Here's how I started with the Elsie program. Note that to get a 50 ohm match on both ends it needs an impractically low value for the coils (.064 uH).
But Elsie lets you specify the coil value. So I then I went with 1 uH. But with this value you don't get 50 ohms at either end. You need a matching network. Elsie provides this too!
I asked Elsie to match my BP filter to 50 ohms. It provided several options to do this -- I went with a simple capacitive impedance divider. But alas, I was now bumping up against the 7 limit of the free version of Elsie so I had to reduce the number of LC elements from 4 to 3. Bummer.
With 3 LC tuned circuits and matched to 50 ohms the plot looks OK. But I would have preferred 4 LC circuits.
The rftools website created a BP filter for me with 4 LC elements, and matched to 50 ohms. Very useful. https://rf-tools.com/lc-filter/
But here's my problem: With both the filter designed by Elsie and the one designed by rftools, I found that the filter passband was too low. It was in the 12 - 13 MHz range. I found that by removing 3 turns from the 1 uH coils I could move the passband up to the desired range. But why the discrepancy? I was measuring the coils and the caps with an AADE meter. I was testing the passband both with a NanoVNA and with a combination of an HP8640B sig gen and a Rigol oscilloscope (with the filter terminated into a 50 ohm resistor). Any suggestions on why these filters should have passbands lower than predicted would be appreciated.
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