Larry Phipps wrote:
You can also fine tune the formula based on DC measurement of the
resistance of the bottom half of the load. The formula assumes an
exact 25.0 ohms for the bottom half of the load.
Also, measure the resistance at the temperature you will be using for
testing. You can see a graph from my HP VNA at
www.telepostinc.com/loads.html for both the DL-1 and a Caddock MP930
thick film resistor mounted on a heat sink. The DL-1 shows some
variation with temp... about 0.5 ohm from room temp to 1 min key down
at 10W. The Caddock showed less, but I seem to have misplaced the
graph for it.
If you take these and Don's caveats into account, the results should
be pretty good, especially near the high power limit of the load where
the diode drop becomes a less significant factor.
Larry N8LP
Don Wilhelm wrote:
Ed,
Yes, the formula in the DL1 manual is off a bit - there was some discussion
about that on the reflector about 6 months ago.
As I calculate, the correct formula is P(watts)=(1.414(V(volts)+0.15))^2/50
Or easier expressed as P=2(V+0.15)^2/50 (I am not certain why the square
root of 2 (1.414) appeared inside the term to be squared - much easier just
to square it and move it outside).
The diode drop must be added to the voltage read on the DMM for low power
measurements.
If you wish better accuracy, measure the actual forward voltage drop across
your particular diode, they will vary a bit from diode to diode. Use the
measured value in place of the 0.15 factor in the formulas above.
73,
Don W3FPR
-----Original Message-----
The formula given in my DL1 manual is not correct, in my view.
The lower the
power, the more incorrect it becomes.
Ed Lambert KD3Y
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