QRP Dummy Load

The QRP dummy load I’ve been using for a few years is four 2-Watt, 200-Ohm resistors soldered between two pennies, with the center conductor of a pigtail section of RG-174 passed through a hole in the bottom penny to the top penny. Thus, the top, or outermost, penny from the pigtail with BNC male connector attached, is the “hot” end. I often connect my scope probe or DVM RF Probe to that point for measurements.

However, when I was asked by Russ, AE4NY to build him a dummy load to use with his NoGaGuppy-WaTTa-PiG-Active Audio Filter that I was troubleshooting and enhancing, I was concerned that a dummy load with it’s RF “hot” parts exposed might cause problems, since it would be attached behind the unit. I was afraid that while Russ was plugging in cables, etc., a dummy load like mine might end up in contact with the chassis of some of the gear, causing an RF short. After several days of ruminating on this while working on the multifunction unit, I took an empty plastic 35mm film container, slit the cap halfway through, and opened up a small hole large enough for my RG-174 pigtail. Now, no chance of a short, but what about cooling? Since my dummy load is air cooled, it is now in an insulated, closed container. I didn’t like that idea for Russ’s purposes, as he has an IC-703 in addition to his new SW40+, whereas I currently have only 1 to 2-Watt rigs. I did like the attractive form factor of the 35mm film can.

For years, I’ve had a homebrew Heathkit Cantenna, with genuine Heathkit cooling oil, in the same form factor, only much larger. The oil-cooled resistors I have in my unit total about 20 watts, and I’ve never had a problem with brief tune-ups at 100 watts output from my boatanchors. Obviously, I just couldn’t fill a film can with oil; even my “cantenna” will leak if inverted. Therefore, I decided I’d transfer the heat from the resistors by conduction to a metal heat sink.

Keeping the form factor in mind, as opposed to the typical rectangular box shape, I cut a section of 1-inch aluminum tube, long enough to contain the length of the resistors, plus clearance for their bent over far ends, mounted on a BNC female bulkhead connector. I then traced the inside of the tube onto a piece of 1/16-inch aluminum sheet. I drew two tabs on each of the two circles opposite each other; these would be bent down inside the tube to form attachment points for sheet metal screws. I cut the top and bottom plates with a hacksaw, using a 24-tooth metal blade. Some final filing was necessary to get a good fit.

Click on the photos for a higher resolution image.

I bent the tabs, and I inserted the top plate into the tube, flush with the top. I then drilled through the tube and tabs, and temporarily attached the screws. Then, I drilled the hole for the BNC bulkhead connector. After fitting and drilling the bottom plate, the assembly was ready. I cut a small square of hobby tin sheet (from Ace Hardware; I use it a lot in QRP projects). The diagonal dimensions of the tin were approximately the same as the diameter of the inside to the tube. I cut into the tin, creating an open hole the size of the threaded end of the BNC. Assembling the unit was easy. I passed the BNC through the top plate, put on the tin collar, BNC washer, and hex nut.

After tightening the nut, I bent up the four corners of the tin…these would be the solder connection points for the ground end of the resistors. I soldered them so that they were spaced about equally around the BNC, and importantly, the outsides of the resistors would be in contact with the inside of the tube when inserted. I bent one resistor’s top lead over 90 degrees, and made then another 90 degree bend so its end would fit into the BNC solder cup, cut to length. I soldered this in, and bent the other resistors similarly, cutting them a bit shorter. I wrapped a small piece of wire around the bundle of clipped leads and the first resistor’s lead, and soldered. Next, I dry fit the tube onto the assembled top plate and resistor assembly I made sure that the resistors were in contact with the inside of the tube. I inserted the screws for the top plate to secure the assembly. I then bent the resistors away from the inside wall of the tube slightly.

Using a paperclip, I put heatsink thermal compound on the outside of the resistors where they would touch the wall, and on the wall of the tube itself. I then bent the resistors back into place against the wall. I used the soft plastic-coated handle of a jeweler’s file to probe down to the bottom of the resistors, pushing them out to ensure contact there as well. After applying the compound, and realigning the resistors, I then mixed up a small batch of 5-minute epoxy and applied it to the sides of the resistors so that it would bond them to the wall of the tube. A few minutes later, I attached the bottom plate. The result is a nice-looking QRP dummy load with a small form factor, and a rating of the nominal 8 Watts of the parallel resistor combination, enhanced with a thermal mass for heat sinking, in lieu of flow-through air cooling as in the case of my 2 cent dummy load. The outside of the case is at RF and DC ground, providing for safe connection.

The KX4OM QRP Dummy Load...neat, huh?