[Elecraft] P3TXMON question?

[Bob McGraw - K4TAX]

A comment here, has anyone thought to use a Bird 43P with a 100 watt HF 
slug?  Or maybe a TelePost LP-100a?  Both are equally good regarding 
power measurements but only when the load is 50 ohms resistive.

73
Bob, K4TAX
K3S s/n 10,163

On 8/21/2015 8:15 PM, Don Wilhelm wrote:
> 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.
>>
>
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