UNIVERSITI MALAYSIA PERLIS SCHOOL OF COMPUTER & COMMUNICATIONS ENGINEERING EKT 341 LABORATORY MODULE LAB 2 Antenna Characteristic 1 Measurement of Radiation Pattern, Gain, VSWR, input impedance and reflection coefficient for given Rectangular Patch and Inverted F Antenna
Experiment 2 : Antenna Characteristic 1 Measurement of Radiation Pattern, Gain, VSWR, input impedance and reflection coefficient for given Rectangular Patch and Inverted F Antenna 1. OBJECTIVES Ability to understand the basic functions of a Rectangular Patch antenna and Inverted F antenna. Ability to measure radiation pattern of E plane and H plane for both antennas. Ability to measure voltage standing wave ratio, input impedance and reflection coefficient for both antennas. 2. COMPONENT AND EQUIPMENT (a) Wave and Antenna Training System (WATS 2002) 2.45 GHz Rectangular Patch antenna 2.45 GHz Inverted F antenna (b) Network Analyzer 3. THEORY (a) PATCH ANTENNAS Patch antennas, as seen in Figure 2.0, are based upon printed circuit technology to create flat radiating structures on top of dielectric, ground-plane-backed substrates. The appeal of such structures is in allowing compact antennas with low manufacturing cost and high reliability. It is in practice difficult to achieve this at the same time as acceptably high bandwidth and efficiency. Nevertheless, improvements in the properties of the dielectric materials and in design techniques have led to enormous growth in their popularity and there are now a large number of commercial applications. Many shapes of patch are possible, with varying applications, but the most popular are rectangular (pictured), circular and thin strips (i.e.printed dipoles). Figure 2.0 : Microstrip patch antenna LAB 2 1
In the rectangular patch, the length L is typically up to half of the free space wavelength. The incident wave fed into the feed line sets up a strong resonance within the patch, leading to a specific distribution of fields in the region of the dielectric immediately beneath the patch, in which the electric fields are approximately perpendicular to the patch surface and the magnetic fields are parallel to it. The fields around the edges of the patch create the radiation, with contributions from the edges adding as if they constituted a four-element array. The resultant radiation pattern can thus be varied over a wide range by altering the length L and width W, but a typical pattern is shown in Figure 2.1. In this case the polarization is approximately linear in the θ direction, but patches can be created with circular polarization by altering the patch shape and/or the feed arrangements. A major application of patch antennas is in arrays, where all of the elements, plus the feed and matching networks, can be created in a single printed structure. The necessary dimensions can be calculated approximately by assuming that the fields encounter a relative dielectric constant of (1+ε r )/2 due to the combination of fields in the air and in the dielectric substrate. Figure 2.1 : Typical patch antenna radiation pattern Figure 2.2 : Polar plot of the electric field radiation pattern of a Rectangular patch antenna LAB 2 2
(a) E-plane (b) H-plane Figure 2.3 : Polar plot of the radiation pattern for rectangular patch antenna array (b) INVERTED F ANTENNAS The Inverted-F Antenna (IFA) is a variation on the transmission line antenna, or bent monopole antenna, with an offset feed that provides for adjustment of the input impedance. The resulting antenna geometry resembles the letter F, rotated to face the ground plane. In order to reduce the height of a monopole antenna, part of the antenna is bent to be parallel to the ground and this kind of antenna is called inverted L type antenna. However, this type of antenna has a very low input resistance and hence inverted F type antenna was introduced to solve this problem. An inverted F type antenna goes resonance when h+l is about 1/4 wavelength and its input impedance characteristics changes depending on the position of a feeding point and a thickness of a feeding path. Adjusting the feeding point's position known as W, desired antenna characteristics can be achieved. Radiation characteristics of this antenna are similar to the monopole and are often used in designing a small size antenna. Figure 2.4 : General structure of an inverted F antenna LAB 2 3
Suppose the ground plane in the Figure 2.4 is unlimited in size, underneath of the ground plane lies image current. Figure 2.4 comprises of real current and image current only. "h" takes the role of a radiation body that radiates wave and the remaining "L" takes the role of a transmitting line. It is at the end of a transmitting line where the current gets to zero and relatively at the radiation section of 2h, current distribution become constant. The magnitude of the current is constant throughout the radiation section putting radiation resistance become large. Increase in radiation resistance is directly connected to the increase in radiation power. Therefore an inverted F type antenna is introduced which is bigger in power size than dipole antenna with length of 2h. Inverted F Characteristics Inverted F antenna in this experiment is structured on the printed board as shown in Figure 2.5. Frequency used is 2.45GHz and to make input impedance at 50ohm, the length of the antenna and its feeding point is selected and capacitor is used for matching purpose. Figure 2.5 PCB pattern type inverted F antenna at 2.45GHz The radiation pattern for the Yagi antenna is shown below in Figure 2.6; (a) The pattern in the y-z plane ( E-plane) (a) The pattern in the x-y plane. (H-plane) Figure 2.6 Radiation Pattern for the inverted F antenna LAB 2 4
