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.