Dayton Hamvention 2017 BUS – QRPme No Solder RF Probe Kit W1REX
52 happy hams got to smash a world record for a QRP kit buildathon on a BUS coming back from Dayton with the Arrow Communications club. I was the only VE3 (actually the chosen ONE – hihi) on the ride and I volunteered to help out with the build. No soldering required and no parts lost.
This is an excellent club project night kit, scouts kit or a beginners kit.
|Often we wish to examine an RF voltage to see if a circuit is “alive,” and perhaps to adjust it. An RF Probe is an important tool to have for electronics and radio troubleshooting. You can measure RF voltage in your solid state circuits, and trace RF through your new designs or malfunctioning circuits. They allow you to probe into different parts of circuits to find where you do or do not have an HF/VHF/UHF signal. With this information, you can find where your signal disappears. In turn, this suggests where there is a bad component or solder joint in the device you are troubleshooting. Do not exceed 50V and will not be useful for Tube based circuits due to High Voltage. You will be able to detect RF voltages in the range of .25v to 50v|
Building the kit is easy once you have figured out the colour code. Pay special attention to the polarity of the diode. I would recommend to actually solder it once tested and install in a small pill bottle with a thick wire for a probe element. This is a perfect tool for troubleshooting your next qrp kit from QRPme.com
Thanks to Joyce, Jim WD8RWI and Shawn KD8YMV for acting as superstars
52 happy builders 10 minutes later – really its that simple
So, how do we use this thing? As per N5ESE
RF Probe is to connected the banana-plug end to the +/- jacks of your DC Voltmeter: (not AC).
To use the RF Probe for signal tracing in a malfunctioning RF circuit or a homebrew circuit, connect the alligator clip to a convenient “ground” or “common” point in your circuit. Often this is the chassis. Most of the time, you’ll be probing at the base/gate, emitter/source, or collector/drain of a transistor, one either side of a coupling capacitor or transformer, or at the input or output of an IC. Because the circuit’s RF must overcome the diode’s barrier potential (of 0.25V, for our 1N34A), voltages much less than that won’t read at all, and voltages less than about a volt won’t read very accurately. Typically, RF and post-mixer-amps in receivers don’t have enough RF voltage, unless you inject a very strong signal at the input.
I recently used my RF probe to troubleshoot my dead TenTec Scout, which had suddenly quit transmitting in mid-QSO. I connected the rig to a dummy load, then keyed it while probing. Using the probe, I was able to follow a steadily increasing RF signal through the transmit chain, from the oscillator through the transmit mixer, to the pre-driver, and the driver. The actual voltage measurements weren’t important, just that they were increasing from stage to stage where expected. Then, (whoops!) the driver’s base circuit had 6 Volts, but the collector circuit only had only 0.1 Volts! The driver transistors had gone south!
You can also use the RF probe to measure RF power with reasonable accuracy, up to about 50 watts in a 50-ohm circuit. By 50-ohm circuit, I mean a 50-ohm antenna system at 1:1 SWR (higher SWRs are not 50 ohms), or a 50-ohm dummy load. Assuming the resistor in your RF probe is sized to match your DC Voltmeter’s input impedance (as explained above), you will get quite reasonably accurate measurements using the following formula:
For example, I want to measure the power out of my TenTec 1340 40-Meter QRP transceiver. I place it on a 50-ohm dummy load, and key down. I generally use a BNC-Tee adapter to gain access to the output line, but I could as easily pop the cover off. Using the RF probe (alligator clip to chassis ground), I measure 12.2 Volts (DC) (and the same RF RMS Volts). Plugging this into the formula above I have PWR= (12.2 + 0.25) * (12.2 + 0.25) / 50 = 3.1 Watts. The rated power for this rig is 3 Watts, so I’ve verified everything is hunky-dorey.
We’ve added the potential barrier to the measured voltage above, but that little trick doesn’t work so well when you get down around a volt, and for voltages less than about a volt, the measurement accuracy suffers greatly. Also, the diode’s response is severely non-linear below the barrier potential, and will generally read much less than expected in circuits where the RF voltage is less than 1/4 volt. So if you see tiny readings in circuits where it’s normal to have voltages less than 1/4 volt RF, don’t get too spun-up about the low readings… it may mean everything is normal. My rule of thumb for guessing at this is as follows: For collector/drain circuits in oscillators or transmit-chain amplifiers in key-down, expect RF Voltages about 20-50% of the applied DC (supply) voltage. This depends on the circuitry, of course, but it’s a reasonable gesstimate. Base/gate and emitter/source circuits will generally be much less, maybe 5-10%. Circuit impedance will affect this too.
QRPme RF Probe: http://qrpme.com/?p=product&id=UKBB
Youtube Video: UK RF Probe Buildathon
N5ESE: http://n5ese.com/rfprobe1.htm and