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 XL=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 about 80 microhenries (80µH) worth of inductance. Doesn’t sound like much but looking at the chart below, we see that even that small amount makes this a 1KΩ load at even the lowest amateur frequency and just gets worse from there.
|Band||Low MHz||High MHz||Center Freq||Ω||SWR||XL|
XL is the calculated reactance for 80µH at the center frequency of each ham band. Add this to the 50Ω resistance to get total impedance. SWR is above 10:1 on all bands; a terrible load.
So it’s pretty obvious that a non-inductive resistor is needed. Capacitance can also cause reactance and non-linear impedance in a load but nowhere near the extent of inductance.
Since all the transmitter power is going into a resistor, the energy is converted to heat that has to go somewhere or risk burning up the load. To dissipate heat, a dummy load might have a large mechanical (finned) heat sink. Alternately—or additionally—forced air can be employed, typically a compact fan or blower. Conduction or convection heat sinks are “dry” loads.
To provide even more heat dissipation, some dummy loads bathe the resistor in a sizeable volume of electrically non-conductive oil. The old Cantenna designs used this “wet” load.
By the way, a dummy load is an inexpensive, simple, and useful DIY project for any ham to develop some practical radio amateur skills.
The November 2018 issue of QST magazine has a nice home brew dummy load project on page 32. It includes a companion digital watt meter if that interests you as well.