A Wideband Printed Directional Antenna Array with Impedance Regulating Load

We proposed a broadband directional antenna array working at mobile communication frequency band, which achieves a relative bandwidth of 50.7%. This binary antenna array is fed by two branches of the balanced microstrip. To enhance the antenna bandwidth, we introduced a section of loading metal strip. The antenna prototype has a 󵄨󵄨󵄨 󵄨 S 11 󵄨 󵄨 󵄨 󵄨 lower than −10 dB within the 1.5 GHz to 2.52 GHz frequency band, particularly from 2.01 GHz to 2.50GHz, the 󵄨󵄨󵄨 󵄨 S 11 󵄨 󵄨 󵄨 󵄨 is lower than −15 dB. The gain varies with relatively small variation within the working band, which is 5.4 dBi to 8.7 dBi.


Introduction
Directional antenna is also called Yagi-antenna.It has features like simple structure and high directionality, but its obvious shortcoming is the narrow bandwidth.In 1998, Qian and other scientists proposed quasi-Yagi-antenna [1].This is a kind of printed antenna with wideband balun, which reaches a relative bandwidth of 17%.Based on this breakthrough, researchers designed a series of improved printed Yagiantennas [2,3].They gradually broadened the bandwidth, but the problem of low gain remained unsolved.So researchers tried adding more directors [4,5] or setting up antenna array [6,7] to increase the gain.Those methods, however, obtained high gain at the cost of losing bandwidth because of the mutual coupling among radiation elements or antenna elements.
In this paper, we proposed a wideband antenna array.The significant broadband property is achieved by introducing a section of loading metal strip at the end of the feeding structure.The antenna prototype exhibits a working band from 1.5 GHz to 2.52 GHz, and the relative bandwidth is 50.7%.The gain varies with relatively small variation in this band.Detailed simulation and experimental results are presented in this paper.

Antenna Structure
The proposed antenna is in the form of double-side printed circuit board as shown in Figure 1.It consists of two uniform antenna elements, which are fed by two branches of the balanced microstrip.Each of the antenna elements is constituted of the balanced microstrip feeding system, the loading metal strip, one reflector, one pair of active dipoles, and one director.The active dipoles are symmetrical dipoles (two metal strips in the same dimensions) printed on the same side of the substrate.The balanced microstrip is directly connected to the active dipoles and served as the feeding structure which could realize balance feed.This structure is similar to parallel double transmission line.One strip of the balanced microstrips, the active dipoles, the director, and the reflector are printed on one side of the substrate.The other strip of the balanced microstrips and the loading metal strip are printed on the opposite side of the substrate.The substrate is made of FR-4 epoxy resin lamina whose relative dielectric constant is   = 4.To explain the working principal of the loading metal strip, we simulated the surface current on the active dipoles and the loading metal strips (shown in Figure 3).Probes are used to measure the current amplitude and phase.Simulation results are drawn in Figure 4.

The Effect of the Loading Metal
Figure 4 indicates that the current on the active dipoles shows sine distribution.Specifically, the amplitudes of  13 and  23 are nearly equal to that of  11 ,  12 ,  21 , and  22 , but the phase differences are significant.In this case, if we define  11 and  22 as currents on resistance, then  13 and  23 are currents on capacitor (or inductance).The antenna's equivalent circuit is drawn in Figure 5.The loading metal strips work as impedance regulating load.As a result of the introduction of reactance for compensation, the impedance of the antenna can keep stable within a wide bandwidth.The bandwidth of the antenna array is broadened.because the currents distribute symmetrically on the antenna array.The simulated radiation patterns shown in Figure 6 can also prove the following.According to Figure 6, we can draw the following conclusions.(1) There is no offset of the maximum radiation direction.The antenna directly radiates towards the direction of the perpendicular bisector of two antenna elements, which means that the currents distribute symmetrically on the antenna elements.(2) The front to back ratio and the sidelobe level of the antenna are over 10 dB, which indicates that this antenna has good directional characteristics.This is because the distance between the centers of two antenna elements is   −  2 = 107 mm that is approximately 0.5 times to 1 time of the working wavelength.This distance will not lead to big side lobes.(3) Gain on 2.0 GHz is 7.3 dBi, while gain on 2.4 GHz is 8.7 dBi.The increase of 1.4 dB complies with the law of gain increasing with the increase of frequency.

Experimental Results
An antenna prototype is produced according to the optimized simulation model as shown in Figure 7.We carried out measurements in the microwave anechoic chamber with Agilent E8363 vector network analyzer.The measurements include reflection coefficient, radiation pattern, and gain.All the experimental results are shown in Figure 8 to Figure 9 and Table 1.The antenna prototype has a reflection coefficient less than −10 dB from 1.5 GHz to 2.52 GHz, which is even wider than simulation results.The measured gain is about 1 dBi lower than simulated ones (shown in Table 1).The main  lobes of the measured radiation patterns closely resemble the simulated ones, but the side lobes show great differences.The discrepancies between measured results and simulated ones can be explained as follows.(1) The dielectric slab is made of FR-4 epoxy resin lamina.This material has great loss; the loss tangent is of the order of 10 −2 ; therefore, it can decrease the reflection coefficient and the gain of the antenna.(2) Welding method and fixture can bring about inaccuracy to the measurements of side lobes.(3) The instability of FR-4 material can also influence the measurement of the radiation patterns.
The antenna proposed in this paper performs a wide impendence bandwidth.The introduction of impedance regulating load eliminates the mutual coupling between two The binary antenna array in [7] has high gain, but this antenna achieves high gain by adopting more directors.As a result, it is longer in the main radiation direction and has smaller K.The comparison results indicate that the proposed antenna has well balanced the bandwidth and gain and acquires a good performance.

Conclusion
In this paper, we proposed a Yagi-antenna array with impedance regulating load fed by balanced microstrip.The loading metal strip is equivalent to a reactive element.It could make a compensation for the resistance deviation International Journal of Antennas and Propagation caused by frequency offset and effectively decline the mutual coupling between antenna elements.As a result, the antenna has broadband and high gain properties.This antenna has a relative impedance bandwidth of 50.7% and gain varies with relatively small variation within this band (5.4 dBi to 8.7 dBi).It can be widely applied in mobile communication.

Figure 1 :
Figure 1: A schematic of the proposed antenna. 1 and  2 represent two metallized via holes.

Figure 2 :
Figure 2: The effect of the loading metal strip length (S) on the antenna reflection coefficient.

Figure 3 :
Figure 3: A diagram of the surface current on active dipoles and loading metal strips.

Figure 3 toXFigure 4 :
Figure 4: Current distribution on active dipoles and loading metal strips (at 2.0 GHz): (a) current amplitude distribution on element 1, (b) current amplitude distribution on element 2, (c) current phase distribution on element 1, and (d) current phase distribution on element 2.

Figure 5 :
Figure 5: Equivalent circuit of the antenna array. 1 and  2 represent loading metal strips, and  1 and  2 represent active dipoles.

Table 1 :
Simulated and measured antenna gain.

Table 2 :
Antenna performance comparison between the antennas proposed in this paper and other references.