Designing RF Probe
In this article we will designing an RF probe. Radio frequency (RF) probe is used to directly measure the level of RF RMS voltage present across two points. It is one of the most useful test instruments for home brewers as well as for communication equipment service/design labs.
Designing RF Probe
RF voltage level being measured provides useful information only when the probe has been designed for use with a specific multimeter. The design of RF probe is a function of the meter we intend to use it with. If a meter with a different input resistance is used with the probe, the reading will be incorrect. The value of RX (refer figure) is so chosen that when this resistor is connected in parallel with input resistance of the multimeter, the peak value is about 1.414 times the RMS voltage. Resistor RX has to drop this excess voltage so that meter indication is accurate. If we know the input resistance of the meter, we can calculate the value of RX with the help of the following relationship:
Determine The Characteristics Of RF Probe
Let meter DC input resistance
X 1.414 = RY
Then RX = Ry – meter DC input resistance For example, if meter input resistance is 20 meg-ohm, Ry = 28.28 megohm and RX = 8.28 meg-ohm.
We can convert the RF voltage level (E) so measured across a given load resistance (R) to RF watts (W) using the following relationship:
Power P = E2 / R watts (W)
For example, if RF probe voltage reading across a load resistance of 50 ohms is found to be, say, 15.85 volts, the power in the load = 15.85 x 15.85 / 50 = 5W approx.
In other words, for 5-watt power in a 50-ohm load, the voltage across the load is 15.85 volts.
The rectified DC voltage at the cathode of diode D1 is at about the peak level of the RF voltage at the tip of the probe. Use shielded cable in between the probe output and meter. It will act as feedthrough capacitance and thus avoid RF interference. The maximum RF input voltage level depends on the peak inverse voltage (PIV) of diode D1. The shielded lead length is too large to give accurate results at UHF.
Please refer tables in the picture above for ready conversion of RF voltage level (RMS) to equivalent power across a 50-ohm load and deduction of R X value for a given meter’s DC input resistance respectively.