RF probes are simple just a diode, capacitor and usually a resistor as well and are quite useful in homebrewing. The output from the junction of the diode and capacitor is a DC voltage equal to the peak voltage of the signal applied But the usual way of refemng to AC and RF voltages is by the RMS value, which is 0.707 times the peak value. We could connect a voltmeter to this point and read the peak value, then multiply by 0.707 but it's simpler to use the resistor. It forms a voltage divider in conjunction with the input impedance of the meter such that the output is approximately 0.707 of the peak,and thus the numbers on the meter correspond directly to RMS witb no conversion needed.
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