Microstrip-Bends-Custom Design Circuit
Board Electronic Pcb
In order to build a complete circuit in microstrip, it is often necessary for the path of a strip to turn through a large angle. An abrupt 90° bend in a microstrip will cause a significant portion of the signal on the strip to be reflected back towards its source, with only part of the signal transmitted on around the bend. One means of effecting a low-reflection bend, is to curve the path of the strip in an arc of radius at least 3 times the strip-width. However, a far more common technique, and one which consumes a smaller area of substrate, is to use a mitred bend.
To a first approximation, an abrupt un-mitred bend behaves as a shunt capacitance placed between the ground plane and the bend in the strip. Mitring the bend reduces the area of metallization, and so removes the excess capacitance. The percentage mitre is the cut-away fraction of the diagonal between the inner and outer corners of the un-mitred bend.
The optimum mitre for a wide range of microstrip geometries has been determined experimentally by Douville and James. They find that a good fit for the optimum percentage mitre is given by
subject to and with the substrate dielectric constant . This formula is entirely independent of . The actual range of parameters for which Douville and James present evidence is and .They report a VSWR of better than 1.1 (i.e., a return better than −26 dB) for any percentage mitre within 4% (of the original ) of that given by the formula. At the minimum of 0.25, the percentage mitre is 98.4%, so that the strip is very nearly cut through.
For both the curved and mitred bends, the electrical length is somewhat shorter than the physical path-length of the strip.