A few days ago, I built myself a simple 30 W dummy load out of Caddock power resistors. As I mentioned in the previous post, I was doubtful about the readings that I was getting from my LP-100 (not that I doubted the LP-100 itself, just my calibration of it). Tonight was my first night back to work after a long break, and I finally got a chance to put the dummy load on a calibrated VNA to verify its performance.
The instrument that I used was an Agilent N5230A. A very nice instrument, but one minor drawback is that its lower frequency limit is 10 MHz. Not to worry, as I suspect that if it does well from 10 to 30 MHz, that the lower bands are probably good enough for my home lab.
Sure enough, it turns out that the performance of the dummy load is great and that my LP-100 calibration is off. The return loss is excellent all the way up to the 2 meter band. There's not much else to say since the plots speak for themselves. If you need a dummy load, you could definitely do worse than to pick up a couple of these resistors on your next Digi-Key order and slap one of these together in a few minutes. Caddock also makes 100 W resistors, which will probably be on my next order for goodies.
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.