H. P. Friedrichs (AC7ZL) Homepage





Radio Room 
Dummy Load 

Radio Room

Building A Dummy Load

Building and adjusting radio transmitting equipment means testing, tinkering, and experimentation. However, radio frequency spectrum-space is limited, shared, and regulated. Therefore, it's an important matter both of courtesy and of law that you don't clutter the airwaves with electronic noise.

What do you do with RF energy you want to get rid of? Dump it into a dummy load. The dummy load can take a transmitter's signal and turn it into heat without allowing any part of it to radiate into space.

You can buy dummy loads easily enough. However, I had some heat sinks and some other odds and ends and decided to fabricate my own. Below are images showing how things came together.

This is an image of some aluminum heat sinks I had scrapped out of a couple of large computer UPS's. In their original application, the manufacturer used them to dissipate heat produced by some large SCR's.



Neither heat sink was, in my estimation, large enough by itself. I decided to link the two together. This image shows an aluminum bar, about six inches in length, that was used to couple the two heat sinks together.



Here we can see the heat sinks coupled together. The "legs" on the heat sinks are plastic stand-offs. I didn't put those there, they came with the heat sinks. I decided to leave them on.




I knew that any wiring associated with the dummy load should be enclosed. After all, I don't want any RF energy escaping. This meant that I needed some lid plates to cover the top and bottom of the assembly. These are two chunks of extruded aluminum that were salvaged from the case of an old test instrument I had taken apart.


This is the photo of good fortune. The lid plates (above) feature tabs that hook inward. The heat sinks each sport a pair of fins with hooks that point outward. By wonderful coincidence, the two features interlock, allowing me to slide the lids into place.



These aluminum plates are used to seal of up the ends of the dummy load assembly. Notice that one end plate has been punched to accept a coaxial connector.





Here, the end plate with connector has been mounted. Notice also the 6-32 threaded studs that will be used to mount a pair of power resistors. Also, note the two ground lugs.




This is an outside view of the end plate with its connector. I should point out that I used lock washers in conjunction with all screws. This was to assure good electrical contact everywhere.




This image shows the power resistors bolted into place, and the wiring complete. I used two 25 ohm, 100 watt resistors (Mouser part number 684-MP9100-25). These resistors were chosen because of their high power and low inductance. Together, they form a 50 ohm load capable of dissipating 200 watts. I used silicone grease to assure good thermal contact with the heat sinks. Also, the round objects you see are small disks of fiberglass circuit board material, used as washers. I was concerned that if I tried to tighten a nut directly against the resistor housing, I might crack it.


Here's a final shot of the dummy load just before I slid the top lid into place. Once set, I screwed on the right-hand end plate, and the dummy load was complete. A quick check with my digital multimeter shows 50.3 ohms resistance.

 Document Revision 1,  xx/xx/2003

Document Revision 2, 04/22/2007