DIY 50 Ohm Oil Cooled Dummy Load – A Smooth Move

I couldn’t help with the laxative one liner. I had a lot of fun working on this dummy load project. It’s been in my hands for about two years. Frank, K9TIE, sent me the assembled can a few years ago, but I never went to the store to purchase the mineral oil (a laxative). I decided that I would build it since I needed a higher-power capable dummy load.

I finally found the mineral oil at my local Wal-Mart, but they didn’t have enough so I went to the Dollar General. It’s cheaper at Wal-Mart by 50 cents. The next thing is that I needed almost a gallon since Frank and I used gallon sized cans for our project. While others built their using quart sized cans, I believe that there is an advantage to using the gallon sized can: increased use time and possible power handing.

I’ll quickly outline the re-build process. I’m not going to hammer out what quite a few others have done. Here’s a few links:

This link is an interesting read related to a Heathkit dummy load. It mentions the effects of getting your oil too hot:
This next link shows a few different Dummy loads and explains a little about heating:

Build Image Gallery

Using calipers to find the center of the lid - Oil Cooled Dummy LoadMarking the drill spots using calipers - Oil Cooled Dummy LoadConnectors fastened to lid -  - Oil Cooled Dummy Load
Fasteners from the top - Oil Cooled Dummy LoadThe diode anode connects to the driven plate - Oil Cooled Dummy LoadThe ground wire connects to the top plate. The driven wire goes through the center of the top plate. - Oil Cooled Dummy Load
The cathode of the diode connects to the red binding post. Ground connects to the black binding post. - Oil Cooled Dummy LoadThe ground also connects to the outside of the RF connector. The driven plate connects to the center of the RF connector. - Oil Cooled Dummy LoadThe finished dummy load being tested. - Oil Cooled Dummy Load

I used calipers to find the center of the lid. Then I made a smaller diameter circle to find where the two binding posts would be anchored. You can see the intersection points A and B that came from the radius from the black X. The holes were drilled, and then I used a stepper bit to make the final hole sizes. The binding posts were not soldered to the lid. I used rubber grommets to insulate them and seal the holes. The top plate shows some of Frank’s original holes. I modified his design a little based on a suggestion from a fellow ham. I lowered the top plate a little so I could use less oil while still keeping the top plate submerged. The rest of the build followed the other instructions found on the web.

BTU vs Oil – Mathematical Data

I’m not exactly sure of the origin or part number of the resistors in my can, but I assume that the data from any 3W metal film resistor should be similar to mine. Judging that the power rating diminishes at 70 degrees Celsius (about 170 degrees Fahrenheit), you probably don’t want to heat your oil to this temperature. This is what I used to judge the CW power rating for my gallon can. I chose 130 degrees Fahrenheit (54 degrees Celcius) as my limit. If you’re driving the dummy load with low power (less than 200 watts) then this math doesn’t matter too much.

First, the 20 resistors at 3 watts comes out to 60 watts with no cooling. Based on my own experiments, you can drive resistors at about ten times their rating for short periods of time without cooling. (See my experiment at from March 2010.) The result of that experiment was this: at 9x the power rating, 1 minute could sustained without damage, 2.5 minutes and 5 minutes showed signs of failure, but were still within tolerance.

Oil Cooled Resistors withstand 20x their power ratingMy experiments with oil cooling performed on May 16, 2014 were a little flawed because I messed up the measurements, but the results did show that exceeding the power rating of resistors is possible using oil cooling. Here’s a brief explanation of the results.

Voltage = 15Vdc, Resistance = 47 ohms 1/4w, Power = 5.13 watts (20x power rating). Outside oil I’ve seen these go up in smoke in just a couple of seconds. They survived all five of my tests in oil.


Five .25w resistors in parallelJust for fun I soldered five 47 ohm resistors in parallel for a resistance around 10 ohms. The first test was to run them at 10 volts for 15 minutes. At 8 times their rating they looked almost new. Then I ran 15 volts through them for 20 minutes. The surface temperature of the oil started at 83 degrees Fahrenheit (28 degrees Celsius) and finished at 170 degrees Fahrenheit (76 degrees Celsius). They were quite charred, but still within their tolerance.

