First off, I want to apologize for the sparse updating on the blog lately. The muse has not been my friend in the last few weeks. I try to keep the content mostly original, but perhaps I will have to turn to that time honored blog tradition of short blog posts repeating something cool I heard on the Internet.
Anyway, on to the good stuff. I remembered in my last Digi-Key order to grab two 100 Ω Caddock power resistors (MP930-100-1%) to make simple dummy load. These were mentioned in a QRP Quarterly article a little while ago as a great non-inductive resistor to use for RF dummy load applications. The datasheet looked good and the price was cheap ($3.51/piece), so I figured I would give ’em a try when I got a few spare moments.
I wasn’t sure what kind of heat sinking was needed, so I used my most scientific method and took a wild guess. The resistors (in TO-220 packages with a ceramic contact pad) were mounted to a piece of copper clad measuring 2″ x 4″. A bit of thermal grease was smeared on the copper clad before mounting the resistors with 4-40 machine screws. I mounted the resistors so that one lead could be soldered directly to the center pin of the BNC bulkhead connector, while the other lead was soldered to the copper clad ground plane. I figured this should minimize stray inductance.
I gave the dummy load a test drive on the IC-718 set for 30 watts power output. A keydown period of 30 seconds showed a nice SWR on the the rig meter, although my LP-100 showed a reading of about 1.8. The dummy load was fairly warm, but could be handled. I wouldn’t want to key it for much longer at that power level, but I bet it could handle <20 watts quite easily. I expected to see a reading of nearly 50 Ω purely resistive on the LP-100, but surprisingly there was a fair amount of inductive reactance (hence the SWR of 1.8). Now I’m a bit doubtful that I’ve calibrated my LP-100 correctly since I wouldn’t expect such such a lousy reading. I’m going to wait to declare the LP-100 reading bad until I can get the dummy load on a calibrated VNA to make sure that it doesn’t really have this problem. Stay tuned for VNA measurements on the dummy load when I am able to make them.