Directional couplers have been used in various telecommunication devices such as wireless and radar systems, and also in power sources for monitoring microwave power. A coupler typically comprises a pair of electromagnetically coupled transmission lines and four ports: input, through, coupled, and isolated. The term “main line” generally refers to the transmission line between the input and through ports. The coupled port of the directional coupler can be used to obtain information such as frequency and power level of the signal on the main line without substantially interrupting the power flow on the main line and independently of the termination at the through port.
The “coupling factor” of a directional coupler relates the power output at the coupled port to the power input at the input port. Similarly, the “directivity” of a directional coupler relates the power output at the coupled port to the power output at the isolated port. High directivity helps to differentiate between the power supplied at the input port and the port reflected back due to mismatch at the through port. A directional coupler is generally designed to exhibit a desired coupling factor and high directivity. Thus, an important goal of a directional coupler is to monitor the power from the source to provide a desired power level at the through port while isolating the variation due to the load at the through port.
The coupling factor and directivity of a directional coupler vary with frequency of the signal to be transmitted and the impedance of the transmission line, which itself varies according to the frequency of the signal to be transmitted. A signal transmitted along a directional coupler has two phase velocities, namely, an even-mode phase velocity and an odd-mode phase velocity. The directivity of the coupler is high when the two velocities are identical or nearly identical. The directivity decreases, however, when the two velocities are different. The even-mode and odd-mode phase velocities can be changed by varying the impedance of the conductors in the directional coupler, but changing impedance can also result in altering the coupling factor so that the desired level of coupling is no longer achieved.
Solutions have been offered to provide high directivity such that a desired coupling factor can be maintained over a large frequency range (i.e. large bandwidth). Some of these solutions, however, require long conductors, typically of length at least one-fourth the wavelength of the signal to be transmitted. A long directional coupler may not be suitable for use, e.g., in a microcircuit or a computer chip and may also reduce the total power at the through port due to dissipation losses associated with a long conductor. Some previous solutions have provided for balancing the even-mode and odd-mode phase velocities, so as to achieve high directivity, when one type of phase velocity is greater than the other—e.g., by altering the even or odd mode transmission line impedance. Unfortunately, this alters the coupling factor from its desired value. Therefore, there is a need for a directional coupler that is small in length and can balance even-mode and odd-mode phase velocities (when either velocity can be greater than the other) over a large frequency range, without substantially altering the desired coupling.