Microstrip Choke
5.1 Design Theory
We will design a multiple-band antenna for GSM 900/1800/1900 and 802.11 b/g applications. The design idea is similar to the dual-band antenna in the previous chapter. We choose the monopole to resonant at 1.8 or 1.9 GHz and let the choke have infinite impedance at 1.8 GHz or 1.9 GHz. Adding a tuning metal line makes the electrical length from the input of the monopole to the metal line end equals
λ / 4
of the 0.9 GHz. Then we add a branch line to the monopole to resonant at 2.4 GHz.There will be a higher order radiation mode of 0.9 GHz in the neighborhood of the 1.8 GHz to 1.9 GHz band. It is important to tune these modes to improve the bandwidth of the 1.8 GHz to 1.9 GHz band.
5.2 Simulated and Measured Results
Figure 5-1 shows the photography of the multiple-band antenna accomplished on a FR4 substrate, whose dielectric constant is 4.7, loss tangent is 0.02, and thickness is 0.8 mm. The feeding line is 50Ω microstrip line. The EM numerical simulators are Zeland IE3D and Ansoft HFSS. We fabricate the antenna in a
26 mm × 25 mm
region surrounding by ground plane. The antenna size is26 mm × 17 mm
. The monopole part of the multiple-band antenna is the inverted-L antenna. The choke is a line structure and the tuning metal line is a spiral shape. The branch for 2.4 GHz is a “quasi-hook antenna” structure. Sine the antenna is a complex structure and each part is quiteclosed to each other, the coupling may induce a higher order radiation mode in the neighborhood of the 1.8 GHz to 1.9 GHz band. Figure 5-2 is the simulated and measured return loss of the multiple-band antenna. It is seen that the multiple-band antenna does exhibit four resonant bands at 0.9 GHz, 1.8 GHz, 1.9 GHz and 2.4 GHz.
The 6 dB bandwidth at 0.9 GHz is 80 MHz, which is from 0.875 GHz to 0.955 GHz.
The VSWR=2.5 bandwidth at 1.8 GHz and 1.9 GHz is 390 MHz, which is from 1.71 GHz to 2.1 GHz. The 10 dB bandwidth at 2.4 GHz is 350 MHz, which is from 2.36 GHz to 2.61 GHz. Figure 5-3 to Figure 5-6 are the current density distributions of the multiple-band antenna in each band. In Figure 5-3 and Figure 5-5, we can find a dual-band performance because of a microstrip choke. In Figure 5-4, we can find the 1.8 GHz resonant point is the higher order mode of the 0.9 GHz and this resonant point improves the bandwidth of the GSM 1800/1900 band. In Figure 5-6, we can find the branch line is dominant the radiation performance of 2.4 GHz band. Figure 5-7 shows the measured radiation patterns of the multiple-band antenna at 0.92 GHz. The maximum gain and average gain of X-Z, Y-Z and X-Y plane are listed below:
X-Z plane Y-Z plane X-Y plane
Maximum Gain 1.50 dBi 1.70 dBi -0.63 dBi
Average Gain -0.57 dBi -1.08 dBi -1.91 dBi
Table 5-1: The maximum and average gain of the multiple-band antenna in the X-Z, Y-Z and X-Y plane at 0.92 GHz.
Figure 5-8 shows the measured radiation patterns of the multiple-band antenna at 1.795 GHz. The maximum gain and average gain of X-Z, Y-Z and X-Y plane are listed below:
X-Z plane Y-Z plane X-Y plane
Maximum Gain -1.43 dBi 0.68 dBi -0.63 dBi
Average Gain -1.40 dBi -1.13 dBi -1.91 dBi
Table 5-2: The maximum and average gain of the multiple-band antenna in the X-Z, Y-Z and X-Y plane at 1.795 GHz.
Figure 5-9 shows the measured radiation patterns of the multiple-band antenna at 1.92 GHz. The maximum gain and average gain of X-Z, Y-Z and X-Y plane are listed below:
X-Z plane Y-Z plane X-Y plane
Maximum Gain 1.54 dBi 2.66 dBi 2.64 dBi
Average Gain -0.14 dBi 0.26 dBi -0.33 dBi
Table 5-3: The maximum and average gain of the multiple-band antenna in the X-Z, Y-Z and X-Y plane at 1.92 GHz.
Figure 5-10 shows the measured radiation patterns of the multiple-band antenna at 2.44 GHz. The maximum gain and average gain of X-Z, Y-Z and X-Y plane are listed below:
X-Z plane Y-Z plane X-Y plane
Maximum Gain 1.18 dBi 1.46 dBi 4.22 dBi
Average Gain -0.63 dBi -0.36 dBi -0.84 dBi
Table 5-4: The maximum and average gain of the multiple-band antenna in the X-Z, Y-Z and X-Y plane at 2.44 GHz.
5.3 Analysis
This multiple-band antenna is similar to the dual-band antenna discussed in Chapter 4. But it is more difficult to design since the wavelength of 0.9 GHz is too long. For 0.9 GHz, this size will perform like a short-dipole antenna and have higher Q-value. So the bandwidth is narrower. It is the most serious problem to overcome.
Decreasing the complexity of the antenna structure or a bit magnifying the antenna size will improve the bandwidth of the antenna at 0.9 GHz band. Similar to the dual-band antenna, the design flow shown in Chapter 4 is also suitable for the antenna.
