{"id":103672,"date":"2021-04-26T19:40:06","date_gmt":"2021-04-26T19:40:06","guid":{"rendered":"https:\/\/www.snapwhole.com\/index.php\/building-the-heathkit-cr-1-crystal-radio-2\/"},"modified":"2023-01-17T01:05:10","modified_gmt":"2023-01-17T01:05:10","slug":"n5ese-rf-probe-construction","status":"publish","type":"page","link":"https:\/\/www.snapwhole.com\/index.php\/n5ese-rf-probe-construction\/","title":{"rendered":"N5ESE RF Probe Construction"},"content":{"rendered":"\n

Perfect for measuring the output of QRP oscillators<\/strong><\/p>\n\n\n\n

Return To Ham Radio Projects<\/a><\/d3><\/center>

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N5ESE RF Probe Construction<\/mark><\/p>\n\n\n\n

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\"N5ESE<\/a>
N5ESE RF Probe<\/figcaption><\/figure><\/div><\/div>\n\n\n\n
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I\u2019ve been building various oscillators recently and I needed a way to measure their RF output<\/p>\n\n\n\n

Luckily, a nice circuit for this is available on the Net, and it includes background on the circuit, and a build article<\/p>\n\n\n\n

N5ESE\u2019s website has an RF Probe Circuit and build article – N5ese RF Probe Page
The circuit is basic and the probe’s read the RF AC Voltage, the circuit converts the AC RF to DC, and the multimeter displays the DC voltage, allowing for a conversion to Watts.<\/p>\n\n\n\n

Although the circuit is simple, it is accurate, the limit of the circuit is the IN34A used to convert the RF to DC, any reading on the multimeter under 1\/4 volt should be treated with caution, above 1\/4 volt should be accurate
Perfect for measuring the output of QRP oscillators
RF Probe Circuit<\/p>\n\n\n\n

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RF Probe Schematic<\/figcaption><\/figure>\n\n\n\n

I decided to build the circuit in an Altoids Tin, with jacks for the probe and the multimeter connections<\/p>\n\n\n\n

I chose the left side of the Tin for the probe connection, and mounted a BNC in the left end<\/p>\n\n\n\n

I chose a probe cable to break out the BNC connector to 2 cables, 1 for the probe(center) and 1 for ground(shield)<\/p>\n\n\n\n

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\"N5ESE<\/a>
N5ESE RF Probe<\/figcaption><\/figure><\/div><\/div>\n\n\n\n
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A shorter cable is better, but a stock 1 ft cable works ok<\/p>\n\n\n\n

So the right side was used for the multimeter connection<\/p>\n\n\n\n

I chose to install banana jacks for the multimeter connection<\/p>\n\n\n\n

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\"N5ESE<\/a>
N5ESE RF Probe – Banana Jacks to connect cables<\/figcaption><\/figure><\/div><\/div>\n\n\n\n
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This allowed me to use a cable with banana plugs on both ends to connect to the multimeter<\/p>\n\n\n\n

I though this would allow more versatility on hookup, and less wire mess when not in use<\/p>\n\n\n\n

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N5ESE Circuit Layout<\/figcaption><\/figure><\/div><\/div>\n\n\n\n
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To simplify construction, I installed 2 small pads of circuit board to mount the components in the middle of the Altoids Tin<\/p>\n\n\n\n

This allowed me to mount the 1N31 diode between the pads, and then connect each pad corresponding end points, greatly simplifying construction<\/p>\n\n\n\n

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N5ESE Circuit Layout<\/figcaption><\/figure>\n<\/div>\n\n\n\n
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So how does it work?<\/strong><\/mark><\/p>\n\n\n\n

Exactly as designed, attach the probe to a an oscillator and the DC equivalent voltage is displayed on the Multimeter<\/p>\n\n\n\n

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N5ESE RF Probe – Working<\/figcaption><\/figure><\/div><\/div>\n\n\n\n
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The Multimeter reading is then converted to Watts using this formula<\/p>\n\n\n\n

Watts = (V_reading + 0.25)^2 \/ R_load<\/p>\n\n\n\n

Example:<\/p>\n\n\n\n

Voltage Reading = 1.31<\/p>\n\n\n\n

(1.31+.25)*(1.31+.25)\/50 = Watts<\/p>\n\n\n\n

In Steps
(1.31+.25)=1.56
1.56*1.56 = 2.4336
2.5336 \/ 50 = .048672
Rounding = .05 Watts
or
50 Milliwatts<\/p>\n\n\n\n

This turned out to be a great tool!
I can build two oscillators and measure the output to compare performance<\/p>\n\n\n\n

I can also build small amplifiers and measure the output to check if the circuit matches the designed output power levels<\/p>\n\n\n\n

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Quick Video of the N5ESE RF Probe in Service<\/p>\n\n\n\n

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