Directional Coupler Design Calculator

Enter Design Inputs

This calculator focuses on electrical targets. Final width and spacing still depend on substrate height, copper thickness, and field solver verification.

Center Frequency

Frequency Unit

System Impedance (Ω)

Coupling (dB)

Effective Dielectric Constant

Electrical Length (deg)

Input Power (W)

Directivity (dB)

Return Loss (dB)

Excess Loss (dB)

Reset

Formula Used

1) Power coupling ratio
Coupling in decibels converts to a linear power ratio using: P_c/P_in = 10^-C/10

2) Amplitude coupling coefficient
The coupled-line impedance equations use the amplitude coefficient: k = 10^-C/20

3) Even and odd impedances
For a symmetric quarter-wave coupled-line coupler: Z_e = Z₀ √((1 + k)/(1 - k))
Z_o = Z₀ √((1 - k)/(1 + k))

4) Guided wavelength and line length
v_p = c / √ε_eff
λ_g = v_p / f
L = λ_g × (θ / 360)

5) Isolation estimate from directivity
The isolated-port leakage is estimated from desired coupled power: P_iso = P_c / 10^D/10

6) Return loss and VSWR
|Γ| = 10^-RL/20
VSWR = (1 + |Γ|) / (1 - |Γ|)

The impedance formulas assume a symmetric quarter-wave directional coupler. If you choose another electrical length, this page rescales line length, but practical coupling flatness still centers near 90 degrees.

How to Use This Calculator

Enter the center frequency and select the correct unit.
Set the system impedance, usually 50 ohms in RF work.
Enter the target coupling in dB, such as 10 dB or 20 dB.
Use an effective dielectric constant that matches your line environment.
Keep electrical length at 90 degrees for classic quarter-wave operation.
Add input power, directivity, return loss, and excess loss values.
Press Calculate Design to show the result above the form.
Review the impedances, line length, power split, and graph.
Download the result as CSV or PDF if needed.
Use these targets in a transmission-line calculator or EM simulator.

Example Data Table

Frequency	Z0	Coupling	εeff	Input Power	Directivity	Return Loss	Excess Loss	Ze	Zo	Quarter-Wave Length	Coupled Power
2.45 GHz	50 Ω	10 dB	2.90	20 W	25 dB	20 dB	0.40 dB	69.3713 Ω	36.0380 Ω	17.9637 mm	1.8240 W

This sample shows a moderate-coupling RF design. Use your actual stackup for final physical geometry.

FAQs

1) What does coupling in dB represent?

It shows how much input power reaches the coupled port. Smaller dB values mean tighter coupling and more sampled power. Larger dB values mean lighter sampling.

2) Why are even and odd impedances different?

A coupled-line directional coupler supports two propagation modes. Their unequal impedances create the phase and amplitude conditions required for directional power transfer.

3) Why is 90 degrees the default electrical length?

Quarter-wave couplers are usually designed around 90 degrees at the center frequency. That electrical length gives the standard directional response for backward coupled structures.

4) Does this calculator produce physical width and spacing?

Not directly. It generates electrical targets like Ze, Zo, and length. Physical dimensions still require substrate height, metal thickness, and a line-geometry solver or EM simulator.

5) How does effective dielectric constant affect the result?

It changes phase velocity and guided wavelength. Higher effective dielectric constant shortens the physical length needed for the same electrical angle.

6) What does directivity change in the outputs?

Directivity mainly affects isolated-port leakage. Higher directivity means less unwanted power at the isolated port and better discrimination between forward and reverse waves.

7) Why are return loss and VSWR included?

They help evaluate match quality at the operating point. Better return loss means less reflected power and a VSWR closer to one.

8) Can I use this for microstrip and stripline projects?

Yes, as an electrical planning tool. Use an appropriate effective dielectric constant, then convert the targets into physical dimensions with the correct stackup model.