Radio amateurs should be familiar with the term dummy load, which is a RF-friendly substitute for an antenna when testing a transmitter or piece of equipment such as a Watt meter.

A dummy load is somewhat generic, also having industrial and commercial uses.  As applied in ham radio, it electrically simulates an antenna to allow a transmitter to be tested without radiating radio waves, typically at 50Ω to match transmitter output impedance.

Dummy loads are rather simple—  just a big resistor and some way to dissipate heat, all in a package that must be non-reactive, meaning it provides insignificant capacitance and inductance.

Why must a dummy load be non-inductive?  Because of impedance (practically speaking, AC resistance), which increases with frequency based on the formula of inductive reactance X_L=2πfL.

Most common power resistors are wire-wound, which have significant inductance.  So RF dummy loads must use resistors with little or no inductance.

As an example, this four-resistor series combination using common Dale metal-clad resistors measures 49.4Ω at 0Hz (DC).  Sounds like a perfect dummy load, right?

Unfortunately  it also has Continue reading →