home Allgemein The Easy Dummy Load

The Easy Dummy Load

Keep stuff simple.

The Build:

I have already seen a bunch of examples on how to build a dummy load with an abundance of same value 1/2 W resistors.

Those examples to my eyes looked like a complete mess. Some of them being of gigantic measurements, some of them are even drowned

in even vegetable oil for extra thermal capacity. I also assume these would as well take hours to build.

Scrolling along the shopping sites of my favourite asian online vendors I came across a tiny but very rugged looking 50 Ohm resistor

stated capable of taking 250W of power!

The price.. extremely affordable, less than 7€.

50 ohm rf-resistor

 

Since the resistor is said to be able to dissipate 250W into the right heatsink, I searched for something suitable in my workshop.

Unluckily there was was really nothing to be found besides an old cpu heatsink wich I recently slaughtered from one of my old pcs.

From internet research I found that ordinary pc heatsinks may dissipate about 40 – 60 Watts of thermal power continiously into the air.

Fair enough for my first experiments and transceiver repairs….

finished dummy load

 

Besides the high power resistor there was nothing extra to be bought.

I used the hand drill to drill the holes through the aluminm to fit the machine screws. Two zip ties secure the oversize fan.

I fitted the spare ending of an ex – antenna coax to the ports of the resistor. As many already know it is almost impossible to solder

onto a device thats designed to spread and dissipate heat. Therefore the shield of the coax had to be soldered to an eyelet first,

then screwed together firmly.

 

finished dummy load

 

The Test:

I tested my new device with the FT-100 Field transceiver with a max rf output power of roundabout 130 Whiskeys.

The internal swr meter didn’t move at all from 1:1 accross all hf bands, 160 to 10m. So far so good.

At 50Watts I could let my finger stay on top of the heatsink without feeling any burns.

At 100Watts I clearly felt the urge of removing my dear finger right after half a minute and no mor seconds.

During the test the Fan is obviously running at 12Volts.

This kind of „Brutal Burns test“ verified the prediction for the pc cpu found on the internet promising 50 Watts of continious heat dissipation.

 

Radio friends asked for a swr measurement for higher frequencies.

I did it with the analyzer.

 

Summed up Thoughts:

  • project cost:  less than 10€
  • project time:  less than 1 hour
  • gained product: dummy load 50 watts cw 1-30 Mhz.
  • gained product: dummy load 100 watts cw 30 seconds.
  • gained experiences: some burns, some fun, some upcoming opportunities in transceiver repair and pa building!

 

Keep building stuff!!

 

73s Simon dl3sps

 

 

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