A QRP operation is an amateur radio transmitter that operates with an output power of only five watts. Ham radio operators routinely communicate around the world despite such low output powers. An operator uses a 50-ohm dummy load when he tunes and adjusts his radio for optimum performance. The dummy load prevents the radio's signal from being accidentally broadcast during testing. Dummy loads also provide the proper impedance for the transmitter.
- Skill level:
Things you need
- 150-ohm 5-watt carbon composition resistors, 3
- Soldering iron
- Heat shrink
- Heat gun
- 14-gauge stranded wire, 2 pieces
Arrange the three resistors so they are parallel. Twist the leads on each end together. Ohms Law states that the total resistance of three equal resistors in parallel is 1/3 the individual resistance or three parallel 150-ohm resistors will equal 50 ohms.
Strip the ends off each of the two wire pieces. Solder one wire to each end of the parallel resistors.
Place a piece of heat shrink over the resistors. Make sure the bare resistor leads are covered.
Shrink the heat shrink with a heat gun.
Solder one of the wire ends to the centre conductor of the SO-239 RF connector. Solder the other wire to the outer conductor of the connector.
Tips and warnings
- Achieve a professional look by installing the connector and parallel resistors in a small electronics enclosure.
- The resistors will get hot if the transmitter power exceeds 5 watts.
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