Figure 5-11 shows the simulated result of the multiple-band antenna without the 2.4 GHz branch from the design flow. We can find the result has less offset than the 2.4 GHz and 5.2 GHz dual-band antenna discussed in Chapter 4. The result from the design flow is more accurate at lower frequency. If we remove the inverted-L antenna, the choke and the tuning metal line, the branch is like a bent monopole antenna. The dash line in Figure 5-12 is the simulated return loss of the bent monopole antenna. Its resonant frequency is a bit lower than the highest resonant frequency of the multiple-band antenna but it has a broad bandwidth. So the branch can dominant the 2.4 GHz radiation performance. If we add the inverted-L antenna to the bent monopole antenna, the dash-dot line is the simulated return loss. It is a dual-band antenna at 1.9 GHz and 2.4 GHz.
(a) Front side (b) Back side Figure 5-1: Photography of the multiple-band antenna
Frequency (GHz)
0.50 0.75 1.00 1.25 1.50 1.75 2.00 2.25 2.50 2.75 3.00
S11 (dB)
-50 -40 -30 -20 -10 0
0.92 GHz
1.77 GHz
2.44 GHz 1.9 GHz
Figure 5-2: Simulated and measured return loss of the multiple-band antenna
(a) Front side (b) Back side
Figure 5-3: Current distribution of the multiple-band antenna at 0.9 GHz band
(a) Front side (b) Back side
Figure 5-4: Current distribution of the multiple-band antenna at 1.8 GHz band
(a) Front side (b) Back side
Figure 5-5: Current distribution of the multiple-band antenna at 1.9 GHz band
(a) Front side (b) Back side
Figure 5-6: Current distribution of the multiple-band antenna at 2.4 GHz band
X-Z Plane
Figure 5-7: Radiation patterns of the multiple-band antenna at 0.92 GHz E-total
E-phi E-theta
X-Z Plane
Figure 5-8: Radiation patterns of the multiple-band antenna at 1.795 GHz E-total
E-phi E-theta
X-Z Plane
Figure 5-9: Radiation patterns of the multiple-band antenna at 1.92 GHz E-total
E-phi E-theta
X-Z Plane
Figure 5-10: Radiation patterns of the multiple-band antenna at 2.44 GHz E-total
E-phi E-theta
Figure 5-11: Result of the dual-band antenna section from the design flow
Frequency (GHz)
0.50 0.75 1.00 1.25 1.50 1.75 2.00 2.25 2.50 2.75 3.00
S11 (dB)
-50 -40 -30 -20 -10 0
Multi-band antenna Dual-band antenna The bent monopole
The inverted-L antenna with the bent monople
Figure 5-12: Comparison with each section of the multiple-band antenna
Chapter 6: Conclusions
In this thesis, three novel printed antennas fabricated on the FR4 substrate are demonstrated.
The printed inverted-E antenna has been reported in Chapter 3. The bandwidth of this compact antenna is 335 MHz. The printed inverted-E antenna has omni-directional radiation patterns. The maximum gain is 2.08 dBi and average gain of certain plane can reach 0.04 dBi. If we add a protecting case to the antenna, the bandwidth is slightly reduced to 240 MHz. The radiation patterns almost keep omni-directional. The maximum gain is 1.43 dBi and average gain of certain plane can reach -0.51 dBi.
The dual-band antenna utilizing a microstrip choke has been reported in Chapter 4. It is a novel antenna to operate in dual-band. The bandwidth of the dual-band antenna in lower band is 350 MHz and that in higher band is 1.05 GHz. The radiation patterns in lower band are similar to a typical monopole antenna and that in higher band are nearly omni-directional. The maximum gain in the lower band is 2.43 dBi, in the 5.25 GHz band is 4.58 dBi and in the 5.77 GHz band is 3.78 dBi. The average gain of certain plane can reach -1.17 dBi at lower frequency, 0.03 dBi in 5.25 GHz band and -0.07 dBi in 5.77 GHz band. The bent metal line can reduce the size of the antenna. The bandwidth of the modified antenna is 520 MHz in lower band and 1.12 GHz in higher band. The maximum gain in the lower band is 3.59 dBi, in the 5.25 GHz band is 6.6 dBi and in the 5.77 GHz band is 6.39 dBi. The average gain of certain plane can reach -1.17 dBi at lower frequency, 0.44 dBi in 5.25 GHz band, and 0.50 dBi in 5.77 GHz band.
The multiple-band antenna utilizing a microstrip choke has been reported in Chapter 5. It is a novel antenna to operate in multiple-band. The bandwidth of the
multiple-band antenna in 0.9 GHz band is 80 MHz, in 1.8GHz to 1.9GHz band is 390 GHz and in 2.4 GHz band is 350MHz. The maximum gain in the lowest band is 1.70 dBi, in the 1.8 GHz band is 0.68 dBi, in the 1.9 GHz band is 2.66 dBi and in the 2.4 GHz band is 4.22 dBi. The average gain of certain plane can reach -1.08 dBi in 0.9 GHz band, -1.13 dBi in 1.8 GHz band, 0.26 dBi in 1.9 GHz band and -0.36 dBi in 2.4 GHz band.
The design of antennas is an art of elaboration. Several recommendations for the design of antennas are dedicated in the following:
1) The simulated and measured results in return loss may have some frequency offsets. But the errors are nearly the same for each experiment. The designers should use the magnitude of the errors to calibrate the offsets between the simulated and measured results so that they can achieve the measured results in expected band.
2) The simulated radiation patterns using HFSS, instead of IE3D, have extremely correspondence to the measured ones.
3) The fabrication of the via-holes on the antennas has enormous influence on the impedance matching of the antennas. The solder on the via-holes must be flatted to the metal.
These recommendations are extremely valuable for the design of antennas. The author hopes that the antennas designers should adopt these as the best regards.
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