I just finished building and testing a coil tester that N4TMI wrote about in 73 Magazine.?
Here's a link to Alan Wolke's W2AEW YouTube channel:?
and
Here's a link to a webpage that is basically the N4TMI article reproduced with comments:?https://www.radio-collector.com/coil-resonance-tester/
Here's a photo of the board undergoing a quick test. I used a 47?H inductor and temporarily soldered a 150pF cap on. I've since changed the soldered on cap to machine pins so I can pull the cap to measure where the circuit resonates without it. I intentionally left the switch off and used machine pins per Alan's video to cut the size of the board down.
Test results
Value of the inductor per Mouser: 47?H
Value of the inductor using the the freq attained (1.84MHz) and the ARRL L/C/F slide rule: 49?H
Value of the inductor using a Keysight U1733C: 44.9?H (probably should have run it through a cal routine)
Calculated value of the inductor using the equations from the article: 49.878?H
Since I couldn't remove C1 I was not able to completely use the equations in the article.
Closeup of the board (it's pretty small). Note that you can also use an SMA connector. In fact, you can mount the SMA connector with the BNC. I right angle connector looking out the side of the board or mounted on the bottom will give you room to connect to it. You can also leave off the power jack. Just add pins or wire directly to the circuit board using the provided holes. You don't need a spectrum analyzer to measure frequency. A good counter will do the job as well as a calibrated oscilloscope.?
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I've included a link to the Eagle files and a PDF of the schematic and board layout. SMD's are 0806 size. If you know how to use Eagle you can swap the parts for through hole. That will cause you to have to reroute the board and possibly increase the size of the board. The J310 MOSFET's were on Mouser. N4TMI used a different MOSFET and those can be found on eBay. I chose to stick with the J310's that Alan used in his video and modify the schematic to follow his (changed one resistor value).
Link to the board on Osh Park's site (no benefit to me. All cost is in what the charge):?https://oshpark.com/projects/gx7Ixj44
Link to GitHub where you can download the Eagle Files: https://github.com/Mike6158/N4TMI-Coil-Tester
It's pretty simple to determine the value of an inductor using this circuit.
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73
NE5U
Mike
