[Elecraft] OT Power Measurements

[Bob McGraw - K4TAX]

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 <kl7uw at 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
> ______________________________________________________________
>