Author Topic: Home Made Dummy Load  (Read 70 times)


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Home Made Dummy Load
« on: September 12, 2021, 04:52:21 pm »
There comes a time when when we're all going to need a dummy load at some point. The easiest option for most is to just simple buy one, however I'm in the category where I say "Build it, Don't Buy It" nevertheless in reality we still need to shell out the cost of components to build one in the first place. In some cases it can be more costly and time consuming, making the first option to buy one a no-brainer.

But for some of us, the components required may have just been sitting around already in a draw or parts bins. Below is a very simply approach to an "Air Cooled" dummy load.


This design consists of 66x 3k3 2W resisters that are sandwiched between to pieces of copper clad. The most time consuming part to this build as I recall was drilling all the holes for the resisters, in this case 66x2 or 132 holes. Originally the holes where the correct diameter needed for the legs, or electrodes as they should be called, but trying to bring the 2 pieces of clad together proved more tricky than I anticipated. The only option was to enlarge them to a more manageable size.

The image below shows a better view of how large the holes where made, and the soldering patterned I chose.


It's important to remember that this type of dummy load (although will work for the job needed) lacks one very important aspect. That is some sort of RF shielding or RF containing. Usually air cooled dummy loads will have whats know as a "Faraday Cage" that's common bonded to negative or braid potential. Not only will this help prevent unwanted signals propagating beyond your work/test bench, but also provided a much needed "Shock Prevention" or "Burn" should you accidentally touch it.

Another option would be to place the the resister sandwich in an old paint can that's filled with some sort of non-conductive oil. Most home brewers will simply use some sort of motor engine oil, as this is usually synthetic and non-conductive.


The finally thing to do, was place a small length of RG213 coax with PL259 plug on the end. Although later I did end up soldering a chassis mount type S0239 onto the copper clad sandwich. Making a more compact design.

One last important point with using home made dummy loads, is they will only work efficiently up-to a certain frequency. You can simply characterised your home made dummy load using a "Vector Network Analyser" or "VNA" for short. I can't remember off-hand as I write this thread, but I think it was acceptable to around 250 MHz.

If you do require a dummy load with a much higher working frequency, it's worth considering a "Chip Resister", these are simply screwed down to a heatsink with a small amount of thermal paste for better heat dissipation and generally work well in-to into the CM wavelengths.

I look forward to seeing other peoples home made dummy loads

73 DE Bill
« Last Edit: September 12, 2021, 10:01:46 pm by 26AM081 »

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