Antenna gain and radiation efficiency.
Limited space is given to antennas in modern portable wireless systems, which means that antennas need to be small in size and compact structures. However, shrinkage of conventional antennas leads to performance degradation and complex mechanical assembly. Therefore, the design of miniature antennas for application in wireless communication systems is highly challenging using traditional means. In this chapter, it is shown that metamaterial (MTM) technology offers a solution to synthesize antennas with a small footprint with the added advantage of low cost and excellent radiation characteristics.
- composite right‐/left‐hand transmission lines
In this chapter, novel and compact planar antennas are presented including ultra‐wideband (UWB) and hexa‐band antennas. The UWB antenna presented in part 2 is based on metamaterial (MTM) transmission lines. In reality, the MTM antenna structure is more accurately described as a composite right‐/left‐handed transmission line (CRLH‐TL) structure due to the resulting parasitic capacitance and inductance effects. The design of the UWB antenna is achieved by embedding E‐shaped dielectric slits in the radiating patches. It is shown that the dimensions of such patches have a considerable influence on the radiation characteristics of the antenna. Parametric study is undertaken to demonstrate how the MTM unit‐cell's parameters affect the antennas performance in terms of gain, radiation efficiency, and radiation patterns. A novel antenna structure referred to as ‘hexa‐band’ coplanar waveguide (CPW)‐fed antenna, which is presented in part 3, consists of asymmetric fork‐shaped‐radiating elements incorporating U‐shaped radiators with a slit. Each of the branched radiators generates triple resonant frequencies within the L‐, S‐, C‐ and X‐bands.
2. Ultra‐wideband antenna
Geometry of an innovative ultra‐wideband (UWB) antenna in Figure 1 shows that it is essentially a patch antenna with a rectangular slot. The antenna is terminated on the right‐hand (RH) side with a grounded 50‐Ω resistive load in order to reduce any impedance mismatch and thereby enhance the antenna's performance. Enclosed inside the slot are three radiating patches on which is etched an E‐shaped dielectric slit. The middle patch of the antenna is excited through a coplanar waveguide (CPW) transmission line. A larger E‐shaped slit is included in the antenna next to the 50‐Ω load, which exhibits characteristics of the simplified composite right‐/left‐handed transmission line (SCRLH‐TL) . In the equivalent circuit of the antenna, the left‐handed capacitance (
The configuration of the E‐shaped slit was determined through simulation analysis. As it will be shown later, the size of the three patches actually has an influence of the antenna gain and efficiency properties. The equivalent circuit of the E‐shaped slit with the absence of left‐handed inductance (L L) and its microstrip structure is shown in Figure 2(a) and (b), respectively. Parameters of the simplified CRLH structure in Figure 2 are given by
The E‐shaped slit structure employed here is shown in Figure 2(b). Based on the resonance frequencies of this structure, the initial magnitude of C L, C R, and L R can be obtained from the dispersion relation of the SCRLH‐TL. This is determined by applying the periodic boundary conditions related to the Bloch‐Floquet theorem, given by
The series impedance of the unit cell is represented by
Negative‐order modes in the SCRLH‐TL structure vanish with the absence of left‐handed capacitance or inductance. Hence, the electrical size of the SCRLH‐TL structure can be reduced by increasing the magnitude of
The antenna was fabricated using standard manufacturing techniques, and its performance measured. The physical dimensions of the antenna are 21.6 × 19.8 × 0.8 mm3, and the corresponding electrical dimensions at 0.7 and 8 GHz, respectively, are 504 × 10-4 λo × 462 × 10-4 λo × 18 × 10-4 λo and 576 × 10-3 λo × 528 × 10-3 λo × 21 × 10-3 λo, where λo is free‐space wavelength. The ground‐plane size of the antenna is 25.2 × 25 mm2.
Anechoic chamber was used to accurately characterize the antenna's performance in terms of its gain and radiation efficiency. This involved applying RF power to the antenna and measuring the EM field radiated from it in the surrounding space. Radiation efficiency was calculated by taking the ratio of the radiated power to the input power of the antenna. Standard gain comparison technique was used to measure the antenna's gain. This involved using a pre‐calibrated standard gain antenna to find the absolute gain of the antenna under test.
