915MHz CIRCULAR POLARIZED PATCH ANTENNA By ARFEL J HERNANDEZ EE172 FINAL PROJECT SPRING 2011 Dr. Ray Kwok San Jose State UniversityABSTRACT A 915MHz circular polarized patch antenna was designed, built and tested. The antenna dimensions were 160mm X 160mm with 40mm truncations at two opposite corners. The antenna was probe fed at a distance of 150mm from the right edge placed at the horizontal symmetry line. The antenna resonated at the desired frequency with a Return Loss of -15dB, a Gain of +11dB and half power beam width of 43 degrees with respect to the normal plane of incidence. The results closely matched the theoretical and simulated by values within 75%. The 25% difference is attributed to the test environment were reflections in the transmission path were several.INTRODUCTION Antennas are all around us, cars, airplanes, satellites, old TV sets, cell phones, radios, laptops to name just a fewover a wide range of frequencies. At either end of a communication link there is some kind of antenna to allow the detection or transmission signal. A simple patch antenna is shown in figure 1. Antennas are all around us, cars, airplanes, satellites, old TV sets, cell phones, to name just a few. Antennas transmit/receive electromagnetic waves over a wide range of frequencies. At either end of a communication link there is some enna to allow the detection or transmission of information embedded in the . A simple patch antenna is shown in figure 1. Antennas are all around us, cars, airplanes, satellites, old TV sets, cell phones, . Antennas transmit/receive electromagnetic waves over a wide range of frequencies. At either end of a communication link there is some of information embedded in theThe dimensions of a patch antenna are obtained using the following equations,   12௥௥௘௙௙௢௢ 2∆ Equation 1. Where fr is the resonant frequency, εr is the dielectric constant of the substrate, µo is the relative permeability of free space, εo is dielectric permeability constant and ∆L is the extended incremental length. 12௥௢௢ 2௥ 1 ௢2௥2௥ 1 Equation 2. Where vo is the free-space velocity of the light and,      ௥௘௙௙ ௥ 12௥ 121  12ିଵଶ Equation 3. In an antenna the width and length are finite and the radiation at the edges undergo fringing as illustrated in figure 2. The fringing field is a function of the ratio of the length of the patch to the height of the substrate and the dielectric constant of the substrate. It influences the resonant frequency of the antenna and must be taken into account.Because of the fringing effect the length has to be incremented by ∆L defined in Equation 1. Therefore the incremental length to height ratio can be calculated using the following equation, ∆ 0.412!௥௘௙௙ 0.3#$ 0.264&!௥௘௙௙ 0.258#$ 0.8& Equation 4 Thus for the actual length of the patch we have,   ఒଶ 2 Equation 5. This represents a length reduction between 1%- 4%. Therefore the effective length Le of the patch becomes, Le = L + 2∆L Equation 6.ANTENNA POLARIZATION There are several polarization patterns but since circular polarization is required for this project it is the one to be discussed. Circular polarizatMagnetic waves traveling in space at a 90° phase di fference from each other as can be seen in figure 4 below, There are several polarization patterns but since circular polarization is required for this project it is the one to be discussed. Circular polarization refers to Electric and Magnetic waves traveling in space at a 90° phase di fference from each other as can be There are several polarization patterns but since circular polarization is required ion refers to Electric and Magnetic waves traveling in space at a 90° phase di fference from each other as can beTo achieve circular polarization in a patch antenna the symmetry of the patch has to be modified in one of many ways possible. Some examples on how to achieve this is presented in figure 5. The position of the fed, F, is critical to minimize reflection and ensure that the antenna’s impedance is matched to the 50Ω coax feed line for maximum power transfer.RADIATION PATTERN Patch antennas radiate maximum power in the direction normal to the patch. An example of such pattern is presented in figure 6 below, From the plot the 3dB beam width represents the point at whichreduces to half. Patch antennas radiate maximum power in the direction normal to the patch. An example of such pattern is presented in figure 6 below, From the plot the 3dB beam width represents the point at which the radiated power Patch antennas radiate maximum power in the direction normal to the patch. An the radiated powerANTENNA GAIN The gain of the antenna is defined as the amount of power gained or delivered by the antenna. It can be calculated with the following formula, G(antenna) = G(reference antenna) – S21(reference antenna) + S21(antenna) where S21 is defined as the transmission coefficient between ports, ie, input and output. FRONT-TO-BACK RATIO This is the ratio of the radiated power in front or behind the antenna, which is the difference of the Directivity. DIRECTIVITY By definition it is the Gain of a lossless antenna which ignores the effects of conduction losses, dielectric losses, mismatches and cable losses.PROCEDURE The dimensions of the half-wavelength patch antenna were determined by the resonant frequency desired and the aid of equations 1 through 6. Based on the results simulations to find the best position of the feed point as to guarantee proper matching, circular polarization and minimum Return loss were run using Microwave Office software from AWR©. Once the best results were obtained a prototype antenna was built using Multipurpose Copper (Alloy 110) Soft Sheet, .027" Thick; 6 x 1.5cm wood lock-stand-offs to guarantee uniform height and one Type-N female connector. The antenna prototype was tested and compared to theoretical and simulated values.TEST ENVIRONMENT The antenna was tested at 2 different orientations . Figure 7 shows one of the settings where the measurements took place. The first measurement was performed at a distance of15.4 ft from the