II. ANTENNA GEOMETRY AND SIMULATION

A) Geometry development from the basic strip monopole

Figure. 1(a), shows the geometry of the basic FGCPW-fed strip monopole antenna, which will be developed into an UWB radiator. By optimizing the parameters of this monopole, various modes can be simultaneously excited and multi-mode operation can be achieved. This fact can be utilized to develop the strip monopole into an UWB antenna. To realize the UWB operation, the strip monopole should be loaded in a manner that lowers the resonant frequencies of the operational mode while increasing their matching bandwidth. In this paper, the proposed approach is to load the basic strip monopole asymmetrically with a rectangular strip making it flag-shaped as shown in Fig. 1(b). To verify the proposed approach, the basic strip monopole and the proposed UWB antenna are both simulated using Agilent's Advanced Design system (ADS) software. The substrate of both the antennas has a relative dielectric constant ε _r = 4.4, thickness h = 0.6 mm, length L _s = 30 mm, and width W _s = 30 mm. In order to have a 50 Ω input impedance, the gap G and the strip width W for both the antennas are calculated as instructed in [Reference Ghione and Naldi14] and are found to be G = 0.35 mm and W = 3 mm, respectively. A ground plane of length L _G = 10 mm and width W _G = 7 mm is found to provide good matching performance. The basic strip monopole is designed to have a strip length L = 18 mm, which corresponds to the first resonance at 3.25 GHz. For the proposed UWB antenna, total strip width W ₁ and the distance T between the loading strip and the ground plane are optimized to achieve the UWB performance. However, the total length L ₁ + T is kept equal to the strip length of the basic monopole. The optimized parameters of the proposed UWB antenna were found to be: W ₁ = 14.5 mm, T = 0.2 mm, and L ₁ = 17.8 mm.

Fig. 1.

Geometry of the FGCPW-fed antennas: (a) the basic strip monopole antenna; (b) the proposed UWB antenna.

B) Simulation results

The simulated reflection coefficients of both antennas are shown in Fig. 2. The multi-band behavior of the basic strip monopole can be observed from Fig. 2 as three resonances centered at 3.25, 8.5, and 13.3 GHz are properly excited. From the reflection coefficient of the proposed antenna in Fig. 2, it can be seen that the proposed loading has lowered the frequencies of the higher two resonances and slightly has increased the first resonant frequency. Also, it has increased the matching bandwidth, and as a result, has achieved the required UWB performance. The current distributions at the three resonances of the flag-shaped UWB antenna are shown in Fig. 3.

Fig. 2.

Simulated reflection coefficients of the FGCPW-fed antennas: (a) the basic strip monopole antenna (antenna 1); (b) the proposed UWB antenna (antenna 2).

Fig. 3.

Simulated current distributions of the proposed UWB antenna at its three resonances.

III. EFFECT OF DESIGN PARAMETERS

In this section, a number of simulated parametric studies are conducted to understand the effect of each design parameter on the proposed antenna's performance as indicated by the reflection coefficient. In each study, only one parameter is varied while the rest are kept unchanged. Also, the value of the varied parameter that offers the best impedance matching will be considered as optimal and will be used in section IV to develop design equations. For the first study, the dimensions are the same as those presented in the previous section.

A) Effect of the loading strip length L₁

In the first study, the effect of the loading strip length L ₁ is investigated. It is varied from 13.8to 21.8 mm in steps of 2 mm. Figure 4 shows the effect of varying the strip length on the reflection coefficient. It can be inferred from Fig. 4 that the increase of L ₁ lowers the frequency of the three resonances. To comply with the FCC regulations, the reflection coefficient of an UWB antenna must be less than −10 dB in the range between 3.1 and 10.6 GHz. However, this is not the case for L ₁ values of 13.8 and 15.8 mm as the reflection coefficient value is above −10 dB at 3.1 GHz for both cases. Although UWB performance is obtained for L ₁ values between 17.8 and 21.8 mm, the value L ₁ = 17.8 mm is selected for compactness.

Fig. 4.

Effect of rectangular strip length L ₁ on the reflection coefficient of the proposed FGCPW-fed UWB antenna.

B) Effect of the total strip width W₁

In the second parametric study, the width of the total strip W ₁ is varied from 12.5 to 16.5 mm in 1 mm steps. The effect of the parameter W ₁ on the reflection coefficient of the proposed UWB antenna is shown in Fig. 5. It can be observed from Fig. 5 that the impedance matching improves with the increasing W ₁ values. It can also be seen that the resonant frequencies are lowered with increasing W ₁ values. The value W ₁ = 14.5 mm offers the best matching at the third resonance whereas W ₁ = 16.5 mm offers the best matching at the second resonance. For compactness, the value of W ₁ is selected as 14.5 mm.

