Unfortunately, the ideal directional coupler does not yet exist. However, system performance will benefit greatly by keeping key concepts and parameters in mind.
The basic directional coupler is a four port junction that is used in a wide variety of microwave systems to satisfy almost any requirement for sampling incident and reflected microwave power conveniently and accurately with minimal disturbance to the transmission line. The basic configuration of a single directional coupler is shown in Figure 1, which illustrates two parallel transmission lines over a length of one-quarter wavelength, corresponding with the center frequency of operation. The main and secondary lines are separated by a calculated physical distance which determines the coupling factor of the device. The physically closer the lines are to each other, the more power will be introduced on the secondary line. The term coupling denotes how much of the input power is sampled to the coupled port and is defined as 10 times the ratio of Incident Power to Forward Power C = 10 log10(Pf/Pi). Typical coupling values found in practice are 3, 6, 10, 20, 30 and 40 dB; however, practically any coupling value may be obtained through proper design.
A dual directional coupler (see Figure 2) is essentially two single directional couplers connected back-to-back sharing a common mainline and providing two output ports with high isolation between those ports. This high isolation is critical for the accuracy of reflectometer setups that simultaneously sample input power to a device or load providing a ratio of signals for the purpose of determining return loss in decibels.
Directional Coupler Theory and Definitions
Consequently, a small amount of backward power will be coupled to the secondary line 180° out of phase from the incident wave canceling power on the secondary line and adding uncertainty to the measurement. The term directivity (see Figure 3) denotes the ratio of forward to backward coupling and is defined as 10 times the common log of the ratio of forward to backward power D = 10 log10(Pf/Pb). The higher the value of directivity, the less backward power is sampled and measurement uncertainty is significantly improved. Directivity is the qualitative benchmark by which couplers are compared.
The frequency sensitivity, or "flatness," of a coupler is a measure of how coupling varies over a given frequency range. Optimum coupling frequency response is achieved by "centering" the design within the specified band of interest. Typical coupling flatness for a quarter-wavelength coupler operating over an octave band is within ±0.75 dB of nominal. All things being equal, stronger coupling factors (3, 6 and 10 dB) exhibit greater flatness than weaker coupling factors (20 through 40 dB). When operating over frequency bands greater than an octave, the flatness tolerance may need to be relaxed due to the inherent characteristics of coupling roll-off.
When specifying a directional coupler with a coupling factor stronger than 20 dB (3, 6 or 10 dB), consideration should also be given to the theoretical insertion loss caused by power coupling from the mainline. Table 1 illustrates the amount of additional loss the device exhibits as a function of the proximity of the two transmission lines. It should also be noted that dual directional couplers exhibit twice the loss of single directional models because there are two secondary lines drawing power from the mainline.
Directional Coupler Solutions
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Table 1. Theoretical Mainline Insertion Loss Due to Coupling Factor (dB)