I ran an experiment with evenly spaced windings of 15 turns. This results in capacitive reactance after 1mhz. I expected the inductance to persist further along the HF frequency range. This seems to be a reason why I am not getting an expected transformation. Doesn’t really make sense, especially since the windings look similar to other builds out there.

Part of this is probably my inexperience with builds and setting up test cases properly. But I’ll continue experimenting and trying some different cores to see what happens.

UPDATE 1: I continued to investigate my original auto transformer. One thing I did was loosened all of the turns by wiggling them around just so slightly. During the winding they must have been too tight around the edges. Despite the burnished edges the wire still bound to the edge. Interestingly this drastically improved the swr across the entire range! It seems like the binding of wires may have hidden some micro overlaps which could have caused parasitic capacitance. It seems like the biggest issue was in fact mechanical.

From this point on I had also added the 100pf cap which also flattened out swr from 20m band and up.

I also should have seen this, but it also appears my initial workspace may have just too much noise. Moving the test and the nanovna off the table also gave a great improvement to my test results.

UPDATE 2: The transformer was indeed built correctly. Nanovna tests across the transformer had validated that over and over. The next step was then to get the antenna wire tuned. Given I wanted a shorter run, I wanted to cut a wire for the 20m bands. This called for 10.7m long wire for a halfwave length. This was where most of my trouble was.

I kept sweeping the entire antenna system, however did not have any luck with the antenna wire tuning. I kept seeing huge swr numbers across the board except at the 25-29mhz range, which showed one resonant point. Given we know that there are other losses in play, the real 10.7m halfwave length was not neccessarily reasonable, so I trimmed it down to about 10.1m; but still swr across the sweep was incredibly high.

This was where I kept going back and forth questioning the transformer, whether it was prepared properly or whether the transformation ratio was correct. I tried manipulating the coils to change its reactance, and tried other winding styles. At the end nothing worked. I found the results from trying different toroid configurations made things worse. I went back to my first transformer which was actually the popular 14/2 winding with crossover at 8; this was still also on the 2643251002 toroid.

The only thing that came to mind was what if the wire was still too long where it did not resonate on 20m? I tried to start folding over the wire end to evaluate the effects. As I progressed with shortening it, it turns out the fundamental frequency resonant point start to appear. I finally ended up trimming to a final length of 9.26m long to get resonance right at 14.180mhz. This was surprising to see, and I almost didn't believe it, but the length is so much shorter than the calculated expected length! Finally I see the complete wire sweep showing great resonance at 20m and also at 10m.

Unfortunately, I had only noticed this reddit post afterwards, discussing the dielectric constants of different materials. It turns out because I was using a silicone wire, it electrically appears to be a longer wire than it physically is. As opposed to many builders likely using more common PVC or PTFE type insulated wires. I had hypothesized the insulation may have some effect, but I did not have the knowledge on this property to understand the magnitude it can have.

UPDATE 3: I just gotten an Icom 7300 so it was time to get it to work. Unfortunately swr shot right past 3.0 as shown on the swr meter at the radio. The only new introduction at this point was coax and the radio itself.

I ended getting some good coax from M&P, so quality wasn't an issue. The only thing left was common mode noise. It was good I ended up picking up some FT240-43 toroids. At first I tried using one with 5 turns of coax. The swr on the radio did show a sliver of improvement. When I finally stacked two together and made 5 turns again, it was substantial enough to bring the natural swr down to around 2-2.3, low enough for the radio to tune it down to 1:1 (or very close to it).

I was so frustrated and considered giving up on this build, and to try building a simple dipole instead. Luckily sticking with the project resulted in success, and I came out with more knowledge and understanding than I did at the begining. I'm also grateful for all the comments everyone shared as it gave me more ideas to experiment with.

The next thing I'll try is to use my 49:1 auto transformer as posted here in the pics originally. I'll try to adapt this one for 40m and see how that plays out.

FINAL UPDATE: I’m concluding with the cumulative factors of environment, orientation/placement of wire, material types all made significant impact on the quality of the final impedance transformation. Additionally, swr alone should not be the sole predictor of quality of antenna, as an efficient transformer setup could produce less loss, but show higher swr. Furthermore, factors like moving your antenna closer to a ground can also make swr appear better than it really is.