Directional couplers and splitters (power dividers) have different names, but they can often be used in the same way. Because power transfer in passive systems is reciprocal, a splitter can work as a coupler and a coupler can work as a splitter.
Splitters usually, but not always, have equal power outputs whereas couplers may have equal power outputs (-3dB couplers) or may tap off just a fraction of the power in a coupled port.
The coupled port of a coaxial directional coupler is usually situated on the side and is near the common port.
A typical application of a directional coupler or a splitter in a wireless system would be to connect two antennas to a radio. The radio would be connected to the common port and the antennas would connect to the thru and coupled ports. Note that if you get it wrong and attach the radio to the thru port you don’t get any power, or hardly any, out of the coupled port.
The terms common port and thru port are often replaced by numbers or names like “IN” and “OUT” but its better to avoid these terms with wireless because the power flow is bi-directional.
A directional coupler will be typically 3dB (50,50) , 6dB (75,25), 7dB (80,20), 10dB (90,10) or 20dB (99,1) . The figures in brackets show the % split in the power.
The through loss of a: 6dB coupler is 1dB, 10dB coupler is 0.5dB and 20 dB coupler is less than 0.1dB.
So, in practice, you would hardly notice the power loss in the thru port with 10dB and 20 dB couplers but the tapped off power may well be just enough for it to be useful.
Typical Coupler Specifications
Frequency Range: 0.8 – 2.5GHz
Coupling Faltness: +/-0.5dB max
Thru Loss: 0.25dB max
Directivity: 20dB min
VSWR: 1.2:1 max
Power rating: 50 W
Typical Plots of Through Loss and Return Loss