An HF 100W dummy load for under £5

(Yes – it’s all been done before)

Our project involves soldering 20 x 2 Watt Metal Film resistors in a container of oil.

The first item you need is a suitable tin for the project, and the lid must be of a material that you can solder to. I already had a container from a previous dummy load project, and this was 16cm tall by 9cm diameter.

It originally held 1000 tablets back in 1993 !!

I sealed all the seams with plumbing silicone sealant, to stop any seepage,

and then attached an SO-239 to the lid, again with sealant..note the extra screws to make sure it was properly sealed.

Now the fun can begin….

You need a piece of tinplate, or another suitable, solderable lid. This must be cut smaller than the container lid, and then drilled with 20 x 1mm holes in a circle to accept the resistors, plus a larger hole in the centre for a wire from the SO-239 centre.

The resistors are soldered between our container lid and our new piece of tinplate, but before doing this, a piece of 16SWG copper wire is soldered to the SO-239 centre pin and through the larger middle hole in our tinplate circle and then soldered, allowing sufficient height from the lid for the resistors.

The resistors are now all soldered up in a circle.

We are almost there!!

Central wire

Using a multimeter, you should measure ideally 50 ohms across the

SO-239.

In Mr. Grumpy’s dummy load, it measured 49.1 ohm, which of course will give a small error in SWR or power readings..this was because the resistors used are only 5% tolerance. Selecting precise values from a batch would be better, or in fact using 1% would be especially nice. Making the resistor leads shorter may also improve things at higher frequencies. For HF this is hardly a problem.

Next fill the container with cooking oil ALMOST to the top, at least so the resistors will be covered. Leave some room for expansion if it gets hot.

I could have put a vent hole in the lid, but thought better of it. If the load tipped over, the bench would have been covered in oil, and we all know that Mr. Grumpy is clumsy.

Carefully lower the resistor unit into the oil and fasten down securely.

SWR:

3.5MHz 1.07

14MHz 1.18

30MHz 1.33

50MHz 1.55

Cost:

Resistors £3.12 CPC

Oil £1.09 Asda

Tin Free