EE 3324 Electromagnetics Laboratory

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1 EE 3324 Electromagnetics Laboratory Experiment #11 Microwave Systems 1. Objective The objective of Experiment #11 is to investigate microwave systems and associated measurement techniques. A precision benchtop microwave system is used to (1) determine the normalized impedance of an unknown waveguide load, (2) implement a single stub tuner for a microwave system, and (3) simulate a microwave radio link. 2. Introduction From transmission line theory, the input impedance of a transmission line (length = l, characteristic impedance = Z o ) terminated with a load impedance Z L is (1) This equation is also applicable to waveguides except that the characteristic impedance of a given waveguide cannot be uniquely specified since it is a function of frequency. To avoid this difficulty, we may normalize all impedances with regard to the waveguide characteristic impedance at a given operating frequency. Thus, using Equation (1), the normalized input impedance of the waveguide is (2) The reflection coefficient at the load is given by (3) Equations (2) and (3) can also be written in terms of admittances to yield the normalized input admittance of the waveguide (4) and the reflection coefficient (5)

2 The normalized impedance of an unknown waveguide load may be determined by simply determining the location of an electric field minimum relative to the load. The normalized input impedance at an electric field minimum is equal to 1/s where s is the voltage standing wave ratio (VSWR). Thus, if an electric field minimum is found a distance d from the load, the input impedance at d is (6) Solving Equation (6) for the load impedance gives (7) Separating Equation (7) into real and imaginary parts gives the real and reactive portions of the load impedance. (8) Just as transmission lines may be matched using shunt or series stubs consisting of shortcircuited or open-circuited transmission lines, waveguides can be matched using the same basic procedure using a reactive stub. A simple probe penetrating the waveguide introduces a parallel susceptance which may be capacitive or inductive (typically capacitive), depending on the length of the probe. The shunt stub matching procedure of a waveguide is shown in Figure 1. Assuming the reactive stub has a capacitive admittance (y stub = jb), we need to find a point along the waveguide with a normalized admittance of (y guide = 1! jb) such that the parallel combination of the stub and the waveguide gives an overall input admittance of (y in = y stub + y guide = 1) for a matched condition. To find the proper point along the waveguide to place the stub, the location of an electric field minimum relative to the load is first measured (the distance d). For points along the waveguide where the electric field is a minimum, the waveguide input admittance is equal to the waveguide standing wave ratio. Then, the distance from the electric field minimum to the position of the matching stub is determined (l m ). The input admittance looking into the waveguide at this point is given by Figure 1. Shunt stub matching. S (9) olving for the real part of Equation (9), equating to unity and solving for the distance l m yields (10)

3 where 8 g is the guide wavelength. 3. Equipment List 4. Procedure Feedback MWT530 Microwave Trainer Warning: Although the microwave power levels generated by this equipment are below 10mW and not normally dangerous, the human eye can suffer damage by exposure to direct microwave radiation. NEVER look directly into an energized waveguide. 1. Waveguide Impedance Measurement. Connect the microwave system shown in Figure 2 making sure that the probe of the slotted line/detector penetrates between 1 and 2 mm into the waveguide. Set the attenuator to approximately 20 o, the X-band source to internal keying and the meter to detector output. Switch on the console power supply and the X-band oscillator. Move the detector carriage along the slotted waveguide section to locate the position of an electric field maximum. Adjust the sensitivity of the detector to provide a near full-scale reading. If necessary, adjust the attenuator setting. Move the carriage as far as possible toward the load. Carefully locate the first electric field minimum. Record the position of the minimum and the corresponding detector current. Move the carriage toward the source and locate the next four consecutive electric field maxima and minima. Record the positions and detector currents for each maxima and minima. Replace the resistive load with the short circuit plate. Locate the positions of the electric field minima with the short circuit load. From your results, calculate the VSWR of the resistive load, the waveguide wavelength, the distance d from the load to the first electric field minimum, and the normalized complex load impedance. Figure 2. Waveguide impedance measurement setup. 2. Load Matching Using a Reactive Stub. Connect the microwave system shown in Figure 3. Determine the location of slotted line probe tuner at which the input admittance looking toward the load

4 is 1! jb according to Equation (10). Note that zero on the scale of the probe tuner carriage is actually 10 mm from the right hand flange. Thus, the probe should be positioned at (l! 10) mm. The first electric field minimum cannot usually be observed due to the constraints of the slotted line scale. Thus, assuming you have located the second electric field minimum from the load, you should add one half wavelength to your distance d. Gradually increase the probe penetration and measure the VSWR at each position. Record the probe penetration referenced to the micrometer scale for each measurement. Obtain the best value of VSWR possible. Compare this value of VSWR with that of the unmatched case and determine the percentage of power transmitted and reflected in each case. 3. Micro Figure 3. Load matching setup. w a v e Radio L i n k. Connect the microwave system shown in Figure 4 where the separation distance between the transmit and receive antennas is 35 cm. Switch the X-band source to internal keying and the meter to detector output. This separation distance ensures that the receive antenna is in the far field of the transmit antenna. Switch on the console power supply and the X-band oscillator source. Set the attenuator setting to approximately 40 o and use the detector

5 sensitivity control for adjustment of the detector current. With the antennas oriented for maximum gain (2 = 0 o ), obtain a near full-scale detector current and record this value. Obtain detector current readings as the angle the angle 2 is varied from 0 o to 40 o in 5 o increments. Since the detector output current is proportional to the received power for small signal levels, a plot of the detector current vs. the angle 2 represents the power radiation pattern for the horn antenna. Use MATLAB to generate a polar plot of the horn antenna radiation pattern assuming that the pattern is symmetric with respect to the angle 2. Determine the half-power beamwidth of the horn antenna from hour radiation pattern plot. Re-align the antennas for maximum gain and vary the separation distance from 35 cm to 75 cm in 5 cm increments. Use MATLAB to obtain a plot of the detector current vs. r and discuss your results. Figure 4. Microwave radio test setup. 5. Additional Questions 1. Check your measured results of part 1 of the procedure using a Smith chart. 2. Check your measured results of part 2 of the procedure using a Smith chart.

ASSIGNMENT: Directional Coupler

ECE 323- MICROWAVE ENGINEERING LABORATORY 1 ASSIGNMENT: Directional Coupler I. OBJECTIVES Know the properties of directional couplers and their applications in microwave transmission and measurement systems.