Figure 4 shows how the vertical dielectric gap between small patches (
Figure 5 shows the proposed antenna's measured and simulation return‐loss performance, where the impedance bandwidth is defined by S11 ≤-10 dB. For comparison purposes, various EM simulation tools were employed to compute the impedance bandwidth of the antenna. Both HFSS™ and CST Microwave Studio (MWS) are powerful three‐dimensional (3D) EM simulators, where HFSS™ is based on finite element method (FEM) technique and CST MWS is based upon finite integration technique. ADS is 2.5D EM tool based upon method of moment (MoM) technique. The impedance bandwidth predicted by the following: (1) ADS is 7.44 GHz (0.8–8.24 GHz); (2) CST MWS is 7.88 GHz (0.4–8.28 GHz) and (3) HFSS™ is 7.65 GHz (0.55– 8.2 GHz). The correlation between HFSS™ and CST‐MWS is excellent. The measured fractional bandwidth is 167.8%, and the simulated fractional bandwidths are 164.6% (ADS), 181.6%, (CST‐MWS) and 174.9% (HFSS™). The measured impedance bandwidth is 7.3 GHz (0.7–8 GHz). This confirms that the antenna can operate over an ultra‐wideband.
The antenna resonates at two distinct frequencies, as shown in Figure 5, which are measured at 4.75 and 7 GHz. Simulation results predict 4.15 and 6.8 GHz (ADS), 4.03 and 6.68 GHz (CST‐MWS), and 4 and 6.6 GHz (HFSS™). The measured results agree well with the ADS prediction. The divergence in the results is attributed to manufacturing tolerances.
The distribution of the current density over the surface of the antenna at the two resonance frequencies of 4.75 and 7 GHz is shown in Figure 6. The current density distribution is symmetrical with reference to the feed line.
Excitation of the proposed antenna structure though the SubMiniature version A (SMA) connector's feed line can result in an imbalance in the current flow over the outer metallization of the antenna, which can severely undermine the radiation characteristics of the antenna. To prevent this from happening, it was necessary to incorporate three smaller patches in the antenna slot. As is evident in Figure 1, only the middle patch is excited through the feed line, whereas the other two patches are used as parasitic radiators. This configuration facilitates the concentration of the EM fields in the proximity of the antenna structure instead of dispersing the field over its ground plane, which would otherwise contribute in unwanted coupling. Figure 7 shows that a larger number of patches can significantly improve the antenna's bandwidth and gain properties. The antenna was implemented on a RT/duroid® RO4003 substrate with total dimensions of about 20 × 20 mm2. Within this size, the maximum number of inner patch arms was restricted to three. The resulting antenna gain and radiation efficiency at its operating frequency range are given in Table 1.
The radiation patterns of the antenna were simulated using ADS, CST‐MWS and HFSS™. The simulated and measured radiation patterns are shown in Figure 8 at the two resonance frequencies.
The antenna radiates similarly to a dipole antenna with a large coverage angle at the two resonance frequencies. Figure 9 shows the simulated and measured radiation gains of the antenna. The measured radiation gain is greater than 1.2 dBi between 0.7 and 9 GHz, with a peak of 4.2 dBi at 4.8 GHz.
Table 2 shows comparison of the proposed antenna with similar antenna structures published in literature to date. It exhibits the largest fractional bandwidth and highest efficiency. It has the second highest gain and its relatively small size is comparable to most previous works. These results clearly confirm that the antenna is great candidate for ultra‐wideband communication systems.
2.1. Effect of the 50‐Ω load on the antenna bandwidth
The return loss of the antenna with and without the 50‐Ω load is shown in Figure 10. It is evident that the 50‐Ω load enhances the impedance match so that the impedance bandwidth improves from 131.8% (without 50 Ω) up to 164.6% (with 50 Ω). Excellent match is discerned near the two resonance frequencies of the antenna, at 4.8 and 7 GHz, thus enabling effective radiation at the resonance frequencies. These results are summarized in Table 3.
