[Elecraft] Accuracy of Wattmeter W2 under 2 Watt

[Don Wilhelm]

All,

Keep in mind that if you use the excellent method Ron describes, there 
is a huge dependency on the precision of the dummy load - just any old 
resistor will not do unless it is characterized first.  It must be 
characterized for not only resistance, but the resistance and any 
possible reactance over the frequency range for which it is to be used.

With the resistance and reactance (hopefully zero) known for the 
frequency of measurement, and the forward voltage drop of the diode also 
known, the power can be calculated with a very small error.

I typically use the RF voltage observed on an oscilloscope connected 
directly across a known dummy load - that voltage allows the power to be 
calculated within 5% if your eyes are good at determining the 'scope 
trace deflection.

73,
Don W3FPR

On 10/25/2015 6:36 PM, Ron D'Eau Claire wrote:
> The DL1 can be very accurate. As a QRP and QRPp enthusiast I have a homebrew
> version that goes back decades before Elecraft, Hi!
>
> A simple RF voltmeter is a common way and inexpensive to measure very small
> power levels. (For the more exotic measurements a bolometer or, optionally,
> an accurate well calibrated oscilloscope is often used.)
>
> The RF voltmeter system (which the DL1 uses) gets inaccurate as you get down
> into the barrier voltage range of the diode. In that range the response of
> the diode is no longer linear. My homebrew version uses a 1N34A Germanium
> diode which is darn near "unobtainium" these days but a good PIN diode such
> as Elecraft supplies in the DL1 is just as good. Both have a barrier voltage
> of about 0.2V.
>
> But you can make very accurate measurements even at very low levels where
> the barrier voltage makes the system non-linear with a little more effort.
> Instead of applying RF to the device, connect an adjustable dc power source
> of about a 1 watt. Use your accurate DMM to measure the voltage and current
> applied to determine the output as you vary the d-c power applied.  The d-c
> power reading is the same as the RF power reading from such a device. So, by
> creating a calibration chart using accurate d-c power levels, you can now
> measure similar RF power levels with excellent accuracy.
>
> 73, Ron AC7AC
>
>   
>