You Can Learn A Lot From A Dummy (Load)

Jul 25, 2023

If you work on RF circuits–even if you aren’t a ham radio operator–you ought to have a dummy load. A dummy load is a non-radiative “antenna” with known impedance that you can use to test your RF circuit without radiating. For radio work, you usually just need a 50-ohm resistor that is non-inductive (at least at the frequencies you are interested in) and that can dissipate the amount of power you’ll expect it to handle (at least for a short time). [VO1PWF] wanted a dummy load and built his own.The Cantenna (not the Pringle’s kind; see right) was a famous dummy load design when Heathkit was in business. It was a single carbon rod immersed in a paint can full transformer oil (which we now know was full of dangerous PCBs; and we don’t mean printed circuit boards). [VO1PWF’s] design is a little more practical, using some resistors in parallel (20 1K resistors), a plastic pipe housing, and mineral oil to keep it all cool.

The reason for the parallel resistors is to maximize the power handling capability. The resistors are 3W units, so the dummy load–in theory–can handle 60 watts. Often, high power resistors are wire wound and thus have a good bit of parasitic inductance that makes the dummy load reactive (not a good thing since that makes the load impedance vary by frequency). They do make non-inductive wire wound resistors, but these aren’t truly non-inductive. The wire winds in two different directions, so the inductance tends to cancel out. We wouldn’t trust them to be a pure resistance in a high-power dummy load design.

There were three things we noticed about the project, though. We’d have matched the resistors since even a small difference in value (or even a little difference in the solder joints) will cause the lowest value resistor to take much more current than the others. For example, if all the resistors were actually 1K except for one that was 5% low (950 ohms), the low resistor will dissipate almost 3.15 watts.

The situation gets worse if the rest were all 5% high, and this is a typical situation since many manufacturers sort out precise values for higher tolerances; in a batch of 5% resistors, none are likely to be within 1% of the nominal value because if they were, they’d have been marked as 1% resistors. Increase the spread (with 10% resistors, for example) and it gets even worse. In practice, you’d probably want to derate the resistors so that by design they stay well below their maximum design power to make the circuit more forgiving of tolerance and differences in wiring. Also, for short durations, it isn’t like the resistors will explode if they slightly exceed their power ratings. However, it probably isn’t wise to try to run the full 60W into this load for any length of time.

The solder job was neat and fairly short. That’s the second thing we noticed: over a Cantenna, the wiring is going to add parasitic reactance and make the dummy load less useful at some high frequency. It would be interesting to know where that frequency is. The bus wire is pretty thick, but using something even larger (like the PCB material in the video below) might increase the useful frequency.

The final thing we wondered about was using ABS for the case. Even with a Cantenna, there are famous stories about operators working DX (long distance) while accidentally hooked to a dummy load and nonetheless getting through. And the Cantenna was in a metal case. We expect that a dummy load in PVC would radiate a lot more. And while this is still better than using an antenna, and electrically this isn’t a problem, we’d love to see some test measurements of the radiated power.

We see quite a few dummy loads that are programmable loads for LED drivers, for example. However, we don’t see many high-power noninductive loads suitable for use with a radio. The video below shows a similar design with a metal can and thicker bus bars that is closer to the old Heathkit design, although it still uses multiple resistors.

Cantenna photo: By [Gerry Ashton]. CC-BY-SA-3.0 via Wikimedia Commons