Classic RF Probe using 1N34A Diode

One of the handiest accessories you can have around the shack is RF Probe. It can be used to measure RF voltage (and power), trace RF signals in a new design, and troubleshoot malfunctioning RF circuits. This RF probe is easy to build.


Complete Classic RF Probe Schematic

For best probe accuracy, size the resistor to match your DC Voltmeter's input impedance:
R = 4.7 Meg for Zin = 11-Meg;
R = 4.3 Meg for Zin = 10-Meg;
R = 430 K for Zin = 1-Meg;

Source: Classic RF Probe

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BAS70 RF Probe Circuit

This RF probe is useful for small RF power levels up through the microwave frequency bands, and simply connects to your DVM (Digital Volt Meter). The voltage shown will not be accurate, since this is a rectifier probe (diode probe), but the measurements are good enough for you to be able to determine where the RF stops, or if a stage is not giving the gain you think it should.

RF Probe Schematic

RF Probe Notes:

1. Maximal rated power is defined by 50 Ohm resistor (use more resistors connected in parallel) and maximal reverse voltage of diode. (for BAS70 70V)
2. Compute output power from measured voltage:

Pout=(( Vmeasured + 0.35 V ) / 2^0.5 )^2 / 50

where:
Vmeasured is RF probe output voltage.
0.35 V is diode forward voltage (actual for BAS70)
/ 2^0.5  is peak to effective voltage
50 is load impedance

Source: RF Detector / RF Probe

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Switchable RF Probe-Watt Meter

Here is one of the most trusted homemade test instruments to have, once you get your FM transmitter up and running. For sustained loads of up to 2 watts. This simple little device is capable of finding your FM transmitters: unloaded RMS output voltage, oaded RMS output voltage, output impedance, and output wattage.

Switchable RF Probe/Watt Meter Construction

First, print out the PCB template to the left. Send the picture to a graphics program, such as Paint, and squeeze and/or stretch to get a printout of the said dimensions. Once that is done, drill your hole for a SPST Sub-mini switch (Radio Shack part # 275-645A). Then mimmick the same routing as to the template. Use a single sided PCB. If you only have a double-sided PCB, etch away the backside.

Once you have etched your PCB, then go ahead and look at the picture below for soldering and wiring all of the components to the PCB.

Once you have completed with all the soldering/wiring of the components to the PCB. You are now ready to understand how this little beauty works ! Do take a look at the picture to the right, it shows my completed RF Probe/Watt Meter. Yours should look similar to that. Check to make sure all copper routings show continuity and that all soldering joints are good. Also make sure your germanium diode is oriented correctly into the circuitry. That is, the cathode (negative) side goes toward the output of the meter.

Begin by hooking up your transmitter, RF Probe/Watt Meter and DVM/Analog Meter as laid out in the picture below.

A DVM (Digital Volt Meter) will always give detailed readings to the nearest hundredths, but it usually isn't that reliable in the VHF arena. The analog meter will have a more reliable measurement...although the reading is taken directly upon where the needle rests on the scale therefore giving a less detailed reading. A DVM seems to always give higher readings then the analog meter, when doing tests with our homemade device. If you have the benefit of having both meters. I would strongly suggest making readings with the analog meter.

Source: The Switchable RF Probe/Watt Meter Project

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RF Test Probe

Like the Classic RF Probe, this RF test probe is used in conjunction with a high-impedance-input Voltmeter or Digital Voltmeter Meter (DVM). What makes this probe unique is?

What makes this probe unique is that it's built inside the shell of a regular ol' ballpoint pen. Besides being conveniently compact, the unit sports a needle-probe suitable for use in probing surface-mount circuits, and good overall shielding. The pen cap protects the needle probe when not in use.

When measuring sinusoidal signals, it should provide RMS-corrected readings, using a 10 or 11-Meg input impedance VTVM or DVM. With a 1-Meg DVM, it reads 25% of the sinusoidal RMS voltage. Reasonable accuracy (+/- 10%) can be expected over the HF/VHF range (2-150 MHz), although this hasn't been verified. When used to measure non-sinusoidal signals, the accuracy will be unknown, but it still affords good relative measurements, and most of the time, that's all that's required. It makes an excellent, compact, and portable accessory for troubleshooting or homebrewing QRP equipment with peak voltages less than 50 Volts (most solid-state equipment)

RF Test Probe Construction
The figure below shows the parts required to build the Ballpoint RF Probe. Click on the image to open an larger, annotated image with parts labled, and construction notes. Pick a ballpoint pen with a non-metalized plastic body, and plenty of room inside. The Papermate Flexgrip model I used had an inside diameter a little over 1/4-inch. We'll use an itty-bitty scrap of double-sided printed-circuit-board to mount the electronic components. Trim the PC board to about 2-12" long and 3/16" wide; don't make it too wide, or it won't fit inside the ballpoint pen. Notch or file a little out of the middle of the pc board, so the 1N34A diode will fit easily inside the pen body. then, on one side only, groove in two places, so as to create 3 lands on the "top" side of the board.

In addition to the rf test probe parts shown, you'll need a 2-1/2" piece of heat-shrinkable tubing to cover the electronic assembly (although electrical tape would do instead), and about a foot of 1/4"-wide adhesive-backed copper tape, commonly available in rolls of 200-300 inches at large hobby stores. Although a chip capacitor is shown in the photo, a very small disc capacitor will do as well.

Source: N5ESE's Ballpoint RF Probe

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