Pattern Regulation Using GECC

There is another useful application, pattern regulation, can be implemented by using the same feature of blocking current. The printed monopole antenna is widely used for its simple configuration. Although many structure variations have been proposed, most of the designs adopted the quarter-wavelength resonance approach.

In the microwave band, the quarter-wavelength monopole is small and can be easily fabricated on the same circuit board of the circuitry. For example, the monopole antenna length for the IEEE 802.11a WLAN system at 5 GHz band is only around 13 mm on an FR4 substrate. As compared to the size of the monopole radiator, the ground plane on the circuit board is usually much larger. For instance, the ground length (about 50 mm) of a small USB dongle is already near one wavelength of a 5 GHz signal. In this section, the radiation pattern of the monopole with long ground plane is to be considered with the application of the proposed GECC.

Consider a 5.25 GHz printed inverted-L monopole antenna fabricated on a 0.8 mm thick FR4 substrate, as shown in Figure 4.8(a). The length L1 and width W1 of the substrate ground plane are 50mm and 20mm, respectively. The monopole strip is with length of lm1 = 7 mm and lm2 = 6.8 mm and width of 1.5 mm. The antenna is fed by a small section of a 50 Ω microstrip line, which, in turn, is connected to a 50 Ω coaxial cable for measurement. Figure 4.8(b) shows the simulated and measured reflection coefficient (S11) of the antenna. It is seen that the inverted-L monopole antenna is well matched around the center frequency. The resultant radiation patterns of co-polarization in the E plane (y-z plane) at 5.25 GHz are illustrated in Figure 4.9(a). Both the simulation and measurement are presented, showing agreement with each other. It is observed that, different from a typical radiation pattern (i.e., a digit “8” pattern in the E plane) of a monopole antenna, a tilted beam is formed in the present pattern. More power is radiated toward the -y direction, which is the direction of ground extension. In addition, there is a radiation null near the broadside direction. This weak broadside radiation and tilted beam pattern is not suitable for most mobile applications.

Antenna feed

Ground plane

x z y

x z y

L

W

l

l

(a)

4 4.5 5 5.5 6

Frequency (GHz) -25

-15 -10 -5 0

Reflection coefficient S11(dB)

Simulation Measurement Simulation Measurement -20

(b)

Figure 4.8 (a)Structure of the inverted-L monopole antenna with long ground plane. (b)Simulated and measured reflection coefficients of the inverted-L antenna.

5 0 -5 -10 -15 -20 -25 -30 -35 5 0 -5 -10 -15 -20 -25 -30 -35

240^o

210^o

180^o

150^o

120^o 90^o 60^o 30^o

0^o 330^o

300^o

270^o

5.25GHz y-z plane

Simulation Measurement Co-polarization (E_θ)

Simulation Measurement Co-polarization (E_θ)

(a)

t = 0 T

t = 0.125 T

t = 0.25 T

(b)

Figure 4.9 (a)Simulated and measured radiation patterns of the inverted-L antenna. (b)Current distributions in different time steps of the inverted-L antenna at 5.25 GHz. T is the time period of the signal at 5.25 GHz. The arrows indicate the current null positions.

To explain the deformation of the radiation pattern, the simulated current distribution on the ground plane in different time steps is plotted in Figure 4.9(b). It can be seen that the monopole radiator indeed behaves as a quarter-wavelength antenna. However, a close inspection on the ground plane current reveals that the ground current is a traveling-wave current, instead of a resonant one, propagating toward the -y direction. This can be observed from the moving of the current nulls with the advance of time. The long ground plane in this case acts like a traveling wave antenna, which contributes to a radiation field toward the (current) wave-propagating direction [60]. The combination of the fields from the inverted-L monopole and the ground plane leads to the radiation pattern shown in Figure 4.9(a).

To regulate the radiation pattern as a broadside radiation one, the traveling wave behavior of the ground plane current should be changed. For this purpose, the proposed GECC is designed to put at the two ground sides, as shown in Figure 4.10(a).

As examined in the above section, the GECCs have the function of reflecting the incident current wave so that a standing-wave current distribution may be obtained along the ground edges. The required structure parameters of the GECC for a resonant frequency at 5.25 GHz are designed as a = 3.5 mm, b = 2 mm, c = 3.9 mm, t

= 1 mm, l = 0.5 mm, and s = 0.6 mm. Also, the position of the GECCs is properly chosen at l_d = 15 mm, which is located near the current null at t = 0 in Figure 4.9(b).

Figure 4.10(b) shows the simulated and measured reflection coefficients of the inverted-L monopole antenna with a GECC-embedded ground plane. The antenna is still well matched and has a return-loss frequency response similar to that without the GECCs. Figure 4.11(a) depicts the simulated and measured radiation patterns of co-polarization in the E plane at 5.25GHz for the GECC-embedded antenna. A significant difference is observed that the pattern becomes a digit “8” pattern that meets the expectation. The antenna pattern has changed from one with tilted beam to a broadside radiation one after the use of the GECCs. To confirm the current blocking effect of the GECCs, the time-averaged current distribution combined with instant maximum current vector distribution are examined and plotted in Figure 4.11(b). The scale of the current level is the same as that in Figure 4.9(b) for comparison. The current along the two sides of the ground plane has been blocked by the GECCs as expected. An effective open circuit caused by the GECC forces

the current turning to be a standing wave, instead of a traveling wave, thus adjusting the radiation pattern to a broadside one.

Antenna feed

Ground plane

x z y

x z y

L

W

l

(a)

4 4.5 5 5.5 6

Frequency (GHz) Simulation

Measurement Simulation Measurement -25

-15 -10 -5 0

-20

R ef le cti o n c o ef fi cie n t S

(d B )

(b)

Figure 4.10 (a)Structure of the inverted-L monopole antenna with GECC. (b)Simulated and measured reflection coefficients of the inverted-L antenna with GECC.

5

Figure 4.11 (a)Simulated and measured radiation patterns of the inverted-L antenna with GECC.

(b)The time-averaged current distribution combined with an instant current vector distribution on the ground plane of the inverted-L antenna with GECC.

4.5 Summary

A miniaturized ground edge current choke (GECC) and its applications have been proposed. The GECC is a parallel combination of an open slit and an overlapping strip implemented on the edge of a ground plane. The slit behaves as an inductor and the strip a capacitor. By introducing the GECC on the ground edge, an effective electrical open circuit provided by the parallel LC circuit can be performed to block the current flowing along the ground plane edge. The design and the measurement of the choke have been presented. The resonant frequency of the choke can be easily determined by the sizes of the open slit and overlapping strip. The proposed evaluation approach for the GECC by using the transmission line method has been proved effective. Two applications of the proposed GECC have been examined. The first one by using the GECC to block the traveling-wave current induced along the ground edge of a small antenna demonstrated the feasibility of the proposed structure on regulating the antenna radiation pattern. And the second application showed the ability of the GECC to enhance the isolation between two nearby antennas. The experimental results agreed well with the simulation.

The proposed printed GECC has the advantages of compact size and ease of fabrication, which can be used in more applications. For a lower operation frequency, such as 900 MHz or 1800MHz, the required inductance and capacitance should be increased for resonance at the design frequency. With the same size, the printed capacitor can be replaced by SMD capacitor to gain larger value for lower operation frequency. However, if only the capacitance is increased but without increasing the inductance, the fractional bandwidth might be too narrow to be used.

To increasing the inductance along the current path, a magnetic material, like ferrite, may be used so as to increasing the inductance and thus the bandwidth.