Dummy Load Project (originally featured in The Component, Feb 2021) #project

Michael Forsman (KEØTCF)

 I originally wrote this up for Ben Schudel (KB0ZOM) over in Hastings, to be featured in the February 2021 edition of The Component, the newsletter for the Amateur Radio Association of Nebraska. I thought it might be interesting for folks here to read who may not be subscribed and didn't get the opportunity to read it then.

One of my favorite things about this hobby of ours, is building things when I can, rather than buying them. So, this month, I decided I wanted to acquire a piece of vital test equipment that my shack has, until now, lacked; that being a dummy load. 

I will not bore everyone with a long explanation of the purpose of a dummy load, because frankly I’m not the right person to explain it and y’all probably know more than I do about it, but my understanding is that the idea is to present your transceiver with a perfect 50Ω resistive load. This will register as a nearly perfect 1:1 SWR to your radio and allow you to run full power from your radio for testing, but without radiating. This allows you to do various tests and calibrations on your radio, amplifier, and other equipment without taking up space on the bands, or worse, accidentally breaking the law. Since the load is made up of 50Ω of resistors, they convert very nearly all of the RF power into heat, rather than radiating it out like your antenna would. 

There are many dummy loads available online, from MFJ and the like, but they are pricey, and as I mentioned, building my own is more fun. And it is dead-simple. I ordered a 1-quart paint can off Amazon, and it came with a 1 gallon can as well, so I can upgrade my dummy load later if I need to. I also got 20 resistors. These are 1KΩ each and rated for 5W of dissipation. When wired together in parallel, their resistance can be calculated by the value of one of them, divided by the number in parallel, since they are all the same value. If they had differing values, this would be more complex. So, 1000/20 gives us 50Ω. Perfect. And, since they are in parallel, their power rating is multiplied by the number, so 20 * 5 gives us 100W of theoretical dissipation. That said, packed together, they probably will not be quite that efficient in air. One final note on the resistors is that they need to be non-inductive ideally. So, no wirewound resistors, because we are trying to avoid inductive or capacitive impedance here, and we certainly don’t want to radiate any RF. We want it all converted to heat. The other parts needed were a chassis mount SO-239 connector, a PCB or some perf-board, and some oil of some kind for cooling. 

Assembly was simple and painless, I soldered all the resistors in parallel, as mentioned above, using the circuit board, cut in half, top and bottom to hold everything where I wanted it. I made sure that none of the resistors were touching each other when finished, so that air or oil can get all around them. I soldered some heavy wire to the top board, and then a longer piece to the bottom, and curved it around to come up next to the top one. 

The can was prepared by cutting a hole in the middle of the lid and attaching the SO-239 connector. This had to be sealed VERY well because mineral oil has a nasty habit of wicking up and finding its way out of any gap. I used silicone originally, but this leaked almost immediately, and I wasted several connectors trying different ideas, and eventually ended up using hot glue, and so far, that is holding. I put hot glue all around the hole in the can liberally, then mashed the connector down into it and let it cool, then bolted the connector down with the screws, to really squeeze it down onto the hot glue. So far, it’s holding. Time will tell.

With the connector mounted, I soldered the short wire from the top of the resistor array to the middle pin of the socket and put a ring connector on the longer lead from the other side and attached that to one of the bolts holding the connector, effectively making the entire can the ground. A quick test with a multimeter read the dummy load within a few tenths of an ohm, so all was well. 

I filled the can about ¾ full of mineral oil. I would have preferred transformer oil, but I have so far not been able to source it. If anyone has a lead on where I could get some, please let me know! In the meantime, I did not want to fill the can all the way with oil, so I filled it just enough to cover the entire resistor array when submerged and sealed. Which is what I did at this point, using a rubber mallet to pound the lid into place, sealing the load up. A multimeter still reads it at almost exactly 50Ω, and in fact even a bit closer than before now that it is all assembled.

I hooked it up to my NanoVNA and swept the load from 1MHz to 30MHz, and at the bottom of the sweep, SWR is 1.02 and at the top of the sweep, it is 1.25. A little higher than I would have liked, but not terribly surprising on the higher frequency, and certainly acceptable! The oil allows for much more power to be poured into the dummy load than just the resistors could dissipate in air alone. I do not know how much, or how to find out, but as of now, I do not use an amplifier, so all I have is the 100W from my IC-7100. The resistors themselves could theoretically dissipate this, so submerged in a quart of oil, they can probably handle that much power for a very long time, and probably quite a lot more power if needed! If, down the road, I need even more capacity, I could always move the load into a gallon size can filled with oil to get much more thermal mass.

If you have any questions or comments, or I did something horribly wrong, please let me know! You can reach me through my website https://ke0tcf.com or email me at om@....

Michael Forsman (KEØTCF)