Dummy Load Ohm Reading

9Track

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Sep 29, 2020
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Hello. I recently purchased a Diamond DL50A dummy load for SWR and power testing. It's supposed to be a 50 Ohm load, but when I check the resistance with multiple multimeters, it's only reading 47 - 48.4 Ohms. I'm assuming this is not going to provide for accurate measurements with the SWR meter?

If it is going to throw it off, I can return for an exchange, but wonder if the design is good. Is there a better one out there that I should consider?

Thanks,

Steve
 

majoco

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Hmmm... the specs say
Impedance: 50 Ohm
VSWR: less than 1.2: 1

and
DC to 1GHz - Max. 100W
So you would expect it to be fairly close to 50ohms with a multimeter BUT it does say "VSWR less than 1.2:1" which infers that at some frequencies it's not quite 50ohms but possibly that's what determines the top frequency - the capacitance of the housing may be changing the impedance. I measured a couple of my loads with an analogue meter which made them both read a smidge under 50ohms and a Fluke 77 digital meter which gave 50.3ohms for both. Personally I wouldn't lose any sleep over it. Just don't feed it with 100watts for more than a couple of microseconds even though they do say you can for 30secs - it probably won't be anywhere near 50ohm after that treament!
 

ko6jw_2

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What is the accuracy of your multimeter? 3%? What is the tolerance of the dummy load resister? 5%? You are well within tolerance. What is the accuracy of your SWR meter? Unless you have a lab grade instrument it probably won't be within 5% anyway. You want to measure the SWR of an antenna and the dummy load is not involved. If you get a low (less than 1.5:1) SWR you will be fine.

Multimeters measure DC resistance which is not the same as impedance at radio frequencies.
 

9Track

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2%, 2% & 3% on the multimeters. If it's close enough, that's good to hear. Just haven't done this before, so didn't know how close the resistance needs to be.

I would think if all three of these MMs are showing 47-48.3 Ohms, it's not 50 Ohms.
 

ko6jw_2

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As I said, multimeters measure DC resistance. They do not measure impedance. They are not the same thing. Will your radio care if the dummy load is 48 or 52 ohms - No. You need to stop obsessing over details that are meaningless in the real world. You could keep exchanging the dummy loads repeatedly until you get one that's right on. You could also get a lab grade impedance bridge to measure the dummy load. However, in the real world it would make no difference.
 
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