Examples of IFAs in the Real World IFAs are commonly used in mobile phones due to their small size (quarter-wavelength). An example of several IFAs in a mobile phone can be seen clearly on the Palm Pre Antennas. These antennas are visible once the back cover is removed, shown in Figure 2.7: Figure 2.7. Palm Pre Antennas viewable by Removing Back Cover. The yellow strip on the left side of the Palm Antenna is the GPS antenna, which is an IFA. Since the GPS frequency is 1.575 GHz, a quarter wavelength is about 1.87 inches (4.75 cm). This is roughly the length of the IFA in the actual product (it is shortened for proper tuning). Two other antennas are visible in Figure 2.7. In the upper right side, there is the diversity cell antenna, which. At the bottom of the device is a dual band IFA. This antenna is the transmit/receive cell antenna, which should operate at the 900 MHz and 1800 MHz bands.the IFA antenna is useful because of its small size and easy construction. LAB 2 5
4. PROCEDURE 4.1 2.45 GHz Rectangular Patch Antenna 4.1.1 E Plane Radiation Pattern Measurement 1. Mount a 2.45 GHz Yagi antenna to the side of the bracket of the transmitting antenna to produce horizontal polarization wave. 2. Mount a 2.45 GHz Rectangular Patch antenna to the side bracket of the receiving antenna. 3. Connect the system as shown in Figure 2.8. 50 Ω terminator needs to be connected to the open antenna bracket. Figure 2.8 System connection diagram for a Rectangular patch antenna E Plane Radiation Pattern 4. Keep the transmitting antenna in parallel with the receiving antenna and the distance of 120 cm between them. 5. Repeat steps from Experiment 1. LAB 2 6
4.1.2 H Plane Radiation Pattern Measurement 1. Connect the receiving antenna to the side of the bracket and rotate the body 90 degrees upward to make it parallel compared to the transmitting antenna as shown in Figure 2.9. 50 Ω terminator needs to be connected to the open antenna bracket. 2. Repeat steps from Experiment 1. Figure 2.9 System connection diagram for a Rectangular patch antenna H Plane Radiation Pattern 4.2 2.45 GHz Rectangular Patch Antenna 4.2.1 E Plane Radiation Pattern Measurement 1. Mount a 2.45 GHz Yagi antenna to the side of the bracket of the transmitting antenna to produce horizontal polarization wave. 2. Mount a 2.45 GHz Rectangular Patch antenna to the side bracket of the receiving antenna. 3. Connect the system as shown in Figure 2.10. 50 Ω terminator needs to be connected to the open antenna bracket. LAB 2 7
Figure 2.10 System connection diagram for a Rectangular patch antenna E Plane Radiation Pattern 6. Keep the transmitting antenna in parallel with the receiving antenna and the distance of 120 cm between them. 7. Repeat steps from Experiment 1. 4.2.2 H Plane Radiation Pattern Measurement 3. Connect the receiving antenna to the side of the bracket and rotate the body 90 degrees upward and rotate the transmitting antenna 90 degrees to get vertical polarization to make it parallel compared to the receiving antenna as shown in Figure 2.11. 50 Ω terminator needs to be connected to the open antenna bracket. 4. Repeat steps from Experiment 1. Figure 2.11 System connection diagram for a Rectangular patch antenna H Plane Radiation Pattern LAB 2 8
4.3 Antenna Characteristics Measurement 1. Connect the system as shown on Figure 2.12 Figure 2.12 System chart for antenna characteristics measurement 2. Start calibration by selecting desired frequency and connector in the network analyzer. 3. Connect the 2.45 GHz Yagi antenna used in the experiment to the network analyzer port. 4. Measure and analyze antenna s reflection coefficient at the resonant frequency. Save and include this graph in your report. 5. Measure the input impedance at the resonant frequency of the antenna in the experiment using Smith Chart. Save and include this graph in your report. 6. Measure the Standing Wave Ratio (SWR) at the resonant frequency. Save and include this graph in your report. 7. Repeat Steps 1until 6 by using Rectangular Patch antenna and Inverted F antenna. LAB 2 9
5. RESULTS AND OBSERVATIONS a) E plane Radiation Pattern (Horizontal Polarization) : Value for AGC Calibration = dbm Please take and note the data as the following: P T = max P R = dbm dbm (used to calculated for normalized values) Normalized (dbm) = P R (dbm) max P R (dbm) Angle P R (dbm) Normalized (dbm) 0 5 10 15 20 30 35 40 45..... 360 The set of above data, should be transfer to EXCEL spread-sheet and use the Radar function to plot the radiation pattern. Maximum receiving power = dbm, Angle = Left half power point and angle = dbm, Angle = Right half power point and angle = dbm, Angle = Half power beam width = LAB 2 10
b) H plane Radiation Pattern Value for AGC Calibration = dbm Please take and note the data as the following: P T = max P R = dbm dbm (used to calculated for normalized values) Normalized (dbm) = P R (dbm) max P R (dbm) Angle P R (dbm) Normalized (dbm) 0 5 10 15 20 30 35 40 45..... 360 The set of above data, should be transfer to EXCEL spread-sheet and use the Radar function to plot the radiation pattern. Maximum receiving power = dbm, Angle = Left half power point and angle = dbm, Angle = Right half power point and angle = dbm, Angle = Half power beam width = Repeat the same steps of 5(a) and 5(b) for Inverted F antenna LAB 2 11
c) Antenna characteristics measurement: 2.45 GHz Rectangular Patch antenna 2.45 GHz Inverted F antenna Please note and capture the figure of measurement (from VNA) of all the following: (a) Return Loss = db (b) Bandwidth = Hz (c) Input Impedance (Value of R= X= ) (d) Standing Wave Ratio (SWR) = LAB 2 12