Also known as: directional coupler, coupler
A directional coupler is a passive four-port network that taps a small, direction-selective sample of a signal travelling on a transmission line.1 Because it responds to waves going one way but not the other, it lets you measure forward and reflected power separately — the basis of power meters, VSWR bridges, and monitoring taps.
Overview
Three numbers describe a coupler. Coupling factor (e.g. 20 dB) is how much weaker the sampled signal is than the main line — a 20 dB coupler taps one-hundredth of the power. Directivity is how well it distinguishes forward from reverse; high directivity means the coupled port reports the forward wave with little contamination from the reflected wave. Insertion loss is the small amount the main-line signal drops by giving up that sample. The main path is nearly transparent, so a coupler can be left permanently in line.
How it works
A directional coupler places a second transmission line close enough to the main line that electromagnetic coupling transfers a small, fixed fraction of the energy into it. The geometry is arranged so the coupling is phase-sensitive: a forward-travelling wave adds up toward the coupled port, while a reverse-travelling wave adds up toward the isolated port. Terminate the isolated port and the coupled port then carries a clean sample of just the forward wave. Swap which wave you sample — or use a dual (bidirectional) coupler with taps for both directions — and you can read forward and reflected power at the same time.
From those two readings you get the reflection coefficient, and hence the load’s return loss and standing-wave ratio — which is exactly how an in-line SWR/power meter works.
Variants
- Single vs dual (bidirectional) — one coupled port, or separate forward and reverse ports.
- Coupled-line, branch-line, and Lange couplers — different geometries and bandwidths.
- Hybrid couplers — the special 3 dB, 90°/180° cases used as splitters/combiners and in balanced mixers.
Relevance to SDR
Directional couplers appear wherever RF power must be monitored or a controlled sample tapped off: transmitter SWR/power metering, antenna-system diagnostics, feedback for automatic gain control and pre-distortion, and bench measurements. On the receive side a low-coupling tap can inject a calibration tone or split off a monitoring feed without appreciably loading the main path.
GopherTrunk is receive-only software and does not include or need a directional coupler. The device is relevant to GT users chiefly as an antenna-system diagnostic tool — a coupler and power meter reveal a bad match or feedline fault that would otherwise show up only as weak, error-prone signals in the decoder.
Sources
-
Power dividers and directional couplers — Wikipedia, on coupling factor, directivity, and forward/reflected sampling. ↩