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You don't want to be connecting a VNA to your rig, ever, only to an aerial without the rig. Nearly all SWR meters measure power too. The ones for CB use a strip line sampler, so above 27 MHz they take too much signal (though you can cut it in 1/2 for 50 MHz or use about 1/3rd for 144 MHz. They can be adapted to measure power only on any band. The HF SWR meters use a toriod transformer with the signal using 1 turn. They may not always be good for 160m or 10m. The CB type meters are OK on 10m, and may be OK 21 MHz to 29 MHz.

If you are measuring power to a dummy load you don't need the SWR circuitry, simply a resistor divider and pair of 1N60 diodes as a peak detector. That then needs calibrated.

An Ardunio or other mpu is only needed for an automatic SWR meter where you don't have to set forward power reference level. All simple SWR meters will also measure forward power as they need that for the cal setting and sensitivity knob, though some may have poor calibration and many only work over a particular range. There were expensive easily damaged wideband HP power meters that could measure almost any frequency and waveform as they used a thermistor. The really expensive Bird meters are more robust and much cheaper!

No cheap VNA is any use to measure power. Mostly the signal generator is no use for anything other than built-in measurements on cheap ones.

One old method was to use two lamps behind a screen that were approximately 50 Ohm at the desired power (tungsten filament lamps can vary 1:10 between cold and hot). Then you connected one to device under test and the other to a PSU. You adjusted them the same appearance/brightness and then the DC PSU readings are your power. Still can be handy above 1 GHz as a rough check.

You can use a cheap CB meter on 28 MHz on the FT817 with a dummy load, if you have a method to calibrate / check it (most have gain pots). That then tells you the FT817 and its inbuilt metering is working (or not), Then assume the FT817 metering is a rough guide. A quality 100 MHz scope may be good enough for HF to see peak to peak on a 50 Ohms dummy load and then you can calculate the power = (peak to peak / 2.828a) squared / 50