Colloquially referred to as a “splitter”, an RF Power Divider is a passive (unpowered) component which divides signal power equally among two or more paths.
A 2-Way Power Divider provides a 50:50% split between the two output ports. This means that signal entering the input port will exit both output ports -3 dB lower than the input power level. For example, a -90 dBm RSRP signal will become -93 dBm at the output ports.
The same logic applies for 3-Way and 4-Way Power dividers, with a 3-Way providing a 33:33:33% split between each output (-4.8 dB), and a 4-Way providing 25% to each port (-6 dB).
When reversing the device it acts as a “Combiner”. When looking at the uplink path, for example, two modems or repeaters feeding back to a shared donor antenna, the power levels depend on the design type of the Power Divider. The most common type of Power Divider is the Wilkinson Divider, which isolates the output ports from one another. This means that uplink signal entering through an output port is again equally split between the other ports, but power towards the other ‘output’ ports is attenuated often by up to 22 dB. This means the uplink path incurs an ordinary -3 dB power level reduction back up the donor antenna, but does not overload any other user devices connected to the other output ports.
Reactive power dividers are a higher performance design capable of handling substantially higher power levels and performing more efficient splits. The downside however is that out ports are not isolated, making them only suitable for the connection of passive devices where inter-port isolation is not important.
It is important to be clear that in reverse a Combiner still incurs -3 dB loss, because 50% of signal will either be dissipated by the resistor (Wilkinson), or exit via the other output port (Reactive).