Caddock Dummy Load Performance

A few days ago, I built myself a simple 30 W dummy load out of Caddock power resistors. As I mentioned in the previous post, I was doubtful about the readings that I was getting from my LP-100 (not that I doubted the LP-100 itself, just my calibration of it). Tonight was my first night back to work after a long break, and I finally got a chance to put the dummy load on a calibrated VNA to verify its performance.

The instrument that I used was an Agilent N5230A. A very nice instrument, but one minor drawback is that its lower frequency limit is 10 MHz. Not to worry, as I suspect that if it does well from 10 to 30 MHz, that the lower bands are probably good enough for my home lab.

Caddock 30W Dummy Load - S11 Plot
Caddock 30 Watt Dummy Load - S11 Plot
Caddock 30W Dummy Load - SWR Plot
Caddock 30 Watt Dummy Load - SWR Plot
Caddock 30W Dummy Load - Smith Chart
Caddock 30 Watt Dummy Load - Smith Chart

Sure enough, it turns out that the performance of the dummy load is great and that my LP-100 calibration is off. The return loss is excellent all the way up to the 2 meter band. There’s not much else to say since the plots speak for themselves. If you need a dummy load, you could definitely do worse than to pick up a couple of these resistors on your next Digi-Key order and slap one of these together in a few minutes. Caddock also makes 100 W resistors, which will probably be on my next order for goodies.

3 thoughts on “Caddock Dummy Load Performance

  1. I discovered the MP9100 50ohm 100w resistor and built a dummy load.
    I put the resistor on sheet copper thence on a big heatsink.
    It was held down with a big fender washer. I used silicon
    heatsink grease.
    My N2PK VNA showed this to be a great dummy load on HF and 2m.

    First, I put it on my 2m rig (70w). Something funny.
    I measured DC resistance. 56ohms. I loaded up again.
    Again measured: 58ohms DC.
    Second, I contacted Caddock and a very helpful rep
    talked with me. He concluded I had a faulty resistor.
    Third, I thought a lot on that. I don’t think so.
    At low wattage the resistor is fine. At above
    50w it must burn away a little of the film because
    the resistance keeps rising.
    I think my problem is I can’t get the heat out of
    the resistor. Pressed onto copper thence onto
    aluminum doesn’t seem good enough.
    Are people able to get the heat away from these resistors?
    If so, it would be great to know how, since they make
    good dummy loads right up to UHF.


  2. Thanks for that information, Tom. That’s a very interesting response from the resistor. How big of a surface area was your copper clad/heatsink combo? I’ve never seen a write-up of anyone using the 100W resistor in ham radio use. I’m wondering if it would work better to parallel two 100 ohm 100 watt resistors, to give you a bit more headroom in heat dissipation?

    73 de NT7S

  3. Thanks for the quick reply. You know the size of the resistor from
    the ones you use. That is the contact area. The heatsink is
    about 2″ by 4″ with fins about 1″. I don’t think that’s terribly
    important because the heatsink stays pretty cool. I.e. the
    heat is not getting out of the resistor. 70w should be
    instantly noticeable with such a small contact area.

    More than one resistor makes it difficult to get it right
    at VHF and UHF since suddenly there is much more inductance and
    capacitance. My resistor connects directly to a BNC female
    fitting. Very short leads.

    I was just hoping someone would find workable solutions since these
    are amazing resistors.


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