[Amps] Dummy Loads & Wattmeters

[Dr. David Kirkby]

David C. Hallam wrote:
> My original posting has garnered a lot of interesting discussion.  To 
> reduce everything to the simplest what I am trying to determine is;
> 
> If I measure the output of an amp into a dummy load with a measured DC 
> resistance of 69/70 ohm using a Bird 43 wattmeter and I read 550W, can I 
> have confidence that this reading is within +/-10% of actual?
> 
> David
> KW4DH

Technically you would need to define confidence.

Is being 10% certain the result is within +/- 10% OK for you? Do you need to be 
50% confident it is within 10%? Do you need to be 90% confident it is within 
10%? Do  you need to be 95% confident? 99% confident perhaps?

Using laymens terms, you should not be confident the result is 10%.

Firstly,  I'm not aware of Bird slugs with power rating W of greater than 500 W, 
but less than 1000 W, though that might not be true now, although it was 20 
years ago.

So you would to use a 1000 W slug. Bird would claim +/- 5% of FSD (50 W on a 
1000 W slug). If you read 550 W +/- 50 W then  according to Bird specs, reading 
should be accurate to +/- 9.1%.

In practice, given I believe Bird specs are optimistic by about a factor of at 
least 2, I'd say it would be 550 W +/- 18 %. So you should not have much 
confidence it is within 10%.

As a very rough guess, I'd say you can be 95% confident the true power is within 
18% of 550 W.

Dave



> Dennis OConnor wrote:
>>  
>> The simplest way for a ham to accurately measure to within 5% power is to measure the RF voltage impressed across a known impedence...
>> There are a number of ways to refine your voltage measurement to be within 5% deviation from NBS, but quick and dirty works just fine for me...
>>  
>> The very easiest is to get a dummy load... And get a 1% precision resistor that is near to 50 ohms -  Mouser, et. al. about a $1.70...  Use the resistor to measure your VOM and calculate a correction factor then measure the dummy load, apply the correction factor from the precision resistor and you know the DC resistance of your dummy with near 1% precision... (ya, I know DC and RF ain't the same - I said quick-n-dirty)
>> Once you know that, you apply the RF from the amp to the dummy load and measure the RF voltage...  That measurement can be direct, with an oscilloscope, or by rectifying the RF to DC and measuring that...  
>> If you are using a Fluke, or other quality VOM, you have a known precision factor for the DC measurement and can directly calculate your supposed precision value - just remember to add the 0.7 volt you lose across the diodes, to the measurement...
>> If you are using an oscilloscope you can take the RF voltage number at face value compared to the scope's built in calibrator  -   or there are other ways, just like the VOM + Precision resistor, to derive a correction factor... It would take a very long post to describe all the ways to do that - but i trust hams to come up with simple solutions...
>>  
>> So for most hams, a dummy load, precison resistor, a couple of diodes and a cap, and a VOM, will get you into the ballpark of laboratory precision for a few bucks...  Then this can be used to tweak your wattmeter to be right on (within your precison range) at the power output you normally run - or to make a correction card for the readings it currently gives (just like the compass on my boat)...
>>
>> denny / k8do
>>
>>
>>       
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>