Fig. 5.

Effect of total strip width W ₁ on the reflection coefficient of the proposed FGCPW-fed UWB antenna.

C) Effect of the ground width W _G

In the third and the last parametric study, the width of the ground plane W _G is varied from 5 to 9 mm in 1 mm steps. The effect of variance of this parameter on the reflection coefficient of the proposed UWB antenna is depicted in Fig. 6. It can be seen that the ground width significantly affects the matching bandwidth of the antenna. Poor matching is observed in the lower end of the UWB frequency range for W _G = 5 and 6 mm. On the other hand, poor matching is observed around the middle of the operational frequency range for W _G = 8 and 9 mm. The only value of the selected W _G range that provides a good matching performance is W _G = 7 mm. It is worth mentioning that variation of the parameters L _G and T is of negligible influence on the antenna performance. Thus, the parametric studies of these two were not included.

Fig. 6.

Effect of ground width W _G on the reflection coefficient of the proposed UWB antenna.

IV. DESIGN EQUATIONS

Design equations of the proposed FGCPW-fed UWB antenna are presented in this section. These equations are developed from the parametric studies discussed in the previous section. The goal of developing such equations is to provide reliable initial values of the parameters L _G, W _G, L ₁, L ₂, and T for any desired substrate. It is up to the user then to optimize these values to obtain the best possible performance. The design parameters in the design equations are given as a fraction of the guided wave length λ _g which is given by:

(1)$$\lambda _g=\displaystyle{{\lambda _0 } \over {\sqrt {\varepsilon _{eff} } }}\comma \;$$

where ε _eff is the effective dielectric constant represented by ε _eff = (ε _r + 1)/2, and λ ₀ is the free space wavelength at the center frequency which is set to be in the middle of the UWB band. More specifically, the center frequency f ₀ = 6.25 GHz. The design equations were found to be as follows:

(2)$$L_G=0.38\; \lambda _g\comma$$

(3)$$W_G=0.27\; \lambda _g\comma$$

(4)$$L_1=0.7\; \lambda _g\comma$$

(5)$$W_1=0.55\; \lambda _g\comma \;$$

(6)$$T=0.007\; \lambda _g .$$

To verify the reliability of the proposed design procedure, three antennas with three different substrates are designed using (2)–(6). The computed dimensional parameters of each antenna are given in Table 1.

Table 1.

Antenna description and computed parameters

The reflection coefficients of the three antennas, which are given in Table 1, are shown in Fig. 7. It can be seen that all the antennas exhibit UWB performances that are close to what is required which means that the proposed design equations give good initial values for the design parameters. Obviously, further optimization is needed to reach the targeted UWB performance.

Fig. 7.

Reflection coefficients of the three antennas given in Table 1.

V. EXPERIMENTAL RESULTS

To verify the simulated results a prototype of the proposed FGCPW-fed UWB monopole antenna, which was shown in Fig. 1(b), is fabricated on a 30 × 30 × 1.6 mm³ FR4 epoxy substrate having a relative dielectric constant ε _r= 4.4. The dimensions of the fabricated antenna are the same as those given in Section II which are: L _G = 10 mm, W _G = 7 mm, W ₁ = 14.5 mm, L ₁ = 17.8 mm, W = 3 mm, T = 0.2 mm, and G = 0.35 mm. A photograph of the fabricated UWB antenna is shown in Fig. 8.

Fig. 8.

Photograph of the fabricated FGCPW-fed flag-shaped UWB monopole antenna.

In Fig. 9, the measured and simulated reflection coefficients of the antenna are shown. It can be observed that the simulation and experimental results match very well. The −10 dB bandwidth is in compliance with the FCC regulations and ranges from 3.09 to 12 GHz. Also, it can be seen from Fig. 9 that the three resonances of the fabricated antenna have center frequencies of 3.45, 6.1, and 8.5 GHz, respectively.

Fig. 9.

Simulated and measured reflection coefficient of the proposed FGCPW-fed flag-shaped UWB monopole antenna.

