Dummy Load confusion

Discussion in 'General Technical Questions and Answers' started by DAMO666, Jul 12, 2021.

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  1. WG7X

    WG7X Platinum Subscriber Platinum Subscriber QRZ Page

    That statement alone will get you a lot of heat here. This is an amateur radio web site and as such, we should not give you a lot of help with citizens band amplifiers and associated equipment.

    That said, you're already getting helpful advice from the fellows. One basic note is that AC and DC work in different ways. That's why using a VOM to test any RF equipment might give you the wrong indications. RF is, at heart, AC at higher frequencies that what comes out of your wall outlet...
     
    K2XT, DAMO666 and WD8T like this.
  2. W4NNF

    W4NNF XML Subscriber QRZ Page


    Yep, right on.
     
  3. W9WQA

    W9WQA Ham Member QRZ Page

    but,a 35 ohm reading is a problem...dc or not
     
  4. K8JD

    K8JD Ham Member QRZ Page

    The looks of the risistor reminds me of a wire-wound resistor, any inductive impedance would meke it "Iffy" as a RF load.
    The device you have is not very reliable , IMHO.

    Wonder what A "CB" guy needs with a 1 KW load, Pruely illegal to run more than a few Watts from a CB radio.
     
    W4NNF likes this.
  5. WA9SVD

    WA9SVD Ham Member QRZ Page


    5 Watts AM, 12 Watts PEP for SSB the last I heard...:rolleyes: (NO other modes are "legal.")
     
    W4NNF likes this.
  6. N4ZAW

    N4ZAW Subscriber QRZ Page

    I'm a ham and an avid CBer who likes seeing CBers here asking technical questions.
    Outside of the dummy load itself, check the coax for heating also. When coax gets subject to high levels of RF, (even RF in-resonance) it's impedance changes, as well as velocity factor, and rf losses. The amount of this fluctuation is, of course, dependent upon the quality of the coax. With quality coax, these fluctuations aren't enough to worry about. But in some poor quality stuff, these changes can be swift and drastic. What I'm trying to say is, try better coax, and inspect all connectors/connections. If no joy, borrow a different dummy load (if you can find one) and re-check the system, coax and all, to see if the problem goes away.
     
    DAMO666 likes this.
  7. WA9SVD

    WA9SVD Ham Member QRZ Page

    A change in a dummy load, both DC resistance and RF impedance can be indicative of an excessive amount of power having been applied.

    1.Did the dummy load EVER have the proper DC resistance,.
    2. Did it ever show a 1.0:1 SWR at it's rated frequency range, (or at least the lower HF region?)
    3. Is it truly able to handle the power it is supposed to handle? And is it continuous power, OR (VERY) intermittent power? (Some dummy loads have inflated ratings, and are only for 10-30 seconds ON, and several minutes OFF.
    4. Was it ever subject to excessive power, either accidentally or on purpose?.
     
    DAMO666 likes this.
  8. WA1UIL

    WA1UIL Ham Member QRZ Page

    So are we testing the dummy load or the power amp?
     
  9. N7EKU

    N7EKU Ham Member QRZ Page

    Hi,

    It states on the label that it should present 50 ohms from DC to 500 MHz (I guess none of the previous commentators noted the "DC" part). With the construction, I really don't see the design as being capable of that (seems to be a lot of inductance on that insulator at the connector end). A good wide-range dummy load is made to show 50 ohms DC resistance with the minimal amount of reatance possible.

    Regardless, with the resistor showing 35 ohms, it is way off the value printed on the resistor itself (50 ohms). So I would say some damage has occurred. Normally, resistance increases with heat.

    73.
     
    DAMO666 likes this.
  10. WA4SIX

    WA4SIX Ham Member QRZ Page

    Call the manufacturer.

    Ed
     

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