You can build a simple RF probe but you need to calibrate it. ?A simple SWR meter does not measure power over a wide range of frequencies. ?If possible, borrow a Bird 43 wattmeter from a local ham.
You can use that device to calibrate the power of your measurement equipment.
Check out kc9om.com. ?He sells some great kits and gives excellent service.
You can also build own watt meter using a Arduino UNO, a power tap and dummy load
See my rather crude video that I produced 4 years ago to see a demo of a talking watt meter.
Ham Radio