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A measurement of my IC7000 on 1.8MHz as per the article Is Zs of a HF ham tx typically 50+j0?.
Using a digital multimeter, I have plotted the DC meter voltage in a directional wattmeter against RF power at 8 points from 10W to 100W, and created a cubic spline interpolation to be able to convert meter voltage with 0.1mV resolution to RF power.
With VSWR(50)=1, the measured forward power is 109W, and of course reflected power is 0W, so the power delivered to the load is 109W.
Now, two tests each with load VSWR(50)=1.5:
So, here are two tests that indicate that equivalent source impedance is, and is not 50+j0Ω. The transmitter clearly exhibits nonlinear behaviour (ie cannot be accurately represented by a Thevenin equivalent circuit) and power in the load depends on something else than just ρ(50). Calculations of MismatchLoss based on ρ(50) (the magnitude of the complex reflection coefficient Γ) are unsafe.
The tell tale sign that equivalent source impedance is not constant at 50+j0Ω is that forward power varies with change in load impedance.
Version  Date  Description 
1.01  01/08/2010  Initial. 
1.02  
1.03 
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