[Elecraft] Elecraft DL1 Wideband Dummy Load

[Bob-W5BIG]

Hi David,

The factor of 1.414 in the numerator is actually the result of two
operations:

(1) The DC voltage at TP1 on the DL1 is  the Peak voltage that appears
across the 25 ohm resistor.
      The diode D1 and capacitor C1 form a peak detector.

      This is converted to RMS by Dividing by the square root of 2.    (
Vrms = Vpeak/1.414 )

(2) Since this is only half the actual output voltage, it is doubled by
Multipying by 2.

Therefore, we have a factor of 2 in the Numerator and 1.414 in the
Denoninator which combine to give
1.414 in the Numerator.

For a quick calculation, when the DC reading is several volts, you can drop
the 0.15 and get
P(watts) approximately = V*V*1.414*1.414/50 = V*V/25

73/ Bob - W5BIG


----- Original Message ----- 
From: "David Pratt" <david at kippax.demon.co.uk>
To: "Elecraft Reflector" <elecraft at mailman.qth.net>
Sent: Friday, June 04, 2004 11:33 AM
Subject: [Elecraft] Elecraft DL1 Wideband Dummy Load


> I am having some difficulty in understanding the formula quoted on the
> DL1 information sheet for the calculation of power from the voltage
> measured at TP1, across one half of the 50 ohm load.
>
> P(watts) = (((V x 1.414)+0.15))^2)/50
>
> I wonder if someone would care to explain it?
>
> I take it that the meter reads r.m.s. and the 1.414 is to convert the
> reading to the peak value.  But what is the 0.15 for and why is the
> numerator divided by 50 when the meter is connected across 25 ohms?
>
> 73 de David G4DMP
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