0.1 ≤ W1/H ≤ 3 0.1 ≤ W2/H ≤ 3 1≤ W2/W1≤3 6≤ε≤13 0.2≤S/H ≤∞ 0.2 Ghz≤f≤18 Ghz W1 is width of microstrip 1 and W2 is width of microstrip 2. S is the gap between them. ε is the permittivity of the substrate.