A dummy load consists of a non-inductive resistor having a dissipation equal to the expected output power. Such a dummy load would have a resistance of about 50 or 70Ω and may consist of a parallel or series/parallel connection of carbon resistors; for example, ten 5W 680Ω resistors may be connected in parallel to provide a 50W 68Ω load which would be satisfactory for a 75W input transmitter. The dissipation of the load may be increased by immersing it in oil. A dummy load should be screened.
On the left, a home made dummy load (screening cover removed). The load consists of 20 x 1kΩ 3W resistors in parallel. The LED near the input connector lights in the presence of over 0.5W of RF. The load is useable up to about 500MHz. To the right is a commercial unit.
