I have displayed and detailed various 500KHz Antenna Loading Coils here that I have experimented with, not all though, it has been fun experimenting.
What I can report is that thin wire coils can be surprisingly efficient if the windings are close spaced and high Q by the introduction of Ferrite. Thin wire spaced coils of course have more DC Resistance and this shows up as a loss.
My Trifila coil reduced the resistance and showed promise but ultimately there was an efficiency problem, was it my imperfect hand twisting?
My first coils were link coupled, my 100mm dia. Helical Coil on a Racal former is a good performer and hard to beat. Long tapped coils need to have taps provided sometimes a long way up the coil, a nuisance if you decide you want to try just one more turn up or down.
In my Loft I had some left over PTFE insulated, Silver Plated wire, 2mm overall dia, I used it for my 40m Doublet, a rough measure out of the remainder indicated that it would just be sufficient for a large dia Loading Coil. A good advantage of a large coil is that the 50 ohm tapping point should lie somewhere between turn 1 and 2, this meant that I could use 2mm Enamelled Copper Wire for the first 2 turns and thus provide good solid tapping points at .0625 of a turn. Needing to be sure that I am scraping every possible dB out of my coil I wound what may be my final coil on a Paint Pot 210mm dia.
The DC resistance is very low, from RF field measurements, as accurate as I can make, the efficiency is better than the Racal set up. Final adjustment is with the same Ferrite Push Rod arrangement as used on all coils. The coil is wound with 1.5mm Enamelled Copper Wire just 22uH without the Ferrite. All coils are interchangeable and thread together, useful for tests and adjustments.
On air tests continue with this coil. I intend to add a weather proof cover to keep water out of the turns as soon as I can use up the paint from a larger pot which will do the job nicely.