Monday, October 5, 2020

Dummy load with chemical impedance matching

Joe WB9SBD has come up with a unique way to build a dummy load.

Rather than a series of standard resistors made to present a 50-ohm load, Joe has mixed up the perfect concoction of household items to achieve a load that presents a perfect 1:1 SWR over a wide range of frequencies.

Those items are a glass jar, water and baking soda.

According to Joe's experiments, straight tap water yielded an SWR of 10:1 at HF frequencies. 

One teaspoon of baking soda dissolved into the water brought the SWR down to 1:1 from from 1.8 to 60 MHz. At 144 MHZ the SWR was only 1.5:1.

With the Miracle Whip jar shown, Joe has used the dummy load successfully for 10 years now, at a power level of 300 watts.

A small additional amount of baking soda allows the dummy load to present a 1:1 SWR at frequencies up to 180 MHz.

Further experiments show that a 100 watt rig, at 100% duty cycle requires 50 minutes to boil the water in Joe's DL.

Full details on Joe's chemical dummy load, and the sequence of events leading to its development, are here (15MB zipped file).

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2 comments:

  1. Hello John, this is a great fun project. So simple but working well. I wonder if the whole jar will radiate some and would like to experiment with WSPR or the new FST4 modes. From the picture I can see how this is constructed but would like to view the powerpoint. However the link you gave is not working here. I've been looking in the QRPlabs IO group and found the actual message from Joe including the same link that is not working. If you are able to download it? Could you send it to my e-mail address? 73, Bas

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  2. Hello Bas - I got a warning when trying to open the file as well and have emailed you the repaired version. Here is a link to it:

    https://www.dropbox.com/s/16sfw8ce1mbj9n4/DummyLoad%20%5BRepaired%5D.pptx?dl=0

    73 - John

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