Why A dummy load replaces the antenna so you can test or tune a transmitter without radiating a signal. It must look like a perfect antenna to the transmitter, meaning a purely resistive 50 ohms with no reactance, so the resistive element is made non-inductive (carbon or thin-film rather than wirewound). All the RF power turns into heat, so the resistor is bolted to a heat sink or immersed in cooling oil to handle the transmitter's output.
Watch out The choice with 50 ohms of inductive reactance is the trap: reactance would store and reflect energy rather than absorb it, giving a poor match and an inaccurate test. Relay and power supply answers describe switching gear, not a load.
Dummy load = 50 ohms of pure resistance plus a way to dump the heat. No coils, no radiation.