Microstrip Directional Coupler

1. Theory

- Direction coupler extracts a portion of a wave traveling in a direction on a transmission line.

- Applications of directional couplers:

Transmitted power monitoring

Reflected power monitoring

Figure: Directional coupler symbol. P1/P2 = insertion loss, P1/P3 = coupling, P1/P4 = isolation, P3/P4 = directivity

Figure: Short-section coupled line directional couplers and equivalent circuit representation.

Figure: Microstrip quarter-wave directional coupler topology. Left: single section coupler, right: multi-section wideband coupler. Port 1: input, Port 2: through, Port 3: coupled, Port 4: isolated

2. Applications

Figure: Directional couplers used in microwave devices.

Microstrip directional coupler for an X-band motion sensor

CIAS MURENA Plus, https://www.cias.it/en/products/sensors/murena/

Spec.: 24-m range, dual frequency Doppler sensor, 10-11 GHz

Figure: Left = MURENA motion sensor, Midde: Intruder detection by motion sensor, Right = Internal view of the 4-element patch array antenna and the motion sensor board.

Figure: Block diagram of the MURENA motion sensor [Sanna(2018)]

3. Laboratory

Figure: Test model of the directional coupler in the MURENA motion sensor [Sanna(2018)]

Figure: A 10.5-GHz direction coupler. Substrate: ε_r = 3.5 , tanδ = 0.003, h = 0.752, t = 0.035; Coupler: W₁ = 1.7, W₂ = 1.4, S = 0.2, L = 4.2, A = 2.2, H = 4.5 (unit: mm). When the port 1 is the input, the ports 3 and 4 are the coupled and isolated ports respectively.

1. Make the structure

2. Simulate the structure with wave-port source.

3. Plot S11, S21, S31, S41 at 9-12 GHz.

4. Find the following values at 10.5 GHz.

Insertion loss = ( ) dB

Coupling = ( ) dB

Directivity = ( ) dB