Dummy Load confusion

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

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  1. DAMO666

    DAMO666 QRZ Member

    I'm into CB'ing and have just got myself a 1KW Zetagi DL61 dummy load for testing various amplifiers, etc'.

    My Zetagi DL61 dummy load shows 48 ohms when checked cold/unused with an ohmeter, but when it heats up after use, checking it again with an ohmeter, the resistance can go as low as ~35 ohms.

    Is this normal?

    Even though the resistance/impedance of the dummy load varies so much, the SWR stays under 1.1:1 throughout HF, and under 1.3:1 at VHF (2M band). SWR stays steady at these levels despite such aforementioned drastic changes of resistance/impedance over the duration.

    Surely an impedance/resistance of ~35 ohms would exhibit a SWR of around 1.6:1 or thereabouts, so am I missing something?

    I say this because a 100 ohm or 25 ohm impedance would be a 2.0:1 mismatch.

    Can anybody shed some light on why I'm seeing no mismatch at 35 ohms when heated, and also, is it normal for this "big resistor" or similar to drop in value when heated up?

    Here is what's inside my dummy load.

    Many thanks.

  2. W1VT

    W1VT Ham Member QRZ Page

    The Seebeck effect? Voltage generated across junctions between two or more metals are used to measure temperature.

    These voltages can upset an resistance measurement.
  3. AJ4GQ

    AJ4GQ XML Subscriber QRZ Page

    It's probably the same type of problem you had with that amp in 2009 where the RF got into the wrong places. You need the T adapter to stabilize the system per the instructions from @W0IS or it might be easier to simply re-hydrate the resistor per @K5RCD's suggestion.
    Just curious, are you in the UK? I ask because the maximum EFRP is 4 W in the UK and one wonders why you would need a dummy load rated for a kilowatt. The B47 amp you were using only puts out around 50 to 100 W so you must have gotten something more powerful.
    Last edited: Jul 12, 2021
    WB5THT likes this.
  4. W0IS

    W0IS Ham Member QRZ Page

    Oops! I think you tagged the wrong person.
  5. G0GSR

    G0GSR Ham Member QRZ Page

    Your Ohm-meter is not telling you the full story.
    RF impedance is a lot more than resistance, it needs to be measured at RF frequency using a suitable device (VNA or similar).
    If you could do this, then you would most likely find that the "R" part of the impedance is different than what you are measuring at DC.

    Its normal bearing in mind the cost and method of construction of that device. But its undesirable.

    DAMO666 likes this.
  6. DAMO666

    DAMO666 QRZ Member

    Hi Frank,

    Thank you for the explanation - I'm very grateful.
  7. AJ4GQ

    AJ4GQ XML Subscriber QRZ Page

  8. KP4SX

    KP4SX Premium Subscriber QRZ Page

    My MFJ dummy load has a snap-in type resistor similar to that one. Resistance went high on it and I had to take it apart and clean the contact surfaces.
  9. W4NNF

    W4NNF XML Subscriber QRZ Page

    If the dummy load works OK, I wouldn't worry about it. A simple ohm meter won't give you the value of impedance the dummy load presents (a combination of resistance AND reactance). :)
    DAMO666 likes this.
  10. WA9SVD

    WA9SVD Ham Member QRZ Page

    It also depends upon the frequency in use. A 50 Ω DC resistance may also show a 50Ω "impedance," on 160 Meters (where a small amount of inductance may be insignificant) ; but depending upon design and actual construction, may be far from a n0ominal 50Ω impedance @ 10 Meters due to reactance introduced by interconnections. (THAT is why dummy loads usually specify a "useful" Frequency range, and a dummy load designed for low, low SWR at HF would almost certainly NOT (or not necessarily) have a low SWR @, say, 440 MHz.
    DAMO666, W9WQA and W4NNF like this.

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