This RF probe circuit uses a simple detector design to measure both the positive and negative halves of an RF signal. It consists of a diode and resistor that converts the RF voltage to a DC voltage that can be read by a voltmeter. The resistor value is selected to match the input impedance of the voltmeter for accurate readings. This probe can be used to measure RF signals from 1 to 400 MHz.

1. RF PROBE
4S6GGS
This configuration is made so that it's a simple
detector to detect both positive and negative half of the RF
signal.
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2. RF PROBE
This configuration is made so that it's a simple detector to detect both positive and negative half
of the RF signal. The resultant DC voltage will be peak to peak at C1 with respect to the input RF voltage
C1 – 0.001uF
D1 – IN34A or similar Germanium diode
Rx - 4.7 ohms. R = 4.7 Meg for Zin = 11-Meg;
R = 4.3 Meg for Zin = 10-Meg;
R = 430 K for Zin = 1-Meg
.
Rx For best accuracy, size the resistor to match your DC Voltmeter's input
impedance:
The value of the resistor Rx
The peak value is about 1.414 times the RMS voltage. Rx has to drop this excess voltage so the
meter indication is accurate. If we know the input resistance of the meter, we can calculate the value of
Rx as follows.
10,000,000 X 1.414 = 14,140,000
Rx = 14,140,000 - 10,000,000 = 4,140,000 Ohms = 4.14 Meg Ohms
4.7 meg ohms is the value chosen in all circuits since digital Multimeters have input resistance of 11 meg
ohms.
RF probe and the DMM resistance. This ratio is 10/(10+4.7) or .68 which is less than 5%
from .707

3. RF PROBE
This RF Probe can use 1 - 400MHz

4. RF PROBE

5. RF PROBE

6. RF PROBE
Power Meter Reading (dBm) Power (Watts) RF Probe Reading (Volts)
-20 10u 0.002
-10 100u 0.013
-6 .25m 0.030
-3 .5m 0.051
0 1m 0.100
7 5m 0.270
8 6.4m 0.350
11 12.5m 0.560
14 25m 0.930
17 50m 1.350
20 100m 2.200
26 0.4 4.500
27 0.5 5.000
30 1.0 8.000
32 1.6 10.400
33 2.0 11.000
34 2.5 12.750
36 4.0 14.500
37 5.0 16.250

7. RF PROBE
38 6.4 18.900
39 8.0 20.300
40 10.0 22.500