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RF Electronics Chapter4: Transmission Line Transformers and Hybrids Page 98 2022, C. J. Kikkert, James Cook University, ISBN 978-0-6486803-9-0. very similar to the performance shown in figure 4.53 and is able to handle 100W of output power. Prepreg sheets are available [12], which can be used to convert single layer PCB's to multilayer PCB's, so that broadside couplers can be constructed. The Roger's RO4403 Prepreg sheets have a dielectric constant of 3.17 and are 0.1 mm thick and can be used to bond two single layer PCB's to form a multilayer PCB. The sandwich is pressed together with a pressure of 2.8 MPa (400 psi) and a temperature controlled profile up to 180 C (350 F) for a cycle time of 2 hours to form the correct bond. For further details, see the RO4403/4450 data sheet. Other manufacturers have similar materials. Coaxial Couplers Transmitter Power Meters For transmitters both the amplifier output and the return loss from the antenna is normally monitored. This can be done using a coupler like shown in figure 4.44 to 4.47. High power amplifiers have coaxial outputs, and the forward and reverse power sensors are normally small coupling loops placed close to the centre conductor of the coax. When power levels are required, a diode rectifier and a capacitor are used to provide a DC voltage or current proportional to the power level. A typical schematic of such a power meter is shown in figure 4.54. In that power meter the coupling loop can be reversed, so that both forward and reverse power can be measured. The plugins, shown on the right of figure 4.54, can provide an RF output, or they cover a specified frequency range and power level. For high power transmitters forward and reverse power sensors are normally incorporated [13]. Figure 4.54. Bird Wattmeter schematic, instrument and plugins [14,15]. Transmission Line Baluns A 180-degree Hybrid or Balun (balanced to un-balanced) is used for microwave balanced- mixers and for converting connect dipole antennae to coaxial cables. At lower power levels and below 1 GHz, ferrites can be used as shown in the TV Balun of figures 3.7 and 3.8. The use of ferrite cores is not desirable for Microstrip or Stripline circuits or for a high power transmitter output. Removing the ferrite core from figure 3.7 results in the Parallel coaxial line Balun of figure 4.55. A Balun can also be made by connecting the outer of a coaxial cable and the centre to two load of half the impedance, as shown in figure 4.56. The floating length of coax is normally 90 degrees in length. A ferrite core around the coaxial cable can be used to enhance the isolation between port 3 and ground. RF Electronics: Design and Simulation 98 www.cadence.com/go/awr

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