100W HF Dummy load

A dummy load is an important tool for testing and tuning equipment. There are a lot of commercially available dummy loads, but simple projects like this help get back to the home brew roots of amateur radio. All that is needed to build your own dummy load is non inductive resistors a connector and a paint can. Total cost of this project was $15. The extra connector and extra resistors are included in that cost. The resistors came in a 100 pack off of Amazon.

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A metal paint can is readily available, for cheap, at any hardware store. I have seen plastic paint cans used, but I like the dummy load shielded. Most RF is absorbed and dissipated as heat, but some RF can be emitted. If you are using the dummy load at a low wattage, this would not be a big issue.

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20 – 3W 1K resistors are used to construct the dummy load. Make sure to use metal film or metal oxide resistors.

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For my dummy load, I am using a SO-239 and a N female connectors. Most operators will not require a N type connector for their dummy load and may prefer a BNC. If you only use SO-239, the second connector is optional.

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The easiest way to install the connectors in the paint lid is with a template. The soft metal used to make the lid tears very easily. To prevent tearing, use very sharp drill bits and step the holes up in size very gradually. When installing the connectors, put some silicone sealant on the base of the connector were it meets the paint can lid.

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Most other builds I have seen, on the internet, used a copper or brass plate to mount the resistors to. I did not have any brass plate so decided to use 16 awg copper wire formed into a circle. A can of hairspray was the perfect size to use as a form.

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When soldering the resistors in place, try to space them as evenly as possible. This will help dissipate heat. If you are wanting to get better results at higher frequencies, keep the legs of the resistors as short as possible. This dummy load will not be used above 21 MHz so I was not to concerned.

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The second ring was formed similar to the first. The wire used to form it was left long and will connect to the center of our antenna connectors.

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The 2 center conductors are connected together. These will connect to the wire going through the middle of the resistor rings. In this build, I cut the wire to size and soldered them together. In future builds, I will form the wire going through the center of the rings and directly connect it to one of the connectors. The other connector will be connected the same as this build.

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The second ring is connected to a mounting screw on the connector. A ring connector was installed on the jumper wire to make it easier to connect. To insure good connection, a jumper was installed between the mounting screws on the connectors. Make sure all the mounting screws are tight and seal the lid of the dummy load with a good silicone sealant.

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Fill your paint can with Mineral Oil. This will transfer heat away from the resistors to help keep them cool. Try to fill the paint can as much as possible.

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Seal the paint can good and we are ready to test the dummy load.

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Testing with the antenna analyzer shows good results. SWR is 1.0 up to around 25 MHz and hits 1.2 at 26.24 MHz. Live tests were done with the Yaesu FT-891 using 40 W FT8 and 100 W SSB. There were no issues and the transmitter stayed happy. Further testing will be done with more extended transmitting and different modes

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Links

https://km4nmp.com/2019/03/30/yaesu-ft-891-review/

https://km4nmp.com/2020/02/02/ps100-antenna-analyzer/

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4 thoughts on “100W HF Dummy load

  1. Nice construction techniques on display, well done.

    Liked by 1 person

    1. 20resistors x 3w =60W.
      Where you get 100w?

      73!

      Like

      1. The mineral oil cools the resistors. You could probably even go higher in wattage for short periods of time. I have tested this dummy load up to 100w ssb and 60w digital signals.

        Like

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