Microstrip Line Directional Coupler
An apparatus, e.g. a radio-frequency (RF) coupler, includes first and second microstrip lines. First and second coupling fingers extend from the first microstrip line into the space, and a third coupling finger extends from the second microstrip line into the space about centered between the first and second coupling fingers. Fourth and fifth coupling fingers extend from the first microstrip line into the space, and a sixth coupling finger extends from the second microstrip line into the space about centered between the fourth and fifth coupling fingers. The second and fourth coupling fingers are adjacent each other with a distance between them along the first transmission line exceeding a distance between the first and second coupling fingers along the first transmission line by at least a factor of about two.
1 . An apparatus, comprising:
a first microstrip line formed on a dielectric surface;
a second microstrip line formed on said surface about parallel to said first microstrip line and separated from said first microstrip line by a space (D s );
first and second coupling fingers connected to said first microstrip line and extending into said space;
a third coupling finger connected to said second microstrip line, extending into said space and being about centered between said first and second coupling fingers;
fourth and fifth coupling fingers connected to said first microstrip line and extending into said space; and
a sixth coupling finger connected to said second microstrip line extending into said space and being about centered between said fourth and fifth coupling fingers,
wherein said second and fourth coupling fingers are adjacent each other and a distance between said second and fourth coupling fingers along said first transmission line exceeds a distance between said first and second coupling fingers along said first transmission line by at least a factor of about two.
2 . The apparatus of claim 1 , wherein a sum of a length of the first and third coupling fingers is no greater than a distance between said first and second microstrip lines.
3 . The apparatus of claim 1 , wherein a sum of a length of the first and third coupling fingers is greater than a distance between said first and second microstrip lines.
4 . The apparatus of claim 1 , wherein a combined length of the first and third coupling fingers is between about 1.0 and about 1.4 times a distance between said first and second microstrip lines.
5 . The apparatus of claim 1 , wherein the distance D s between said first and second microstrip lines is about 1.25 mm, a length D 1 of the first coupling finger is about 0.79 mm, and a length D 2 of the third coupling finger is about 0.57 mm.
6 . The apparatus of claim 1 , wherein said first microstrip line has a width of about 0.78·D s or less.
7 . The apparatus of claim 1 , wherein said second microstrip line has a width of about 0.68·D s or less.
8 . The apparatus of claim 1 , wherein said first coupling finger is spaced about 1.125 mm from said second coupling finger.
9 . The apparatus of claim 1 , wherein said first microstrip line is coupled to a transmitter configured to provide a signal having a frequency in a range between about 1800 MHz and about 2200 MHz.
10 . The apparatus of claim 1 , wherein said substrate comprises alumina.
11 . The apparatus of claim 1 , wherein said substrate comprises Rogers Laminate 4350B.
12 . The apparatus of claim 1 , wherein a coupling between first and second ends of said second microstrip line is about −29.3 dB at about 1950 MHz.
13 . A method, comprising
forming a first microstrip line over a dielectric surface;
forming a second microstrip line over said surface about parallel to said first microstrip line and separated from said first microstrip line by a space (D s );
connecting first and second coupling fingers to said first microstrip line and extending into said space;
connecting a third coupling finger to said second microstrip line, extending into said space and being about centered between said first and second coupling fingers;
connecting fourth and fifth coupling fingers to said first microstrip line and extending into said space; and
connecting a sixth coupling finger to said second microstrip line extending into said space and being about centered between said fourth and fifth coupling fingers,
wherein said second and fourth coupling fingers are adjacent each other and a distance between said second and fourth coupling fingers along said first transmission line exceeds a distance between said first and second coupling fingers along said first transmission line by at least a factor of about two.
14 . The method of claim 13 , wherein a sum of a length of the first and third coupling fingers is no greater than a distance between said first and second microstrip lines.
15 . The method of claim 13 , wherein a sum of a length of the first and third coupling fingers is less than a distance between said first and second microstrip lines.
16 . The method of claim 13 , wherein a sum of a length of the first and third coupling fingers is less than about 0.85 times a distance between said first and second microstrip lines (Ds).
17 . The method of claim 13 , wherein the distance D s between said first and second microstrip lines is about 1.25 mm, a length D 1 of the first coupling finger is about 0.7883 mm, and a length D 2 of the second coupling finger is about 0.5683 mm
18 . The method of claim 13 , wherein said first microstrip line has a width of about 0.78·D s or less.
19 . The method of claim 13 , wherein said second microstrip line has a width of about 0.68·D s or less.
20 . The method of claim 13 , wherein said first coupling finger is spaced about 0.4125 mm from said second coupling finger.
21 . The method of claim 13 , wherein said first microstrip line is coupled to a transmitter configured to provide a signal having a frequency in a range between about 1800 MHz and about 2200 MHz.
22 . The method of claim 13 , wherein said substrate comprises alumina.
23 . The method of claim 13 , wherein said substrate comprises Rogers Laminate 4350B.
24 . The method of claim 13 , wherein a coupling between first and second ends of said second microstrip line about −29.3 dB at 1950 MHz.