Free RF Directional Coupler & Directivity Calculator

RF Directional Couplers: Directivity, Coupling & Uncertainty

A directional coupler is a passive 4-port microwave device that samples a precise fraction of the forward traveling wave while isolating against signals traveling in the reverse direction. They are central to RF power monitoring, transmitter ALC (automatic level control), and return loss/VSWR reflectometer bridges.

Fundamental Coupler Equations

• Coupling Factor ($C$ in dB): $C = 10 \log_{10}\left(\frac{P_{in}}{P_{cpl}}\right)$
• Coupler Isolation ($I$ in dB): $I = 10 \log_{10}\left(\frac{P_{in}}{P_{iso}}\right) = C + D$
• Directivity ($D$ in dB): $D = I - C = 10 \log_{10}\left(\frac{P_{cpl}}{P_{iso}}\right)$
• Mainline Insertion Loss ($IL$ due to coupling): $IL = -10 \log_{10}\left(1 - 10^{-C/10}\right)$

Why Coupler Directivity Causes VSWR Measurement Errors

In a dual directional coupler or SWR bridge, the reverse coupled port detects the signal reflected by the antenna ($|\Gamma_L|$). However, because directivity is finite (typically 20 to 35 dB), a tiny fraction of the massive forward wave ($|\Gamma_D| = 10^{-D/20}$) leaks directly into the reverse port.

Because the phase between the real reflection and the leakage signal is unknown (it rotates with cable length and frequency), the two signals add vectorially at unknown phase angles, creating an uncertainty envelope: $$|\Gamma_{meas,min}| = |\Gamma_L| - |\Gamma_D|, \quad |\Gamma_{meas,max}| = |\Gamma_L| + |\Gamma_D|$$ For instance, if you test an antenna with a 20 dB Return Loss (1.22:1 VSWR) using a coupler with only 20 dB directivity, the measured return loss can display anywhere from 14.0 dB (1.50:1 VSWR) to >40 dB (1.02:1 VSWR)!