I have been working on getting my LCD display for my 600W EB104 based
amplifier (most parts in, but no solder flowing yet) working. I am
using a Wavenode RF sensor (gives forward voltage reflected voltage) (
http://www.wavenode.com/accessories.htm ) to feed the PIC (Microchip
16F877). The information is displayed on a 2 x 16 back-lit LCD
display.
For SWR, I simply sample the forward and reverse voltage and use SWR =
(Vf + Vr) / (Vf - Vr). For forward power, I multiply the sample by
4.88mV/bit to get the actual voltage (5V reference, 10bit A-D). From
the Vforward chart at Wavenode (
http://www.wavenode.com/power_cal_3.htm ), I take the forward voltage,
square it, and then multiply by 12.5 to get a value in Watts. This
works great when the SWR is low and my load is close to 50ohms.
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. However, I
am not sure if that is the right way to do it. The MJF seems to do a
fine job of measuring the forward and reverse power at low and high
SWRs so it seems like I should be able to get my setup accurate.
Any suggestions are appreciated. I have read the "SWR Protection"
thread from a few months ago.
Thanks,
Matt
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