Hanbo Tao 2x2 Dipole Antenna Array 1 San Jose State University EE 172 - Microwave 2x2 Dipole Antenna Array Dr. Ray Kwok by Hanbo Tao May 25th, 2011Hanbo Tao 2x2 Dipole Antenna Array 2 Abstract This report will explain the theory of dipole antenna, and a 2x2 half wave dipole antenna will be designed at 2.4 GHz frequency. The antenna will also be built, and it has three main parts: four half wave dipole antennas, four baluns, and one power divider. I would simulate my design with the software, which’s called 4nec2. In addition, I would test my antenna in the laboratory. The return loss would be measured, and I would also measure its radiation pattern and its dB gain by comparing with the standard 10dB gain antenna from the Dr. Kwok. I would also test the power combiner. In the end, the simulated gain is 9.08 dB, and the measured gain is 8dB. Introduction According to the article from Electronics and Radio Today, dipole antenna consists of two equal length poles, and the power is applied to the center feeder as shown in the Figure 1. It is a very basic construction of a single dipole antenna, and the power transfers to or receives from the antenna through the feeder. In this report, I would build an ideal dipole antenna, which is same to the half wave dipole antenna shown in the Figure 2. Moreover, to get the maximum power that is transferred between the feeder and the antenna, the impedance of the antenna and the feeder has to be matched; in this case, the impedance is 50 Ω. Figure 1. Basic Construction of a Dipole Antenna Figure 2. Half-wave Dipole Antenna As we can see on the Figure 2, the total length of the dipole is a half wavelength, so it is half the length of the original dipole antenna shown in the Figure 1, this makes each section the dipole a quarter wavelength long in the Figure 2.Hanbo Tao 2x2 Dipole Antenna Array 3 Figure 3. Radiation Pattern of a Dipole Next, let’s look at the Figure 3 ("RADIATION PATTERN OF A DIPOLE"), the radiation pattern of a dipole. We can see clearly the 3D radiation pattern and its horizontal and vertical pattern in the Figure 3. When we look at the vertical pattern, we can see that direction of maximum radiation is at right angles to the axis of the antenna, which are 90 and 270 degrees in this case. Theory and Procedure The design of the 2x2 half wave dipole antennas is shown in the Figure 4 below: Materials needed: - 50 Ω coax cable(RG58A) with BNC connector - BNC flat mounts - 22 AWG wire - Solder wires - Copper sheet - Brass shim stock sheet - BNC female connector to female N type connector - #4 screws - Sufficient tools (drills, tapes, solder iron, clamps, knife, third hand, and others) Figure 4. 2x2 Half-wave Dipole AntennaHanbo Tao 2x2 Dipole Antenna Array 4 As I mentioned previous, the requirement for this antenna is to be operated at 2.4GHz, so I used the following equation to calculate the length of the wire. Length of the half wave dipole: Length of the quarter wave dipole: The factor A in the above equation is the ratio of the length of the antenna to the thickness of the wire or tube used as the element. The Figure 5 shows the results of the ratio of the length and thickness ("Electronics and Radio Today"). Figure 5. Factor A I would also need to use balun to get the maximum power that is transferred between the feeder and the antenna. In the Figure 6 ("Antenna-Theory.com"), the pink line is the inner conductor of the coax cable, grey is the outer conductor, red line is the antenna wire, and the green is the balun. We want to make the current ID travels in the opposite direction to the current IC; therefore, the voltage on the inner conductor is out of phase with the voltage on the outer conductor are equal in magnitude, then the currents will cancel in the region below the balun. The balanced operation is restored. Figure 6. Current FlowHanbo Tao 2x2 Dipole Antenna Array 5 The equation is shown below calculate the length of the balun for 2.4GHz half wave dipole antenna: Length of balun: The velocity factor in the above equation for the coax cable RG-58A is 0.68, and according to Wikipedia, “The velocity factor (VF),[1] also called wave propagation speed or velocity of propagation (VoP or vP),[2] of a transmission medium is the speed at which a wavefront passes through the medium, relative to the speed of light. For optical signals, refractive index is a similar quantity.” ("Wave propagation speed") After I required the correct measurement and the materials shown in the Figure 7, I started to build the antenna. At first, I built one antenna without the balun, and the measurement of S11 was not good enough; therefore, I added the balun on all four of the half-wave dipole antenna. The finished single dipole antenna is shown in the Figure 8. Figure 7. Materials to Build Antenna Figure 8. Single Half-wave dipole Antenna Then I continued to build the rest of the antennas. After I successfully finished building all four of the antennas, I went to the laboratory to measure the S11 on each one of them. I have to make sure they were getting the similar return loss, and then I could further continue my construction of this project. The Figure 9 shows the measured S11 values of four each antenna, and they are -25dB, -26dBd, -30Bd and -30BdB. As I expected, they are close enough and in the acceptable range.Hanbo Tao 2x2 Dipole Antenna Array 6 Figure 9. S11Values of Four Single Half-wave Dipole Antenna After I finished building and testing the four single antennas, I went on with the four ways power divider. The impedance of my four single half-wave dipole antennas is 50Ω, so the four way power divider has to match 50Ω indeed. The Figure 10 shows the impedance diagram of this power divider. Figure 10. Impedance DiagramHanbo Tao 2x2 Dipole Antenna Array 7 As we see in the Figure 10 (Whitsel), the 50Ω//50Ω=25Ω on the two sides of the WR 90 waveguide, and I used the equation below to get the impedance in the middle port. Z0 = (Z1* Z2)^1/2, Where Z1=Input Impedance, Z2=Output Impedance