Coupled Lines Directional Coupler

Directional couplers are four-port circuits consists of through, isolation, coupling ports. An ideal directional coupler is lossless and has all ports are matched. One of the most common couplers is formed by a pair of coupled transmission lines. When two unshielded transmission lines are in close proximity, power can be coupled from one line to the other due to the interaction of the electromagnetic fields. They usually consist of three conductors in close proximity, although more conductors can be used. A geometry and port designations for a single-section coupled line coupler is shown in Fig. 1

Figure 1: Geometry and port designations of a single-section coupled line coupler.

A single-section couple line requires a transmission line of λ/4 in length. This causes a very limited in bandwidth for the coupler. This issue can be overcome by using multiple sections to obtain wideband coupler as shown in Figure 2.

The multi-section coupled line couplers are generally made with an odd number of sections. In this video, we will use N = 5 to design a 10 dB coupled line coupler with a binomial (maximally flat) response, a system impedance of 50 Ω, and a center frequency of 5 GHz. We will plot the response from 1GHz to 9 GHz.

Figure 2: N-section coupled line coupler

The calculated values for the even impedance and odd impedance for each section are:

Section 1: Z0e(1) = 50.38320340 Ω, Z0o(1) = 49.61971116 Ω
Section 2: Z0e(2) = 53.69294350 Ω, Z0o(2) = 46.56105322 Ω
Section 3: Z0e(3) = 78.78484240 Ω, Z0o(3) = 31.73199214 Ω
Section 4: Z0e(4) = 53.69294350 Ω, Z0o(4) = 46.56105322 Ω
Section 5: Z0e(5) = 50.38320340 Ω, Z0o(5) = 49.61971116 Ω

Below is the video on how to design using FILPAL EDS