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
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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