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Measuring Coil Inductance with an MFJ Antenna Analyzer

With my limited resources I’m attempting to design, test, and build a minimalist QRSS transmitter. While working on the output filter, I’ve done a lot of reading and thinking and have determined that a pi filter (a 3 pole Chebyshev filter) will give me the low-pass characteristics I’m looking for to eliminate harmonics of the QRPP output.


This is the filter (with values and AADE-generated gain plot) I’m shooting for. It has about a 12dB reduction by the time it gets to 14m. My major goal is suppressing harmonics, but I thought I’d be polite to the 20m crew. The filter uses standard 1nF capacitors and an inductor of ~.44uH which according to this chart I can get by 12 turns around a small yellow T37-6 toroid. Although I’d like to use an air coil because of the cost savings, I’ll admit that I understand why toroids are used. That’s the filter, as modeled by AADE, software gracefully recommended to me today by David (VK2/VK6DI).

David also suggested that I not rely on standardized values, but rather measure inductance myself. While an inductance meter is out of my budget (of about $10), I was able to check-out W4DFU’s MFJ antenna analyzer, which can measure inductance. However, my readings were not as expected. In fact, with a total short (center connector directly to ground) it read a very high inductance measurement. Knowing that series inductance can be added to get total inductance, I suspected that this could still be useful. I used a T37-6 toroid I had on hand and wound it from 0-25 times, checking the inductance reading after every turn. After plotting and curve fitting, I corrected each value by subtracting the y-intercept and compared these points with those discussed in this chart and, whew! They’re a good match. To measure inductance with this meter, I have to measure inductance with the straight wire, then subtract this value from the final measurement.


All right, back to work. Dental school homework is due tomorrow [rolls eyes]

update: this is the antenna analyzer I used:


UPDATE 2 I built the proposed filter with wire randomly coiled around an unknown toroid (oh the challenge!), added a 50 ohm (51 ohm, close enough!) resistor as a dummy load, and hooked it up to the SWR analyzer. I noticed a swr minimum around 8mhz… As I unwound loop by loop, I got higher and higher… 9.15, 9.64, and finally BOOM! 10.215mhz swr 1.0. 10.140mhz swr was 1.1. I assume that a low SWR means that the filter passes maximum signal of that frequency into the dummy load, so by “tuning” this filter into a dummy load to minimize SWR by adjusting the coil at a fixed frequency, I maximized gain at that desired frequency. Here’s a photo of the completed circuit. The capacitors are “102”, 0.001uF and the toroid is unknown, but 9 turns seems best.


About the author

Scott W Harden

Scott Harden has had a lifelong passion for computer programming and electrical engineering, and recently has become interested in its relationship with biomolecular sciences. He has run a personal website since he was 15, which has changed names from HardenTechnologies.com, to KnightHacker.com, to ScottIsHot.com, to its current SWHarden.com. Scott has been in college for 10 years, with 3 more years to go. He has an AA in Biology (Valencia College), BS in Cell Biology (Union University), MS in Molecular Biology and Microbiology (University of Central Florida), and is currently in a combined DMD (doctor of dental medicine) / PhD (neuroscience) program through the collaboration of the College of Dentistry and College of Medicine (Interdisciplinary Program in Biomedical Science, IDP) at the University of Florida in Gainesville, Florida. In his spare time Scott builds small electrical devices (with an emphasis on radio frequency) and enjoys writing cross-platform open-source software.

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