Seems that I somewhat stirred the pot on this topic with my suggestion
of using a Bird 43P or LP-100a. In this regard with a Bird 43
instrument, proper choice of a slug is necessary. Thus if one wishes to
measure 10 watts or so then a slug of the proper full scale range, both
in power and frequency, should be used. In no way would I suggest or
imply a 100 watt slug be used to measure 10 watts.
We seem to overlook the point the Bird meter, as well as most other ham
type power meters, are voltage devices. In this regard, the accurate
power measurement relies on the calibration at precisely 50 ohms. Any
other Z, resistive or reactive basically negates the accuracy of these
devices. Now used as a means to determine SWR, they work OK as one is
comparing two different power values obtained with the same instrument
at the same point in the feed line and using a calculator or nomograph
to determine actual SWR values.
One of the few instruments to calculate the true power is the LP-100a in
as much as it has two sensors which are used to measure both voltage
and current. At this point the Z component is insignificant to a larger
degree.
As to using an oscilloscope, again the calibration is a critical point
in accuracy plus the resolution accuracy or the ability of one to
determine the actual value of the trace as displayed. And too, the
accuracy of the dummy load becomes a significant factor. As a rule we
presume most dummy loads are 50 ohms, but are they? I have four
different ones, all showing a difference in R value, yet are a
"supposed" to be 50 ohm loads.
As someone else stated, all of this is determined by the accuracy of the
items being used to measure power and the operator understanding and
knowing the limitations of the measurement system.
73
Bob, K4TAX
K3S s/n 10,163
On 8/22/2015 9:00 AM, Hank Garretson wrote:
On Fri, Aug 21, 2015 at 11:19 PM, Edward R Cole <kl...@acsalaska.net> wrote:
When I need to know accurate power readings my best instrument is either
scope and good 50-ohm termination, or using my HP432A mw power meter. Its
about 1/2 dB accurate.
Isn't "1/2 dB accurate" the same as "12 percent accuracy"?
73,
Hank, W6SX
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