An RF power divider creates two or more controlled output paths from a common input; a directional coupler extracts a defined fraction of a travelling wave while preserving a main through path. Equal dividers are selected by port count, theoretical split, excess loss, amplitude/phase balance, output isolation, return loss and power. Couplers are selected by coupling factor, coupling flatness, directivity, main-line loss, return loss, orientation and power.
Use a divider when multiple branches need known relative amplitude and phase, such as parallel receiver paths, local-oscillator distribution or coherent test channels. Use a directional or bidirectional coupler when the system must monitor forward or reflected power, level a source, detect mismatch or feed a measurement channel without taking half of the main-line power.
Do not substitute one only because the connector count looks similar. A coupler normally provides unequal outputs and a defined isolated port; a divider normally provides comparable branch outputs. The RFQ should state signal direction, every port function, system impedance, full band, required ratio, loss definition, isolation or directivity, match, average/peak power, mismatch condition and the termination used on unused ports.