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a cheap 250 watts dummy

Discussion in 'Homebrew and Kit Projects' started by 9H1FQ, May 10, 2017.

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  1. 9H1FQ

    9H1FQ Ham Member QRZ Page

    go to ebay and find a 250 watts 50 ohms resistor. I bought two for around $5
    then, get an old pentium aluminium heatsink, complete with fan, and instal the resistor!
    thats it. It can withstand over a 100 watts, without overheating! Possibly more!
  2. WA7PRC

    WA7PRC Premium Subscriber QRZ Page

    The ONLY way to know if it's overheating (or not) is by MEASURING the temperature and comparing it to the datasheet.

    My homebrew DL uses four 50Ω/400W flange-mount RF power resistors (Anaren pn RFP400-50R, datasheet), connected in series-parallel and mounted to two heatsinks (140 sq in each). A 100 cfm tubeaxial (aka Muffin™) fan forces air through the heatsinks:
    (click for yuuuge images)
    The close-fitting sheetmetal housing plays an important role in cooling by forcing most of the air through the heatsink fins. Not shown is a perforated strip of sheet metal between the heatsinks to further focus airflow where it will make the most difference. Mounted betwen two resistors is a LM35DT 'Precision Centrigrade Temperature Sensor' IC (datasheet). It produces 10mV/°C. When I tested the DL, I used a 660W/40.68 MHz RF source. Based on heatsink temperature rise, the DL should handle 1500W at 100% duty cycle continuously w/ a few degrees to spare. It easily takes all the RF my Heathkit SB-220 amplifier can throw at it.

    As tested on a VNA, VSWR is well under 1.5:1 thru 54 MHz.

    vy 73,
    Bryan WA7PRC
  3. AI0K

    AI0K Ham Member QRZ Page

    Just ensure you get a carbon composition or other non-inductive resistor. Most high-wattage resistors are wire-wound, which creates an inductive load at RF frequencies.
    Another way to get a 250 watt dummy load is to remember resistor wattage ratings are for free air, and continuous duty. They will withstand higher power for short times. But you can get a significant increase in power rating by immersing the resister in oil - food/medical quality mineral oil or transformer oil are recommended. Using mineral oil, a Heathkit Cantenna (basically a paint can with a 90 watt resistor) could handle 1KW for up to around 10 minutes.
  4. 9H1FQ

    9H1FQ Ham Member QRZ Page

  5. 9H1FQ

    9H1FQ Ham Member QRZ Page

    actually these resistors are white in colour and are mounted on a small metal bar, with holes on either side. Can they be ceramic?
    Anyway, when connected to my good old TS850 at full power, there was absoluetely not a trace of swr from top to 28Mhz . If there were any reactances would have been reflected in swr, however small!
  6. WA7PRC

    WA7PRC Premium Subscriber QRZ Page

    The resistors I used are specified for use at RF, up to 500 MHz. Some are rated much higher. I used resistors from Anaren but, there are others. The RF Cafe ( lists several.
    The resistors I used are flange-mounted to a heatsink or heat spreader. They came to me from an application that used a water-cooled heat spreader. They list only percent of rating vs temperature. IMO, it's NEVER a good idea to exceed that.
    The resistor I used is meant only for flange-mounting to a flat surface, not directly into liquid. Mineral oil is not nearly as good as transformer oil at removing heat. And then, in the case of my olde Heathkit HN31 'Cantenna', the steel can is heated, and after hitting it w/ significant RF power, I have to wait for it to cool down. OTOH, my air-cooled DL never runs out of air. :p
    Likely beryllium oxide ceramic w/ an alumina ceramic cover.
    Though the four resistors are rated to 500 MHz, my DL looked good "only" past 54 MHz. Chalk that up to the interconnections. The shorter/fewer the interconnections, the better. My first attempt at a DL in the 1970s used many small carbon resistors. VSWR at 28 MHz wasn't so good. :oops:

  7. AI0K

    AI0K Ham Member QRZ Page

    Yes, but not everyone understands that.

    No, it is never good to exceed specifications, no matter what they are.

    It can still be immersed in oil for better cooling. And while transformer oil is a better heat conductor than mineral oil, it is hard to find, especially in small quantities. Mineral oil is readily available from many local sources. And how often does a ham need to put a KW into a dummy load for hours at a time? In almost 50 years of being a ham, I have *never* needed to do that. The most is for maybe a minute while tuning and checking the output power of an amplifier - and even then, tuning is done at lower power then raised long enough to check output.

    And your four resistors were more expensive than a single 100W or so resistor immersed in oil. I believe in good equipment, but I also don't believe in spending more than I need to. It limits the number of toys I can buy :)
  8. WA7PRC

    WA7PRC Premium Subscriber QRZ Page

    ANYTHING can be immersed in oil for better cooling. How well heat is removed is related to they surface area. Liquid cooling works well until there's a spill. OTOH, I've never heard of a problem with spilling air.
    When I refurbished my Heathkit HN-31 'Cantenna', I found a gallon of FREE transformer oil at my power company's transformer shop 2 miles away. It took only a couple of telephone calls to contact the supervisor. The only difficulty was getting their gnarly fill pump to supply just ONE gallon of oil.

    Incorrect. The four flange-mount RF power resistors were scrapped parts from my employer. In fact, ALL parts were scrapped. Yes, my total cost was ZERO. For those not that fortunate, eBay sellers such as Henry Radio ( typically offer usable parts for dummy loads at reasonable prices. Right now, I see a 50Ω/250W terminator available for $16 shipped (link):
    Add a heatsink and small fan, and you're in business up to 1GHz. Need more power? How about 50Ω/800W (only 500 MHz) for $50 shipped (link):

    The MFJ pn 115-1500A 50Ω/90W carborundum resistor (equivalent to the Kanthal/Globar #886SP used in the Heath HN-31, MFJ-250, et al) will run you $41 plus shipping.

    In my nearly 50 years as a ham, I also have never needed to put a KW into a dummy load all day. However, it's not expensive to buy/build a DL capable of handling it. And if you need it, you HAVE it. In drag racing, we say "it's easier to turn the horsepower screw down than it is to turn it up". However, hams tend to build/buy stuff that can BARELY meet the minimum requirement... and then push it harder. Hence, ICAS ratings.

    I too believe in good equipment. I also believe in not being pennywise and pound foolish.

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