# A test of Zs on an IC7000

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:

• Adjusting the load impedance by adjusting the input capacitor of the ATU until VSWR(50)=1.5, forward power is 85.9W and reflected power is 3.4W, so the power delivered to the load is 82.5W, or a loss of 1.2dB due to load impedance. Measurement of the IC7000 on this load with VSWR(50)=1.5 yields a power reduction of 1.2dB, whereas the classic MismatchLoss formula would yield 0.18dB, less than one sixth the actual loss. The considerable error in the MismatchLoss formula’s prediction is proof that the equivalent source impedance of the transmitter is not 50+j0Ω.
• But wait a minute, swinging that capacitor the other way for VSWR(50)=1.5, forward power is 109W and reflected power is 4.4W, so the power delivered to the load is 104.6W, or a loss of 0.18dB due to load impedance. Measurement of the IC7000 on this load with VSWR(50)=1.5 yields a power reduction of 0.18dB, where the classic MismatchLoss formula would yield 0.18dB, exactly the same. The low error in the MismatchLoss formula’s prediction is proof that the equivalent source impedance of the transmitter is 50+j0Ω.

So, here are two tests that indicate that equivalent source impedance is, and is not 50+j0Ω. The transmitter clearly exhibits non-linear 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.

# Changes

 Version Date Description 1.01 01/08/2010 Initial. 1.02 1.03

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