Dummy Load

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.

Dummy1   Dummy2   Dummy3

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.

T7C01-2018

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.

T7C12-2018

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.

XL

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?

DSCN4385

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
DC 0.000 0.000 0.000 49.4 1:1
160 1.800 2.000 1.900 >1K >9.9 955.04415
80 3.500 4.000 3.750 >1K >9.9 1884.9556
60 5.331 5.404 5.368 >1K >9.9 2697.9997
40 7.000 7.300 7.150 >1K >9.9 3593.9819
30 10.100 10.150 10.125 >1K >9.9 5089.38
20 14.000 14.350 14.175 >1K >9.9 7125.132
17 18.068 18.168 18.118 >1K >9.9 9107.1
15 21.000 21.450 21.225 >1K >9.9 10668.848
12 24.890 24.990 24.940 >1K >9.9 12536.211
10 28.000 29.700 28.850 >1K >9.9 14501.591
6 50.000 54.000 52.000 >1K >9.9 26138.05
2 144.000 148.000 146.000 >1K >9.9 73387.603
70 cm 420.000 450.000 435.000 >9.9 218654.84

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.

Dummy5

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.

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