The measured principal plane radiation patterns of the proposed UWB antenna are plotted in Fig. 10. The patterns are measured at the center frequencies of the three resonances. In the E-plane, it is observed that the radiation is nearly omnidirectional, which is a typical feature of monopole antennas. The H-plane patterns, however, vary with frequency in the manner shown in Fig. 10. The proposed antenna exhibits an acceptable cross-polarization performance. It can be seen from Fig. 10 that the cross-polarization levels are generally low and do not exceed −9 dB.

Fig. 10.

Measured radiation patterns of the proposed FGCPW-fed flag-shaped UWB monopole antenna at frequencies: (a) 3.45 GHz, (b) 6.1 GHz, and (c) 8.5 GHz.

The measured gain of the antenna in the entire operational frequency band is shown in Fig. 11. It is observed that an average gain of 2.45 dBi is obtained for the proposed FGCPW-fed flag-shaped UWB monopole antenna. Radiation efficiency of the antenna is also measured using the UWB Wheeler cap method [Reference Schantz15] and is plotted in Fig. 12. An average efficiency of 91% is obtained for the proposed antenna.

Fig. 11.

Measured gain of the proposed UWB antenna.

Fig. 12.

Measured efficiency of the proposed UWB antenna.

VI. TRANSIENT ANALYSIS

In this section, the time-domain characteristics of the proposed antenna are discussed. First, the distortion introduced by the antenna is investigated. This is done by setting up a typical transmitting/receiving antenna system using two identical antennas each of which is the proposed UWB antenna. Then, the transmission coefficient S ₂₁ is measured for the face-to-face and side-by-side orientations of the received antenna. The transfer function H(ω) is calculated from the measured S ₂₁ for each orientation using the following formula [Reference Sörgel and Wiesbeck16]:

(7)$$H\lpar \omega \rpar =\sqrt {\displaystyle{{2{\rm \pi }Rc} \over {j\omega }}S_{21} \lpar \omega \rpar \, e^{\,j\omega R/c} {\rm \; }}\comma \;$$

where R is the far-field distance between the transmitting and receiving antennas, and c is the speed of light in free space. The source pulse is selected to be a fourth-order Rayleigh pulse because its spectrum matches the entire UWB band directly. To qualitatively evaluate the distortion performance, the received pulses at the two orientations need to be calculated. This is done in two steps for each orientation. First, the inverse fast Fourier transform is applied on H(ω) and calculated from (7). The result is then convolved with the source pulse to obtain the received UWB pulse [Reference Chen, Wu, Li, Yang and Chia17]. In Fig. 13, the waveforms of the transmitted and received pulses are shown. By comparing the transmitted and received pulses, it can be observed that the received pulses maintain the shape of the transmitted pulse to a great extent. As a result, it can be concluded that the distortion caused by the antenna to the transmitted pulse is feeble.

A figure-of-merit to quantitatively evaluate the distortion introduced by an UWB receiving antenna is the fidelity factor (FF) [Reference Ma and Jeng18, Reference Lamensdorf and Susman19]. Basically, it is the cross-correlation between the input and the received pulses. When the value of FF is unity, the input and received pulses are identical, and no distortion occurs. The FF is a function of the received antenna's orientation. In our case, the FF is measured in the H-plane (θ = 90°) for ϕ between 0° and 360°, and the result is shown in Fig. 14. The value of the FF is greater than 0.9 in all measured orientations which supports the conclusion made earlier that the proposed antenna introduces minimal distortion effects to the transmitted signal.

Fig. 13.

Waveforms of the input and received pulses.

Fig. 14.

Measured FF of the antenna.

The radiation limits set by the FCC for indoor and outdoor data communication applications are expressed in terms of the effective isotropic radiated power (EIRP). Therefore, the EIRP of the proposed UWB antenna needs to be calculated to check whether it complies with the FCC regulations. The face-to-face H(ω) of the transmitting/receiving antenna system, which was computed earlier, is essential to the EIRP calculation. The EIRP of the proposed antenna can be found using the following expression [Reference Mirshafiei, Abtahi, LaRochelle and Rusch20]:

(8)$$EIRP=P_T \lpar f\rpar \sqrt {H\lpar \omega \rpar } \lpar 4\pi Rf/c\rpar \comma \;$$

where f is the frequency of operation and P _T(f) is the transmitted pulse power level. The EIRP of the proposed UWB antenna is calculated and compared to the FCC indoor and outdoor masks as shown in Fig. 15. It can be asserted from the comparison that the EIRP emission level of the proposed UWB antenna is FCC-compliant.

Fig. 15.

EIRP emission level of the proposed antenna.