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RF Electronics Chapter4: Transmission Line Transformers and Hybrids Page 96 2022, C. J. Kikkert, James Cook University, ISBN 978-0-6486803-9-0. Example 4.4: 100 Watt 3 dB Broadside Coupler A 3 dB coupler was required to operate at a power level of about 100 Watt at a frequency of 100 MHz. At the time of the design, MWO did not yet exist and the equations 4.23 and 4.24 and Figure 4.50 were required to complete the design. Two 12.6 mm thick pieces of Polyethylene, which has a dielectric constant of r = 2.26, were used for the top and bottom dielectric sheets for this coupler. Polyethylene sheets of this thickness can conveniently be purchased as a cutting board for bread from most local supermarkets. For a 3dB coupler, C = 0.707 and Z OE = 120.7 and Z OO = 20.7 . Since these impedances must be evaluated iteratively, it is the easiest to enter the above equations for Z OE and Z OO into an Excel or other spreadsheet. Using the values B = 29 mm, s = 2.9 mm, w = 12.7 mm, gives 1 . 0 B s and thus from the table gives 56 . 0 fe C and 15 . 1 fo C . This then results in Z OE = 119.7 and Z OO = 20.8 , which is close enough. At 100 MHz the length of a quarter wavelength line is given by: � �� � √ �� ����� Eqn. 4.25 Figure 4.51. Terminating line-width calculation. As shown in figure 4.51, the width of 50 terminating lines can be calculated using TXLine or equivalent programmes as 23.2 mm. A 2.9 mm thickness of Polyethylene is not readily available. However, a 3.17 mm thickness of Perspex, with a dielectric constant of approximately 3.42 was available and was used. The effect of this substitution cannot readily be calculated using the above techniques. This coupler was constructed and performed as expected, even though the line spacing was not ideal. The use of SBCPL in CADENCE AWR DE gives much more flexibility and allows a more accurate determination of the performance. The actual cutting boards used are 12.6 mm (0.5 inch) thick, pieces of Polyethylene. They are used for the top and bottom dielectric and a 3.17 mm (0.125 inch) thick Perspex sheet is used to separate the tracks. RF Electronics: Design and Simulation 96 www.cadence.com/go/awr

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