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How to Build an Antenna Dummy Load

A dummy load is a device to which a radio transmitter is connected to test power output in place of performing the test over the air on an antenna. Dummy loads are also used to test the power output of an amplifier when tuning up a radio or measuring the SWR (Standing Wave Ratio). The dummy load will dissipate the heat safely and prevent damage to equipment. Constructing a jar filled with salt water and the right connector is an inexpensive way to build a homemade dummy load.

Things You'll Need

1-gallon glass jar with a plastic lid
Drill
5/8-inch drill bit
1/8-inch drill bit
1 SO-239 connector with nuts and screws
Screwdriver
Needlenose pliers
2 feet of solid copper wire, 12 gauge
Soldering iron
Solder
Wire cutters
Butter knife
1 package of table salt
SWR analyzer

Instructions

- 1
  Remove the lid from a gallon glass jar and place it flat on a table. Drill a 5/8-inch hole in the center of the lid. Insert an SO-239 connector into the lid from the top so the female side of the connector with the threads is facing up.
- 2
  Drill a 1/8-inch hole through the existing holes (using these holes as a template) in each corner of the connector and through the lid. Insert a screw into each hole and tighten down a nut on each one to secure the connector to the lid. Use a screwdriver and pliers to tighten the nuts on each screw.
- 3
  Cut a 2-foot length of 12-gauge copper wire into halves. Bend a small loop in the end of one of the wires. Attach this end under one of the nuts on the connector and tighten it back down.
- 4
  Solder the one end of the remaining copper wire to the tip on the bottom of the connector, which will be inside the jar when the lid is screwed on.
- 5
  Insert the wires into the jar and spread them apart along opposite sides of the jar walls. Cut the bottoms of the wire so they are about 1/2 inch clear of the bottom of the jar when the lid is screwed down tight.
- 6
  Screw the cable from an SWR analyzer onto the female end of SO-239 connector.
- 7
  Fill the jar with tap water. Screw the lid back on. Using the analyzer and following the manufacturer's guidelines, check the SWR reading of the water. The reading should be fairly high.
- 8
  Unscrew the lid and use the butter knife to place to a small amount of salt in the water. Screw the lid back on and recheck the SWR. Repeat this step until the SWR reads at a ratio of 1:1.
- 9
  Disconnect the analyzer from the dummy load and attach the cable from the transmitter. Test the radio for power output, using the functional dummy load and following the manufacturer's guidelines for your model of radio.

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