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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>
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