and 2.55 GHz, respectively. The peak is about 3 dB at 2.48 GHz. When w1 is increased to from 1.05 to 1.25 mm, the two dips shift up to 2.42 and 2.63 GHz. In these two cases, the peak is about the same at 2.8 dB, so w1 can also be used to adjust the frequencies of both dips.
(a) (b) Fig. 5 (a) Microstrip-fed patch antenna and (b) line-fed slot antenna
III. ST UDY OF M ET ASURFACE From the optimization process, it is found that the parameters Px , Py , a, b and w1 are most sensitive to the two lowest ARs (dips) at around 2.4 and 2.6 GHz, regardless which source antenna is used. Thus a parametric study is carried out on the effects of these parameters on the AR using MS Ant 3 and results are shown in Fig. 6(a), (b), (c), (d) and (e). In these figures, the blue lines show the optimized performance using Px =26.25, Py =24.15, a =23.1, b =22.05 and w1=1.05 mm in the MS. It can be seen that the AR has two lowest dips of 1 and 0 dB at 2.42 and 2.58 GHz, respectively, and a peak of 2.8 dB at 2.52 GHz between the two dips. Fig. 6(a) shows that, when Px is decreased from 26.25 to 25.75 mm, the two dips move away from each other to 2.32 and 2.6 GHz with AR= 1 and 2.5 dB, respectively, but the peak shoots up to 7 dB at 2.5 GHz between the two dips. When Px is increased to 26.75 mm, the two dips merge together with a minimum AR of 1 dB at 2.54 GHz. Thus Px can be used to adjust the ARBW. When Py is increased from 24.15 to 24.65 mm, Fig. 6(b) shows that again the two dips move away from each other with AR=0 and 0.2 dB at 2.4 and 2.66 GHz, respectively, and the peak shoots up to 7 dB at 2.56 GHz. When Py is decreased to 23.65 mm, the two dips move towards each other, but the peak shoots up to 13 dB at 2.55 GHz. So Py has the similar effect to Px on the ARBW, but in an opposite way. When a is increased from 23.1 to 23.5 mm, Fig. 6(c) shows that only the lower-frequency dip shifts down to 2.3 GHz. The higher-frequency dip remains unchanged at 2.58 GHz with
(a)
(b)
(c)
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10 dB) of only about 50 MHz (a fractional bandwidth of only 2%). With the use of the MS, the RLBWs are substantially increased to 380 MHz (a fractional bandwidth of 15.5%), from 2.3 to 2.68 GHz, for MS Ant 1, and to 430 MHz (17.6%), from 2.27 to 2.7 GHz, for MS Ant 2. Note that both MS antennas have similar performances (but different polarizations as shown later) for the reason described previously. For the slot antenna alone, Fig. 11 shows that the measured RLBWs are only about 200 MHz. However, when the MS is used, the RLBWs for MS Ants 3 and 4 are substantially increased to 630 MHz (25.7%), from 2.05 to 2.68 GHz, and to 615 MHz (25.1%), from 2.075 to 2.69 GHz, respectively. Again, both MS antennas have similar performances (but orthogonal polarizations as shown later). These results show that the RLBWs can be substantially increased by using the MSs. The reason for the improvements of RLBW, in fact, can be seen in the results of Figs. 10 and 11, which indicate that the source antennas have only one resonance in the frequency band from 2 to 3 GHz, but the MS antennas have more than one resonance across the same frequency band. Thus to increase the RLBW, the dimensions of MS antenna should be adjusted to generate more resonances at the appropriate frequencies.
(a)
(b) (c) Fig. 9 Equivalent circuits of (a) new unit cell with diagonal strips removed seen by
E1
seen by
E2 .
and E 2 , (b) new unit cell seen by
E1 , and (c) new unit
cell (a)
V. SIMULAT ION AND MEASUREMENT RESULT S The patch antenna, the slot antenna and the MS antennas have been studied, designed and optimized using computer simulation. For verification of the simulation results, these antennas have been fabricated using our prototype machine and measured using the antenna measurement system, Satimo Starlab. A. Return- Loss Bandwidth The simulated and measured return losses (RLs) of the patch antenna, MS Ants 1 and 2, are shown in Figs. 10, and the RLs of the slot antenna, MS Ants 3 and 4 are in Fig. 11. It can be seen that the simulated results agree well with the measured results. The differences are due to 1) the fabrication tolerances of the prototype machine in our laboratory (which could make the narrowest microstrip line of only 0.2 mm), 2) the alignment tolerance of the source antenna with the MS in assembling the MS antenna, and 3) the measurement uncertainty. Fig. 10 shows
(b) Fig. 10 Simulated and measured RLs of patch antenna only and (a) MS Ant 1 and (b) MS Ant 2
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