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July 11, 2003 R.F.Probe
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Latest project came about with the need to measure the output of my FT-817 at the 500 milliwatt level. I had made a contact from Colorado to California with 1/2 Watt on 2 meter sideband and was eligible for a couple of awards, but I needed to confirm the actual power. (By the way, turned out to be 590 mW actual power for 1,434 miles per Watt.)
I took the basic schematic from the ARRL Handbook (2002) and the use of copper tubing from N5ESE's great web pages. Two capacitors, a resistor and a VHF/UHF Schottky diode. RF is converted to a DC voltage (RMS voltage due to the ratio of the divider resistor to the meter's impedance) that is read on a digital voltmeter. A few taps on the keypad of a calculator and I have Peak Envelope Power.
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Below is a photo of the entire setup. The 817 is coupled directly through a "tee" which also joins the probe.
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Closeup with a bit more details.
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Right:
This is a set of instructions I taped to the side of the tube to remind me how to convert rms voltage into power. I have to include the (rms) voltage drop (.35 x .7071=.25v) across the diode to reflect the true total voltage. ( Consider this a note-to-self for future reference.)
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