Watts to BTU to Temp Change

formulaChangeInTemperatureThe fun part of this experiment was figuring out how fast the oil would be heated using these resistors. (Specific Heat of mineral oil is about 0.4, weight is in pounds, power is in watts, time is in hours, and the temperature is in Fahrenheit.) The experiment variable that I did not control was the amount of heat lost on the surface of the oil, but despite that my calculations were close. Using this formula I estimated that 10 watts would raise the temperature of 8 ounces of oil about 39 degrees Fahrenheit in 15 minutes.  The actual measurements: start @ 90 F (32 C) and end @ 123 F (51 C).

The second test was at 25.5 watts. The calculation came out with a result of an increase of 133 F in 20 minutes for 8 ounces of oil. My actual result: start @ 83 F (28 C) and end @ 170 F (77 C). The increase was only 87 F. I imagine that something happened that I didn’t see or was not able to measure. My open container may have cooled some of the oil since the air temperature was about 80 F.

I’d like to do this experiment again with different resistors. Based on my calculations, 7 pounds (almost a gallon) of mineral oil would have the following heating rates at these given powers and times (assuming ambient temp is 80 F:

  • 100 watts @ 60 minutes: Ambient temp + 112 F = 192 F
    100 watts @ 30 minutes: Ambient temp + 66 F = 146 F
  • 200 watts @ 30 minutes: Ambient temp + 112 F = 192 F
    200 watts @ 15 minutes: Ambient temp + 66 F = 146 F
  • 400 watts @ 15 minutes: Ambient temp + 112 F = 192 F
    400 watts @ 5 minutes: Ambient temp + 37 F = 117 F
  • 600 watts @ 9 minutes: Ambient temp + 112 F = 192 F
    600 watts @ 2 minutes: Ambient temp + 22 F = 102 F
  • 1200 watts @ 4.5 minutes: Ambient temp + 112 F = 192 F
    1200 watts @ 1 minute: Ambient temp + 22 F = 102 F
    (Not really recommended, though.)

I concluded that I would only run about half of those times (or less). If the ambient temp was 80 F, you’d be close to water’s boiling point, and well past the acceptable power derating of the resistors. I wouldn’t operate past an increase of 60 F above the ambient temps, especially if you’re operating at anything above 200 watts. The resistors will be getting close to a thermal runaway when it nears the 170 F point when operating past their power rating.

If someone has the time, and they want to see if this would work, I’d be happy to hear about the experiment. I don’t have anything that exceeds 60 watts output, so I won’t be trying it anytime soon. I’ll be revisiting this experiment again myself just to try to confirm how much torture these home built dummy loads can handle.

10 Meter Mobile

HTX-100 on the Jeep's dashI realized on October 12, 2013 that it had been eleven months since I had last made an HF contact of my own. I made some during Field Day 2013, but those weren’t really mine. There were several reasons why I couldn’t get on the air. I had lots of receive noise, little time, and an uncomfortable ham shack. I believe that I have solved these problems.

The noise that I was receiving was, by far, the worst inhibiting factor. My inverted vee dipole was picking up RF from the Ethernet wiring in my neighborhood and home. It was causing birdies every 25kHz on 10m. To add to that annoying S9 noise, powerline noise plagued me during most of the day. Turning on my Realistic 100HTX was a painful and disappointing task.

Since I moved to my new home, I was hoping all that would change. Boy, was I wrong. I attempted to mount my dipole in the attic, and it fit perfectly. The only problem: S9 noise across the entire 10m band! Even using a random wire antenna resulted in the noise. After a week or two of being frustrated by this, I decided to do a test.

I took the radio and the random wire antenna to the Jeep. I hooked up the radio to the power supply and inserted the random wire antenna. To my surprise, it was quiet. I heard a little birdie every once in a while, but it was hardly audible. I knew what I needed to do.

