> The problem occurs when I use my antenna tuner to purposely mis-tune
> to give me high SWR and high reflected voltage/power. I am using the
> LCD display on my MFJ-993B antenna tuner for comparison. The Wavenode
> sensor is put in between my IC-718 and the antenna tuner so it sees
> the mismatch I create. The antenna tuner feeds a 50ohm dummy load and
> I use the tune function of the IC-718 to keep the power down to about
> 10-12 watts.
> With high SWR, the MFJ still shows about the same forward power (12W)
> with a high reflected power (6-8W). However my PIC LCD shows much
> lower levels (8W forward and 3-4W reflected). This must be caused
> because the Wavenode and my calculations assuming a 50 ohm load and
> the load is no longer 50ohms. I was thinking about trying to use the
> SWR to correct the power reading since SWR is just a ratio of forward
> and reverse voltage and that ratio won't change over load.
The problem is that you don't know the phase. 2:1 VSWR can be 25 ohms or
100 ohms resistive, or an infinite combination of R+/-jX. When the SWR
is high, you can't assume that the forward power the rig delivers is the
same as it is into 50 ohms.
My guess is that what you're seeing is the difference in directivity
between the coupler systems. Couplers don't have perfect isolation
between the forward and reverse samples they take. If some of the
forward power finds its way to the reverse output, you get an error (and
vice versa). The phase between them determines whether the reading is
higher or lower it should be.
As a simple experiment, make a 25 ohm load from resistors, or two dummy
loads in parallel then connect it to the output of each coupler through
a variety of cable lengths from short up to 1/2 wavelength to vary the
phase. The indicated powers will vary, but a decent meter should show
the VSWR as between 1.8 and 2.2.
Directional power measurements are fraught with error. A fancy coupler
to make accurate measurements in a lab will cost as much as a posh rig.
Steve
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