N4GA.com

Dummy Loads

If you’re going to be building and testing amplifiers, you’ll need a high power dummy load or attenuator. The latter basically being the former with a way to tap off a highly attenuated signal, to feed a Spectrum Analyzer, for example.

These dummy load attenuators can get very pricey. Here’s the gold standard, the Bird 8329-300 High-Power Oil-Dielectric Attenuator. This is about the size of your typical old school home radiator and is rated for “2kW continuous.” It’s $2923 in the Bird Online Store, but can often be found used on eBay for less.

It’s a BEAST and I’m sure it’s really nice. However…

I decided to roll my own by taking a heavy duty heat sink, twenty 50Ω, 250W power RF resistors, added in OHM’s law and made this:

The heat sink was $40 and the resistors came off eBay from a seller selling them in lots of 10 for something less than $100. I think I spent maybe $70 on the resistors.

Here’s the heat sink. These are also the heat sinks I use for all my amps.

The hardest part was drilling and tapping all the holes. What a chore.

I obviously did not use any heat spreader or any thermal compound. This baby gets really hot in use but I’ve never had any issues with it all the way up to almost 3kW.

UPDATE: this home made dummy load sold on May 31, 2020. Enjoy AG7YY!

Comments

21 responses to “Dummy Loads”

  1. thomas Avatar
    thomas

    Hello N4GA, I have been wanting to make a dummy load the same exact as you have and have already attempted twice with terrible luck. First I tried a suposly 1600w 50ohm resistor and mounted it with heatsink compound since it had 2 leads and it fried quickly.

    Then I tried 2 x 25 ohm 800w resistors in series and the same thing happened. I am curious now if the devices I was using just were not the right type.

    Is there anything that you can share with me as in the exact type of Resistors that you ordered. I will go order a lot of them. I want to make a 100 Percent Duty cycle dummy load to where I don’t have have to worry about it over heating around 1500 to 2000w. 24/hour a day lol I know I know. I have many huge heatsinks and I have fans. I’m ready!!! Thanks ain advance.

    Reply
    1. N4GA - Rob Avatar
      N4GA – Rob

      Tom,

      I used 25 Florida RF power resistors, each 50 ohms, 250W. This divides the power out and also mechanically dissipates the heat over a much larger space than 1 or 2 resistors. I found them on eBay in lots of 10.

      Reply
      1. thomas Avatar
        thomas

        Thank you very much. Out of curiosity, why didn’t you use heatsink compond? was there a benifit for you for this specific project to not? Thanks in advance and Please keep adding projects to your page as you go along. I really love your content sir.

        Reply
        1. N4GA - Rob Avatar
          N4GA – Rob

          Thanks a lot Tom!

          I decided to try no copper spreader or compound on this one. Figured with this many resistors spreading the heat around, it would be sufficient.

          So far for my use, it’s fine. But I do not hit this dummy load with constant high power for long periods of time. I’ll go like CW at 2kW for just long enough to get the measurements I need. Maybe 30 seconds at a time.

          With SSB I can give it all I have for as long as I want. But SSB is easy, right!

          Reply
  2. Jim Kinney WE4S Avatar
    Jim Kinney WE4S

    Hey Rob,

    Nice job. Have you checked the frequency response to see how high you can go in frequency before the SWR starts changing?

    Thanks,
    Jim WE4S

    Reply
    1. N4GA - Rob Avatar
      N4GA – Rob

      Jim, how are ya.

      SWR starts slowly creeping up above ~80Mhz due to all the interconnects.

      Just rechecked it, because I have a new analyzer and look for any excuse to use it. I’m at 1.4SWR at 72Mhz, pretty flat below that.

      Some day I’ll get one of the 2kW Bird ones that look like a house radiator.

      Reply
  3. HZ1MT Avatar
    HZ1MT

    Hi Rob,

    Thanks for sharing this valuable info! How can i turn this dummy load into an RF attenuator? Say 30dB? I need to connect my 1500w ldmos amp to spectrum analyzer
    Thank you

    Reply
    1. N4GA - Rob Avatar
      N4GA – Rob

      MM,

      I would make a separate RF sampler. Like this one.

      Reply
  4. John Lawrence Avatar
    John Lawrence

    Hi Rob!
    Trying to order the pieces-parts to build this.
    What are the measurements on the het sink?

    Reply
    1. N4GA - Rob Avatar
      N4GA – Rob

      John,

      Here’s the heat sink: https://www.heatsinkusa.com/10-080/

      It’s just over 10 inches.

      Reply
  5. Blayne Marek Avatar
    Blayne Marek

    Shows there out of stock.. is it 10x10x2.5 ? As in 2.5″ is the height of the fins? Thank you for all your time and info you put into this Rob. I want to make a few of these for a few friends and I and have 100 of the 250w resistors just like this. I figure build them in a nice case. Also throw some fans on the heatsinks and a temperature sensor to turn them on and off. Just for extra cooling. Thank You again for all the info 🙂

    Reply
    1. N4GA - Rob Avatar
      N4GA – Rob

      Yes sir, the one in the link. 73!

      Reply
    2. Dean Avatar
      Dean

      Care to sell 25 of these?
      I’d be extremely interested!
      73
      Dean K2WW

      Reply
      1. N4GA - Rob Avatar
        N4GA – Rob

        25 resistors or 25 complete dummy loads?

        Doesn’t matter because I can’t do either. But I can sell the entire dummy load to you, as is. I’m no longer using it.

        Reply
        1. Dean Avatar
          Dean

          I never saw this reply Rob,
          Do you still have the loAd?
          73
          Dean K2WW

          Reply
          1. N4GA - Rob Avatar
            N4GA – Rob

            Dean,

            I’m sorry, its sold. The only thing I’ve ever sold off this site!

            73! Rob

  6. Herb Brasington Avatar
    Herb Brasington

    Rob,
    I would be interested in purchasing the heat sink if it is not already sold.

    Herb AG7YY

    Reply
    1. N4GA - Rob Avatar
      N4GA – Rob

      Herb,

      You can get that heat sink here, any length:

      https://www.heatsinkusa.com/10-080-wide-extruded-aluminum-heatsink/

      73s!

      Reply
  7. Brandon Avatar
    Brandon

    Would power handling be improved by submerging this design in oil for better cooling? I was told the oil makes the power dissapation ten fold… Any truth to it? Love the design by the way!

    Reply
    1. N4GA - Rob Avatar
      N4GA – Rob

      Anything that removes heat will improve power handling. Not sure you would want to dip this particular DL into oil, there’s a lot of exposed connections and I don’t know the conductivity of oil at RF. Thanks for the compliment, this little DL worked great. Someone since purchased it from me.

      Reply
  8. Victor Koren Avatar
    Victor Koren

    From RF matching point of view it is better to arrange the branches in a star formation, with the connector at the middle.

    Reply

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