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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VHF RF Power Meter

Here's a simple instrument for the homebrewer to measure RF power well through VHF. interests. The reference instrument incorporated a built-in analog meter and provision to connect an external DVM. It also utilized a conversion chart to relate
meter readings to RF power.

The DVM (Digital Volt Meter) also displays the correct polarity sign. There’s neither rocket science nor smoke and mirrors here folks: It’s all done simply with op amps and resistors! The schematic tells all.

In the process of designing these basic of power meter circuit functions, it occurred to me that other simple additions would add a lot to the utility of the instrument. Thus, the project grew “on the fly.” One of these extras is a gain-change option that includes an external OFFSET control for the analog display.

For more information about component parts list, power meter process design,  and calibration-operation, please download An Advanced VHF Wattmeter (in Pdf).

Source: RF power meter project by Wes Hayward, W7ZOI and Bob Larkin, K7PUA that appears in QST, June 2001.

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