Gumdrop mounted to the luggage rack of the JeepI searched MFJ for a 10m vertical and a hitch mount. Since my birthday just went by, I had a little gift money left over. When it arrived I immediately did a quick install and ran 50 feet of LMR-400 to the radio in my garage. I fired it up to find a very quiet receiver. It was also midnight, and there were no stations to talk to either.

The next day I was able to make two contacts. It is not as easy to burn through pileups with 25 watts and a non-ideal vertical behind a large vehicle, but it sure beats the heck out of S9 interfering noise.

After testing that out, I routed coax through the Jeep. I placed my radio on the dash. I still had two problems. Having the antenna behind my vehicle still resulted in a good amount of noise. It is even worse with the engine running. After a little research I decided that there wasn’t a good enough ground plane under the antenna.

My Jeep Commander mobile stationIt was time to “ditch the hitch.” I took the gumdrop antenna connector off the hitch mount. I drilled a hole through the rear luggage rack and mounted the gumdrop antenna connector to the rack. I put the rack back on the Jeep and routed the coax. Wow! That made a world of difference.

Now I am able to make mobile contacts without a “hitch.” The only problem is that my Jeep is almost 11 feet tall. Drive through service will not be possible. My wife doesn’t really like it either, but she said, “well, if it works.” That means that it is what it is.

2 Meter Stealth Yagi

Standing in the attic looking down after installing antennas In August 2013 I moved into a nice neighborhood that has a nice yard with very few objects to mow around. I decided that it would probably be a good idea not to put up my two sections of Rohn 25. This decision was based on two factors:

  1. I didn’t want to mow around it.
  2. I didn’t want my neighbors to freak out.

With that in mind, I gave my tower sections to a ham friend, Andy. Now, I’m left with a dilemma. I don’t have any way of communicating from my new garage ham-shack using my ham radios! After a little brainstorming and exploring the weird attic space, I decided that I could mount my Arrow 146-4 Yagi in the attic! I ran 50 feet of LMR-400 to a junction box that I built using an aluminum box and two SO-239 jacks. From there, I used another length of random coax to run the final length to the antenna. I secured it to some 2×4 wood pointing North. North just happens to be the higher population of wanted repeaters. It works towards the South fairly well, too. I’m sure that the East-West performance of the antenna is as expected (null?).

Yagi antenna installed in the atticWhile I was in the attic, I also rescued my 10m dipole. It didn’t fair so well amongst the Ethernet, power, and phone cables running up there. I had S9 noise and birdies scattered all over the 10m band. This solution is for another post to follow. Check back as I’ll show you next how I get on HF without ticking the neighbors off with huge, permanent antennas.


xastir_screenshot_10_9I’ve mentioned Xastir many times before, and it seems to be better and better every time I use it. My original issue was that my KAM TNC would not recognize much of the information that was passed to it. The developers added a delay that accommodates my slower TNC.

The other day I noticed that I could not send any messages through Xastir. I contacted one of the developers, but right off hand he didn’t have an answer. I continued to poke through the settings, and I finally found the setting that fixed this issue. I went to “File -> Configure -> Timing”. The setting that I changed was “Serial Inter-Char Delay (ms)”. I changed it from 1 to 30, and now messages go through as they should without my TNC hiccuping. Be sure that when you find something that works that you “Save Config Now!”.

Welcome to my new ham radio blog

(old call: KK4FGM)

I have had quite a few changes lately. I applied for a vanity call sign in July 2013, and I was granted my new call sign and purchased a new house. Many things in life have changed, so now I’m blogging from scratch. If you want to view any of my past adventures, check them out at

I’m in a new home, and I live in the city with a tiny yard and neighbors, so things will be much different. Such is the life of a ham that wants to keep the surroundings happy. No more crazy towers and and the like. Everything has to be incognito now.

Hope to hear you on the air!

Robbie, W1RCP