I was reading a little more in the ‘who invented the Transistor’ debate recently. It was most likely a case of parallel paths.
Likewise my Loading Coil for 600m experiments. I had seen the Air Cored Coil, the Ferrite Cored Coil, the Variometer and the variable Capacitance ATU types.
I consider the Variometer may be a lossy device, in anything other than ‘add all Inductance’ it must be lossy. When it is in a subtract situation, bucking some of the primary Inductance it must be worse.
I tried large air cored coils, these seemed less efficient maybe due to wire resistance or capacitance to ground. Smaller coils with less wire seemed better, I can use Ferrite as the QRPp levels are unlikely to cause saturatation.
With a Ferrite Cored Coil the Inductance can be varied to tune the value precisely. I designed and built a system where the main part of the Coil is air cored (on a PVC former) and a Ferrite Rod is inserted in the end for trimming the Inductance value. I even called it a Pump System due to its resemblance to a Bicycle Pump. It also resembles and uses the same construction as a small Spotter Telescope which I built many years ago.
Then someone posted a reply with reference to Litz Wire, the write up by Mitchell Lee has loading coil designs. One has an Air Core Coil with a separate Ferrite coil for adjustment this he calls a Push Rod Tuner.
Amazing, no need to draw mine, just make the Ferrite Coil the same diameter as the Main Coil and tip it up on top of the main one. Oh and mine uses a 5 turn link coupling not a tapped
Mine is out in the garden on test it will come in for waterproofing and I will take photos later.