Chapter 3 Miniaturized Antennas with Circuit Resonances
3.2 One-Eighth Effective Wavelength Slot Antenna
3.2.2 One-Eighth Effective Wavelength Slot Antenna Design
Feed
C
M2C
M1w
Sl
Ml
TFeed Ref.
plane
Ref.
plane
l
Sw
Sl
Tl
S(a) (b)
Figure 3.18 (a)The configuration of a conventional microstrip-fed slot antenna with open end. (b)The configuration of the proposed slot antenna. Both the configurations have the identical slot size.
Z
0, θ
SCZ
0, θ
OCR
AC
AC
1L
S FeedC
2Figure 3.19 The corresponding equivalent circuit model of the antenna configuration in Figure 3.18.
Based on the equivalent circuit of the directly-fed open-end slot antenna, the
conventional slot antenna with microstrip coupled structure was then studied. Figure
3.18(a) shows the conventional microstrip-fed slot antenna, where a 50Ω microstrip
line is used to couple the input power to the open-end slot radiator. As well known, its
operation frequency is mainly determined by the slot length l
S, which is quarter
wavelength at the resonant frequency. The feeding position l
Mand open stub length l
Tcan be designed to achieve critical coupling for input impedance matching. Also,
Figure 3.18(b) shows the configuration of the proposed antenna, where two capacitors,
C
M1and C
M2, are loaded to the 50Ω microstrip feeding line. With choosing the proper
values, the antenna can operate below the resonant frequency of the slot. Both types
utilize capacitive coupling for feeding the slot. Therefore, there will be a series
capacitor between the feeding port and the slot for equivalent circuit. Considering
the shunt capacitor CM1 takes the same place of C1 in equivalent circuit. CM2 and the open stub are same for the capacitive coupling, which is a series capacitor in equivalent. The conventional type and proposed type share the same equivalent circuit model, as shown in Figure 3.19.
In this study, the identical slot, ls = 9 mm and ws = 1.5 mm, for the conventional and proposed design were designed and fabricated on 0.4 mm FR4 substrate for comparison. The same ground plane size of 65 mm by 42 mm and 50 Ω microstrip line are used for both antennas with the slot radiator located in the middle of the long edge of the ground plane. The conventional type utilizes the natural resonance of the slot that possesses the resonant frequency determined by the slot length. lM = 3.2 mm and lT= 1.1 mm are designed for impedance matching. The proposed type is designed to operate in 2.45GHz, in purpose, by using two SMD capacitors loading on the feeding line. The capacitors CM1 and CM2 are properly designed as 2.4 pF and 0.6 pF by using HFSS simulation.
Figure 3.20 is the comparisons of the results of circuit model calculation and full-wave simulation for both conventional type and proposed antenna. Since the identical slot is used, the circuit model parameters of the slot are the same as that of the directly-fed slot, Z0 = 140 Ω, RA = 650 Ω, CF = 0.12 pF, and θSC + θOC = 39o. For the conventional type, equivalent θOC is 26o for lM = 5 mm, θSC is 13o, and C2 = 0.26 pF. For the proposed type, equivalent θOC is 3o for lM = 0.5mm, θSC is 39o, C2 = 0.63 pF, and C1 = 2.35pF. The results of full wave simulation and equivalent circuit calculation match each other very well. This approves the accuracy the circuit model. It can be found that the conventional type possesses an anti resonance around 5GHz, which is the natural resonant frequency of the slot length. In the other hand, the purposed type has a resonance at 2.45GHz for the operational frequency, i.e.
the slot antenna is miniaturized. The operation frequency is at 2.45GHz that is half of the conventional although identical slot size is used, as shown in Figure 3.21.