|Freq. (GHz)||Simulation (ADS)||Measured|
|Gain (dBi)||Efficiency (%)||Gain (dBi)||Efficiency (%)|
|Papers||Dimensions at 1 GHz||Bandwidth||Gain (max)||Eff. (max)|
| 4 ||0.047 λo × 0.021 λo × 0.002 λo 14.2 × 6.32 × 0.8 mm3||104.8% (1–3.2 GHz)||2.3 dBi||62%|
| 6 ||0.064 λo × 0.021 λo × 0.0027 λo 19.2 × 6.32 × 0.8 mm3||123.8% (0.8–3.4 GHz)||2.8 dBi||70%|
| 5 ||0.056 λo × 0.02 λo × 0.005λo 16.7 × 7 × 1.6 mm3||82.9% (7.7–18.6 GHz)||3.1 dBi||58%|
| 6 ||0.06 λo × 0.02 λo × 0.027 λo 18 × 7 × 0.8 mm3||74.4% (7.5–16.8 GHz)||2.1 dBi||44%|
| 7 ||0.072 λo × 0.02 λo × 0.005λo 21.7 × 7 × 1.6 mm3||87.2% (7.8–19.85 GHz)||3.4 dBi||68%|
| 8 ||0.075 λo × 0.02 λo × 0.027 λo 22.6 × 7 × 0.8 mm3||84.2% (7.25–17.8 GHz)||2.3 dBi||48%|
|||0.2 λo × 0.05 λo × 0.003 λo 60 × 16 × 1 mm3||116.7% (0.67–2.55 GHz)||4.74 dBi||62.9%|
|||0.06 λo × 0.06 λo × 0.005 λo 18 × 18 × 1.6 mm3||26.5% (1.8–2.35 GHz)||3.69 dBi||20%|
|||0.2 λ o × 0.017λ o × 0.017λ o 60 × 5 × 5 mm3||103% (0.8–2.5 GHz)||0.45 dBi||53.6%|
|||0.06λ o×0.06λ o×0.021λ o 18.2×18.2×6.5 mm3||66.7% (1–2 GHz)||0.6 dBi||26%|
|||0.04λ o×0.04λ o×0.011λ o 12×12×3.33 mm3||8.2% (2.34–2.54 GHz)||1 dBi||22%|
|||0.07λo×0.08λo×0.003λo 20×25×0.8 mm3||8.3% (3.45–3.75 GHz)||2 dBi||27%|
|Proposed antenna||0.072λo×0.066λo×0.002λo 21.6×19.8×0.8 mm3||167.8% (0.7–8 GHz)||4 dBi||80%|
|Unloaded||1.5–7.3 GHz, Δf = 5.8 GHz ≈ 131.8%|
|Loaded||0.8–8.24 GHz, Δf = 7.44 GHz ≈ 164.6%|
2.2. Effect of the SMD load on the radiation performance
Antenna gain and efficiency performance, with and without the 50‐Ω impedance load, are shown in Figure 11, and the results are tabulated in Table 4. These results show that a small improvement is achieved with loading the antenna.
3. Hexa‐band antenna
Configuration of a novel hexa‐band CPW‐fed antenna, shown in Figure 12, comprises three asymmetric fork‐shaped‐radiating stubs with U‐shaped‐radiating elements. Embedded within the U‐shaped elements is a dielectric slit. The tri‐branched radiator generates three distinct resonant frequencies within the L‐, S‐, C‐ and X‐bands. The U‐shaped element also generates resonant frequencies at the lower band of the antenna. The asymmetrical fork‐shaped stubs improve the impedance‐matching characteristics of the antenna and reduce its stopband. The antenna is fabricated on an FR4 substrate with relative permittivity ε r = 4.4 and thickness
The antenna was excited through a 50‐Ω coplanar waveguide transmission line. Figure 13 shows that it resonates at 1.3, 1.75, 3.35, 4.85, 6.5 and 7.6 GHz. There is good agreement between simulated and measured results. Any divergence between the simulated and measured results is ascribed to a number of factors, that is (1) the non‐uniform current density distribution over the antenna structure, (ii) imperfect equivalent circuit models, (iii) undesirable EM coupling and (iv) manufacturing errors. This antenna is shown to function between 700 MHz and 11.35 GHz, and has a fractional bandwidth of 176.76% that includes several communication standards, in particular, GSM, DCS, PCS, Bluetooth, WLAN, WiMAX and WiFi along with the major parts of the C‐ and X‐bands. The measured E‐plane and H‐plane radiation characteristics at 1.3, 1.75, 3.35, 4.85, 6.5 and 7.6 GHz are shown in Figure 14. The measured results show that the antenna radiates omnidirectionally at these frequencies.
