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”

thomas

January 27, 2018

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
  
  January 28, 2018
  
  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
    
    January 28, 2018
    
    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
      
      January 29, 2018
      
      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
Jim Kinney WE4S

June 13, 2018

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
  
  June 13, 2018
  
  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
HZ1MT

November 18, 2019

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
  
  December 5, 2019
  
  MM,
  
  I would make a separate RF sampler. Like this one.
  
  Reply
John Lawrence

December 14, 2019

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

Reply
1. N4GA – Rob
  
  December 22, 2019
  
  John,
  
  Here’s the heat sink: https://www.heatsinkusa.com/10-080/
  
  It’s just over 10 inches.
  
  Reply
Blayne Marek

March 8, 2020

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
  
  March 8, 2020
  
  Yes sir, the one in the link. 73!
  
  Reply
2. Dean
  
  April 13, 2020
  
  Care to sell 25 of these?
  I’d be extremely interested!
  73
  Dean K2WW
  
  Reply
  1. N4GA – Rob
    
    April 21, 2020
    
    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
      
      March 25, 2021
      
      I never saw this reply Rob,
      Do you still have the loAd?
      73
      Dean K2WW
      
      Reply
      1. N4GA – Rob
        
        March 27, 2021
        
        Dean,
        
        I’m sorry, its sold. The only thing I’ve ever sold off this site!
        
        73! Rob
Herb Brasington

May 29, 2020

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

Herb AG7YY

Reply
1. N4GA – Rob
  
  May 29, 2020
  
  Herb,
  
  You can get that heat sink here, any length:
  
  https://www.heatsinkusa.com/10-080-wide-extruded-aluminum-heatsink/
  
  73s!
  
  Reply
Brandon

July 26, 2020

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
  
  July 31, 2020
  
  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
Victor Koren

May 26, 2021

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

Reply