[Elecraft] P3TXMON question?

[Phil Hystad]

I have an LP100A as well as TX Mon for the P3.  But, I also do admit that my reason for having the LP100A is not to have an accurate reading of power but rather to have a reasonable good indicator of power and SWR.  I got the LP100A long before I had my K3 or P3 so if I were doing it all over again, the power meters on the KPA500 or the P3 TX Mon are good enough.  I like to know whether I am putting out 100 watts or 2 watts.  I really don’t care to know if it is 100 watts or 95 or 105 watts.

I think these days, the best thing about the LP100A is that it is yet another lighted up display that looks cool for any visiting guests.

73, phil, K7PEH


> On Aug 21, 2015, at 7:38 PM, Bob McGraw - K4TAX <rmcgraw at blomand.net> wrote:
> 
> 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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