The conception of using the loading capacitors for impedance matching is quite intuitive. Since the open-end slot antenna is short as compared to the wavelength at 2.45 GHz, it behaves as an inductive component as seen from the feeding microstrip line. Therefore, by the proper design of C1 and C2, it is possible to match the short
C
1 and C2 can be easily found.1.5 2.5 3.5 4.5 5.5
Frequency (GHz)
Input impedance – Conventional type
-400 -300 -200 -100 0 100 200
Circuit model HFSS simulation
Real part Imaginary part
Circuit model HFSS simulation
Real part Imaginary part
(ohm)
(a)
1.5 2.5 3.5 4.5 5.5
Frequency (GHz)
Input impedance – Proposed type
-100 -50 0 50 100
Circuit model HFSS simulation
Real part Imaginary part
Circuit model HFSS simulation
Real part Imaginary part
(ohm)
(b)
Figure 3.20 The comparison of input impedance between circuit model calculation and full-wave simulation. (a)The conventional open-end slot antenna. (b)The proposed open-end slot antenna.
3.2.3 Experimental Results of One-Eighth Effective Wavelength Slot Antenna
For experiment, the antennas were fabricated and measured. The Murata SMD capacitors of 0.6 pF and 2.2 pF with part number GRM1555C1HR60CZ01 and GRM1555C1H2R2CZ01 are used in the proposed antenna for CM2 and CM1. A 50Ω cable is connected to the microstrip line for measurement. Figure 3.21 shows the simulated and measured return losses of the conventional and proposed open-end slot antennas. The measured 10-dB bandwidth of the proposed type is from 2.4 to 2.509 GHz that is sufficient for 802.11b/g application. The conventional type possesses quarter wavelength resonance operates around 4.8GHz twice than the proposed type.
Therefore, the proposed type can be treated as roughly one-eighth effective wavelength open-end slot antenna. Figure 3.22 shows the radiation patterns of the proposed one-eighth effective wavelength slot antenna at 2.45GHz. The measured peak gain is 1.89dBi on yz-plane. The averaged gains are -3.8dBi, -1.1dBi and -1.2dBi on xy-, xz-, yz-plane respectively. It means the radiation efficiency is fairly good although the slot does not resonate at the operation frequency. In addition, since the resonance of the antenna is determined by the circuit synthesis, which is an inductive slot and two capacitors, the resonant frequency should be stable against ground size, as mentioned in Chapter 3.1.3. Figure 3.23 shows the measured return losses a set of antennas with the identical slot and capacitors but with different ground sizes was fabricated and measured. The resonant frequencies are the same against different ground sizes, as prediction.
1.5 2.5 3.5 4.5 5.5
Figure 3.21 The simulated and measured return losses of the proposed antenna.
-30 -25 -20 -15 -10 -5 0 5
Figure 3.22 The measured radiation patterns of the proposed antenna.
Type C
(170 mm X 250 mm) Laptop panel size
Type A
(20 mm X 50 cm) USB dongle size
Type B
(40 mm X 70 cm)
(a)
1 2 3 4 5
Frequency (GHz)
Return Losses
20 15 10 5 0
Type A Type B Type C
(dB)
1 2 3 4 5
Frequency (GHz)
Return Losses
20 15 10 5 0
Type A Type B Type C Type A Type B Type C
(dB)
(b)
Figure 3.23 (a)One-eighth wavelength antennas with identical slot and capacitors with different ground sizes. (b)The return losses of one-eighth wavelength antennas with identical slot and capacitors with different ground sizes.
3.2.4 Summary
The input impedance of directly-fed open-end slot antennas has been explained by a equivalent circuit model. The results of circuit model calculation quite agree the result of full wave simulation over a wide bandwidth. Based on the slot model, the circuit model of conventional quarter wavelength open-end slot antenna was also developed. Also, using the same circuit model, we proposed a one-eighth wavelength slot antenna. The proposed antenna utilizes a short open-end slot, roughly one-eighth effective wavelength in 2.45GHz, with two chip capacitors. The values of chip capacitors can be designed through the circuit model. In operational frequency where the antenna is matched, the slot possesses inductive impedance that does not resonate.
The comparison between the conventional and the proposed slot antennas using identical slot size was given. The results of the circuit model calculation, full-wave simulation and measurement show agreements. In this study, the proposed antenna designed to operate at 2.45GHz has measured 10dB return loss of 109MHz. The antenna size is only 9 mm by 1.5 mm. Measured radiation pattern is omni-directional with peak gain of 1.89 dBi that means the efficiency is still fairly good even the slot line does not resonate.