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Homebrew dummy load readings

Discussion in 'Antennas, Feedlines, Towers & Rotors' started by KN4YRM, May 31, 2021.

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  1. KN4YRM

    KN4YRM XML Subscriber QRZ Page

    I've never used a dummy load before so trying to understand if I've had successful build or not...

    I made a low power dummy load using 2 100Ohm resistors in parallel, each rated for 100W.
    Housed inside an aluminum case typically used for guitar pedals.
    Multimeter reads 50.1 Ohms.

    On VHF FM I put 5W then 10W into it, SWR meter shows ~2.3.

    On HF (40m & 20m) SWR is fairly flat around 1-1.5 as I speak into the mic, but occasionally seems to "blip" up to 3 but comes down.

    Is this the way a dummy load behaves, or should it be dead on 1:1 at all frequencies?
  2. W4HWD

    W4HWD Premium Subscriber QRZ Page

    Assuming the rigs are 50 ohms, you could have a heating problem; heating the resistors can change SWR; increasing time and increasing frequency will increase heat. Heathkit had a Cantenna, which consisted of a non-inductive resistor floating in (I think) mineral oil, sealed in a 1 gallon paint can. That thing was rated at 1kw for a few minutes, and the can would get warm. Others use air cooling; such designs would be rated for lower power and shorter intervals.

    Also, the resistors must be non-inductive and the wattage rating has to be high enough to dissipate heat at a certain power level.

    Now I know when you parallel resistors you get half the resistance; I don't know if that halves the power rating too, so someone more smarter needs to chime in.
  3. KN4YRM

    KN4YRM XML Subscriber QRZ Page

    These are the resistors I had around

    I very much doubt heating, but the non-inductive qualification you gave could be a problem as they are described as "Wirewound Resistor" which I assume implies some kind of coil inside?
    PU2OZT likes this.
  4. W4HWD

    W4HWD Premium Subscriber QRZ Page

    If you google "non-inductive resistor" they will came up. The last time I bought some I think I got them from DigiKey. Those wirewound resistors are probably inductive if they're not specifically labeled as non-inductive.
    KU3X and PU2OZT like this.
  5. WB2WIK

    WB2WIK Platinum Subscriber Platinum Subscriber QRZ Page

    Those "non-inductive" resistors are non-inductive at very low frequencies, like 60 Hz through maybe the audio spectrum.

    They are not non-inductive at radio frequencies, especially above a few MHz.

    Resistors used for power RF dummy loads are never wirewound, they're film deposited on ceramic substrates if they're surface-mount or "Globars" (just really pure carbon cylinders, no wires) if they're not. It's also possible to use twenty 1K Ohm 3W carbon film resistors in parallel, immersed in a can of oil, to make a 50 Ohm load that's good for 100W.

    My 50 Ohm dummy loads all measure VSWR <1.1:1 at 500 MHz or higher; the Bird 1200W load I use is <1.2:1 at 1000 MHz. But if you don't need a VHF-UHF load, things are a bit less critical. Still, the "non-inductive" wirewound resistors will never be a good RF load.
    KI4ZUQ and W4HWD like this.
  6. W4HWD

    W4HWD Premium Subscriber QRZ Page

    See I knew someone who knows more about it would pop along quick sticks.
  7. CX3CP

    CX3CP Ham Member QRZ Page

    Best for radio from DC to 500 MHz 800 Watts for sale on ebay $55 each

  8. VK2TIL

    VK2TIL Ham Member QRZ Page

    "Proper" RF resistors are readily available on ebay etc and from suppliers such as Mouser; this is the first "hit" from a Google search;

    A substantial heatsink is required to achieve their power rating; my home-made load uses a 250 watt KDI rf resistor on two heatsinks with a combined thermal resistance of 0.22C/W; it handles 100 watts comfortably; that's as much as I have put into it but I think it might manage about 150 watts continuous.

    Photo 1.JPG

    SWR is better than 1.2:1 up to 500 MHz and is 1.5:1 at 750 MHz, after which it deteriorates rapidly.
  9. AG6QR

    AG6QR Premium Subscriber QRZ Page

    When you combine two resistors, they each retain their original power limits. If they have equal resistances, and are combined in parallel (or series, for that matter) the power will split equally between them.

    So two resistors of 100 ohms rated at 100W, when wired in parallel will have a resistance of 50 ohms and a power handling capacity of 200W.
  10. KI4ZUQ

    KI4ZUQ Ham Member QRZ Page

    20 X 3 = 60. Right?

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