Bends
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 the 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. Note that for the minimum of 0.25, the percentage mitre is 96%, 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.
Read more about this topic: Microstrip
Famous quotes containing the word bends:
“Silent is the house: all are laid asleep:
One alone looks out o’er the snow-wreaths deep,
Watching every cloud, dreading every breeze
That whirls the ‘wildering drift, and bends the groaning trees.”
—Emily Brontë (1818–1848)
“What did you hit her for?
I’ll let you in on something, she’s a lush, the lady. After she bends the elbow a few times, she begins to see things. Rats, roaches, snakes, bats, you know. A sock in the kisser’s the only thing that’ll bring her out of it.”
—Richard Brooks (1912–1992)
“He bends to the order of the seasons, the weather, the soils and crops, as the sails of a ship bend to the wind. He represents continuous hard labor, year in, year out, and small gains. He is a slow person, timed to Nature, and not to city watches. He takes the pace of seasons, plants and chemistry. Nature never hurries: atom by atom, little by little, she achieves her work.”
—Ralph Waldo Emerson (1803–1882)