[Elecraft] P3TXMON question?

[Don Wilhelm]

In a slightly related note - for those who are using an oscilloscope to 
measure the RF voltage at the output, instead  of converting everything 
to RMS and doing the power calculation, I can give you a shortcut.  The 
formula for power by observing the RF voltage across a 50 ohm dummy load 
is Vp-p squared, and then divided by 400.  If the load is not 50 ohms, 
then it is Vp-p squared and divided by 8 times R.

The derivation is left to "the student" - Hint, use SQRT 2 in your 
derivation rather than 1.414 or .707 because the radicals will cancel 
out - the numbers will only cause confusion, but will produce a similar 
result.
I use this easy formula at the workbench often when determining power 
output, it is especially useful at power levels of 10 watts and below.  
Yes, I do have an oscilloscope probe permanently connected directly 
across my dummy load just for this purpose as well as for looking at 
relative RF voltages during an alignment.

73,
Don W3FPR

On 8/21/2015 3:34 PM, Bill Breeden wrote:
>
> It's important to remember that RF power expressed in watts is always 
> based on RMS values.  If RF watts are calculated by measuring the 
> voltage across a known load, the voltage must be expressed as an RMS 
> value to correctly calculate the power in watts.  Watts are watts.  
> There is no such thing as "peak to peak" or "RMS watts" when 
> expressing RF power in watts.
>