ELEC4604 RF Electronics Experiment MICROWAVE MEASUREMENT TECHNIQUES 1. Introduction and Objectives In designing the RF front end of a microwave communication system it is important to appreciate that the signals received by the antenna can be very, very weak. Thus, any reflection loss caused by mismatch between the antenna input impedance and the characteristic impedance of the feed line (microstrip line, waveguide, etc.) must be kept as small as possible. Similarly, the performance of a microwave amplifier will be affected by the degree of mismatch at its input and output ports. All RF engineers must know how to measure the reflection coefficient, which determines the degree of mismatch when different components are connected together. In this experiment we will learn the basic principle of measuring the reflection coefficient when a particular load is connected to a waveguide/transmission line. From the reflection coefficient we can then determine the load impedance. In addition we will study the performance of a circulator. Circulators are used to isolate the transmit and receive paths when a single antenna is used for transmitting and receiving signals at the same time, especially when the transmitted and received signals have the same frequency. Practical circulators, however, do not provide a perfect isolation, and a small fraction of the transmitted signal may find its way into the receive path with detrimental effects. The second part of this experiment is to study the leakage of a given circulator the amount of unwanted signal that leaks into the wrong path. The amount of leakage will vary with frequency. From the results you obtain you will be able to assess whether a device/component under test will be suitable for use over a given frequency band.. Procedure.1 Calibrate the frequency as a function of micrometer reading, given the plot provided.. Over a frequency band from 9 to 10GHz, carry out the following: a. Measure the impedance of: the Horn antenna
a short circuit a matched load A Smith chart is attached. b. Measure the leakage of a given circulator, and hence calculate the isolation (in db) achievable with this circulator over the frequency band considered..3 Explain the function of each piece of equipment or component used in the experiment. 3. Wave guide Theory Waveguides are metal tubes used to transport microwave signals. Due to the presence of the metal conductors, the propagation of electromagnetic waves is considerably more complex than in free space. One of the key differences is that the electric and magnetic fields are not limited to oscillate in directions that are mutually perpendicular and also perpendicular to the direction of propagation more complex propagation mode can exist. The simplest modes that exist in waveguides are called respectively Transverse Electric (TE) and Transverse Magnetic (TM) waves modes where the Electric and Magnetic fields, respectively, are transverse to the direction of propagation down the guide. Rectangular Waveguide Here we present some important results for a rectangular waveguide, having a cross section of a x b. The cut off frequency for the (n,m) mode TM wave, TM : f n, m c. o. 1 a m n b Electromagnetic waves below the cut off frequency cannot propagate down the waveguide. The dimensions of the waveguide are typically chosen so that only the mode with the lowest cut off frequency (called the dominant mode) can propagate. When the frequency is larger than the cut off frequency, then TM waves can propagate down the waveguide. Its wavelength inside the guide is: g k m n where. a b
For a rectangular waveguide with a > b, the dominant mode is the TE 01 mode. Its cut off frequency is Circular Waveguide 01 f c. o. For a circular waveguide of radius r the dominant mode is the TE 11 mode, with cut off frequency: The wavelength inside the guide of the 11 f c. o. a r 1 1.841 TE mode can be found from: g 1 f c. o. f Note that inside the waveguide, the ratio of the Electric field amplitude to the Magnetic field amplitude for the electromagnetic wave, termed the characteristic impedance of the waveguide is: Z TE E H g where η = 10π is the characteristic impedance of free space.