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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