In Figure 13, the impedance bandwidth of the lower passband at 1.3 GHz is 800 MHz (0.7–1.5 GHz) with a corresponding fractional bandwidth of 72.72%. The impedance bandwidth of the next passband at 1.75 GHz is 1100 MHz (1.6–2.7 GHz) with S11 better than 15 dB and functions at GSM (upper), DCS, PCS, WiFi (lower), Bluetooth, WiMAX (lower) and WLAN (2.4–2.484 GHz). The impedance bandwidth of the passband at 3.35 GHz is 1.75 GHz (2.85–4.6 GHz) covering the WiMAX (upper) band. The subsequent passbands at 4.85 GHz, at 6.5 GHz and at 7.6 GHz have impedance bandwidths of 1.35 GHz (4.65–6 GHz), 0.95 GHz (6.25–7.2 GHz) and 4 GHz (7.35–11.35 GHz), respectively, covering WiFi (upper) band and significant parts of C‐ and X‐bands. The impedance bandwidth, passband and stopband of antenna‐I are given in Table 5.
|Total Imp. BW||176.76% (0.7–11.35 GHz)|
|Passband 1||72.72% (0.7–1.5 GHz) →f r1 =1.3 GHz|
|Stopband 1||1.51–1.59 GHz|
|Passband 2||51.16% (1.6–2.7 GHz) →f r2 =1.75 GHz|
|Stopband 2||2.71–2.84 GHz|
|Passband 3||46.97% (2.85–4.6 GHz) →f r3 =3.35 GHz|
|Stopband 3||4.61–4.64 GHz|
|Passband 4||25.35% (4.65–6 GHz) →f r4 =4.85 GHz|
|Stopband 4||6.01–6.24 GHz|
|Passband 5||14.12% (6.25–7.2 GHz) →f r5 =6.5 GHz|
|Stopband 5||7.21–7.34 GHz|
|Passband 6||42.78% (7.35–11.35GHz) →f r6 =7.6GHz|
Table 6 shows a comparison of the proposed antenna with similar antennas published in literature. Parameters compared are (i) the number of distinct bands, (ii) the communication standards being covered and (iii) the sizes of the antennas. It is evident that the proposed antenna operates in the following communication bands: GSM (880–960 MHz/1.85–1.99 GHz), DCS (1.71–1.88 GHz), PCS (1.71–1.99 GHz), Bluetooth (2.402–2.480 GHz), WLAN (2.4/5.2/5.8 GHz), WiMAX (2.3–2.4/2.496–2.690/3.3–3.8 GHz), WiFi (2.412–2.4835/4.9–5.9 GHz) and parts of the C‐ and X‐bands. The antenna provides substantially greater coverage than other antennas reported in Refs. [9, 10, 14, 15]. Dimensions of the proposed antenna are comparable to other antennas.
|Multiband antennas||Proposed antenna|||||||||||||||
|Number of separated bands||6||3||3||3||4||3||2||2|
|Max. gain (dBi)||5.3||3.06||2.57||3.6||5.0||6.7||5.5||5.2|
|Dimensions (mm2)||35 × 26||32 × 28||25 × 38||20 × 20||44 × 56||50 × 50||54 × 52||32.5 × 25|
To summarize, a compact antenna comprising three asymmetrical branched fork with U‐shaped strips is shown to exhibit hexa‐band characteristics. The antenna meets multi‐communication standards including L‐, S‐, C‐ and X‐bands for GSM, DCS, PCS, Bluetooth, WLAN, WiMAX and WiFi applications. Slits in the U‐shaped strips are shown to excite additional resonant bands. The proposed structure reduces stopbands and improves the antenna's impedance‐matching performance. The antenna exhibits good return loss, gain and radiation patterns, which makes it an excellent candidate for multiband and broadband communication applications.
The Authors would like to give their special thanks to faculty of microelectronics for the financial support.
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