If anyone is curious here are the characteristics of the Directional Coupler I just posted. The top line is the dBm on the forward port and the bottom line is the dBm on the REFL port for a 0dBm input signal. The markers for the common radio bands.
Over the past few days I built 5 different Directional couplers. Each one uses either a new design even tierly or a new winding ratio. After all the testing I've finally figured out the ideal design for the frequency range I'm targeting. Time to build the final prototype.
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It took me 3 hours but I rolled two perfect 126 turn Toroid inductors. These will be two halves of the two transformers in a Tandem Match Circuit (Directional Coupler). Should have infinitesimal insertion loss
Decided to build another tandem match circuit based directional Coupler. This time I used a 21:1 winding ratio instead of the last one at 8:1.
It seems to have reduced unwanted coupling at lower frequencies. But more important it decreased insertion loss from about 1.15dB to only 0.03dB at 2 MHz and 0.4dB around 50MHz.
I think I prefer this particular winding ratio for my design.
So my Directional Coupler performed fairly well. Here is the output from my spectrum analyzer. Basically one trace is for the forward port when the antenna is matched, the other is when the antenna is an open circuit. Then the other two traces are the same condition on the reflected port.
As you can see all the powers show a nice flat -20db coupling. Only exception is the one cyan trace that shows the reflected port when the antenna is matched. Since there are no reflections as expected the response is much less. It is also not linear which is to be expected.
Pretty similar results to my last attempt. Though going with fewer windings this time (8 instead of 12) seems to have results in a slight stronger coupling (-20 dBM rather than the -23 dBM I got before).
All in all I'd say its a success.
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Was running my new project in demo mode where it just continually increases the phase and magnitude of a load's complex value then plots it on a Smith chart. I was pleasantly surprised when I noticed it produced a Celtic knot pattern.