RF Voltage Probe

I like to build equipment, which includes transmitters, receivers and transverters. I also repair equipment from time to time. I found myself in need of a simple RF Voltage Probe. They can be very useful to constructors. They can trace RF through a circuit. Read the Power in parts of a circuit or at the output. They can be used to find faults. Looking through some of my Radio related books. I found a few descriptions of projects others have built.

This is the circuit I decided to go with. It is basically a voltage sampler. It reads the Peak AC voltage and converts it to DC.

From left to right we have the probe which feeds to a 10 nF capacitor.  The job of this capacitor is to block DC voltage and to charge up to the Peak voltage of the applied RF AC signal. Your probe could see DC voltage and not the AC RF voltage. so its vital to include it.

Next we have the 1N60P which is a germanium diode. There are lots of Germanium diodes around and you could substitute another in the place of the 1N60P. I like to performance of the 1N60P and keep a small supply for projects like this and others. Like diode detectors or RF sensing.

The next component is a 4.7 MegaOhm resistor. This probe will create a DC voltage that will be read by a digital multi meter  (DMM ) in DC reading mode. DMM’s can read AC voltages but are normally limited to a few tens or at best a couple of hundred Hertz and wont give useful readings for radio. The input impedance of my DMM is 11 MegaOhm, many commercial DMM’s are either 10 MOhm or 11 MOhm but some can be as low as 1 MOhm. The resistor in this circuit creates a potential divider with the DMM. Which will act to give us an RMS reading. The Peak to Peak reading is 1.414 times the RMS. We usually work with the RMS value in respect to AC.

If your meter has a different impedance, here are some common values :-

4.7 MOhm for 11 MOhm DMM

4.3 MOhm for 10 MOhm DMM

430 KOhm for 1 MOhm DMM

Next we have another 10 nF capacitor, its job is to smooth out ripples when we try to take readings at higher frequencies. Without it, we would be limited to between 25 Mhz and 30 Mhz. Including it, means we can push that higher.

Some notes about the components and construction:-

The 1N60P has a voltage drop of around 0.24 volts. Its why we use Germanium diodes for RF detectors, Crystal radios etc. We will need to take the 0.24 volts in to account.

I used 630 Volt capacitor’s. At QRP voltage levels ( less than 10 watts ), the probe should only see 25 volts or under but if you might use it above 10 watts, you should leave yourself extra room. I had a number of 630 Volt 10 nF capacitors. so they were to hand,

I have used RG58 low loss shielded coax to connect the RF probe to the DMM. This keeps us from receiving stray RF on connecting wires and helps when we want to calculate power in watts from a transmitter.

How do we take readings :-

Once we connect this probe to a DMM, we put it on DC volts and take a reading. Which will give us the AC RMS of the RF we have sampled.

How do we calculate power ? We need to connect our transmitter to a 50 ohm dummy load and take a reading. Once we have our reading across the dummy load, we need to add 0.24 volts to it. This replaces the voltage lost across the Diode.  e.g :-

We take a reading of 15 Volts across the dummy load and add 0.24 Volts. Giving us 15.24 Volts.

Next we need to use power law to calculate the power. We know a voltage and we are using a 50 Ohm dummy load. So we can use :-our voltage is 15.24, which we square, giving us 232.2576. Which we divide by our 50 Ohm resistance. Giving us, 4.64 Watts !

Constructing my probe :-

I had a 12 Volt tester used for automobiles in my garage. It has a light inside and a ground connector. Once its connected to the car ground. The bulb will light when it comes in contact with 12V. Its big enough to fit the circuit inside. It is not ideal for two reasons.

The first is the way the probe connects to the circuit. This is achieved with the Spring on the right. Which could be loose or not make contact. As you can see. I have soldered it to a piece of copper strip board. The spring on the left is to force the board against the probe and overcome that problem.

The second issue is that the body of the 12 V tester is plastic and RF can pass through it and give us stray readings. I will over come this with aluminium tape once it is completed. You could also put it into a metal container. In reality if you are only using this occasionally, it should be fine.

The above image shows the RG58 and ground wire soldered to the circuit and being placed in the Probe.

Above you can see the circuit inside. With the end screwed on, the circuit is pressed against the probe.

My probe has no insulation, so I have added heat shrink. This stops me touching the probe and affecting readings or getting shocked. The heat shrink I used is capable of withstanding 1 KVolt. so its good for anything I will use it on

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