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This small dummy load is very useful working on qrp rigs and portable for field day. Even though the ideal dummy load
would be at 50 ohms impedance, the standard series of resistors makes it hard to find a good combination of resistors
without spending an arm and a leg. After searching the local market, eight 390 ohm 2 watt 2 percent resistors were used
in parallel. These resulted in a nominal impedance of 48.75 ohms. This is 2.5 percent of the target 50 ohms. Using available
inexpensive resistors, this about as close as you can get. The useable power level will be discussed later on.
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First thing to do is locate an available container for the dummy load. A near empty Minwax Wood Finish can was on hand
and was elected. It is important to select a container that provides good sheilding. Many long years ago, a station about
200 miles away was worked on 40 meters using five watts with a light bulb dummy load and six inch leads.
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The Minwax can was stripped of the labeling and cleaned up. Might as well have the hardware look good.
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A hole was drilled in the ex-Minwax lid to accept a BCN connector. The BCN connector was inserted and snugged down
tight
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A view of the lid on the can shows that the BCN connector was soldered to the top of the lid.
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A circle was cut out thin tin. Eight notches were cut around the edge of the circle and a hole drilled in the
center. The eight 390 ohm resistors were soldered in the edge notches. The leads were kept short. The bottom
leads were bent at right angles away from the circle center.
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A lead was soldered to the BNC connector. Each of the bent resistor leads were snipped short. The BNC lead was
fed through the center hole of the tin circle and the snipped resistor leads set flush on the inside can lid. The
center BNC lead was soldered to the tin circle. Each bent resistor lead was soldered to the inside can lid.
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The power level of the dummy load now was considered. Eight two watt resistors gives 16 watts dissipation. However,
they are contained in a very small space, so it should safely dissipate eight watts. Filling the can with a heat
absorbing dielectric oil should safely dissipate twenty to twenty five watts. Good old fashioned mineral oil economicallyv
fits the bill.
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Put the lid on the can and you have a small dummy load.
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