Virtual Laboratory for Microwave Engineering
Understand the fundamental operation and physical construction of directional couplers
Analyze S-parameters and scattering matrix representation of 4-port networks
Calculate coupling factor, directivity, and isolation using measured power levels
Visualize power flow and wave propagation in coupled transmission lines
Design couplers with specific coupling values for microwave applications
Evaluate performance metrics: Return Loss, Insertion Loss, and VSWR
A directional coupler is a 4-port passive device that samples a small amount of microwave power for measurement purposes. It consists of two transmission lines (main line and coupled line) with electromagnetic coupling between them. The device is reciprocal, matched, and lossless (ideally), with power flowing from Port 1 to Port 2 (through), Port 3 (coupled), and ideally zero power to Port 4 (isolated).
For a symmetric, reciprocal, matched, and lossless directional coupler, the S-matrix is:
Where P = Power transmission coefficient, C = Coupling coefficient, P + C = 1
Ratio of input power to coupled power (in dB):
Typical values: 3dB, 6dB, 10dB, 20dB, 30dB
Ratio of coupled power to isolated power:
Higher is better (typically 20-40 dB)
Ratio of input power to isolated power:
Power loss in the main line:
Connect the directional coupler to the signal generator (Port 1) and spectrum analyzer. Set the operating frequency to 2.4 GHz (ISM band) or desired frequency. Ensure all ports are terminated with 50Ω loads except the measured port.
Apply input power P₁ to Port 1. Measure power at Port 3 (coupled port) using P₃. Calculate coupling factor: C(dB) = 10 log₁₀(P₁/P₃). Record values for different frequencies across the band.
With input at Port 1, measure power at Port 4 (isolated port) using P₄. Calculate directivity: D(dB) = 10 log₁₀(P₃/P₄). This indicates how well the coupler discriminates between forward and reverse waves.
Measure power at Port 2 (through port) using P₂. Calculate insertion loss: IL(dB) = 10 log₁₀(P₁/P₂). Verify that IL ≈ Coupling Loss (theoretical minimum based on coupling factor).
Measure reflected power at Port 1 to calculate return loss and VSWR. Ideally, input return loss should be >20 dB (VSWR < 1.2) for a well-matched coupler.
Perform frequency sweep from 1-4 GHz to observe bandwidth characteristics. Plot coupling, directivity, and isolation vs. frequency. Identify the center frequency and 1-dB bandwidth.
Safety Note: Ensure power levels do not exceed device ratings (+20 dBm typical). Use proper ESD precautions when handling microwave components.
Note: For an ideal lossless coupler, total output power equals input power minus coupling loss. In practice, add ~0.1-0.5 dB for conductor and dielectric losses.
Submission: Submit PDF report via course portal. Include screenshots of simulation results and photos of actual measurements if hardware lab was performed. Due date: 1 week after lab session.