The loading coils for electrically short MF and HF antennas that I have seen over the decades are all air cored. Ferrites have got better but air has not .Clearly a ferrite cored coil would be smaller but would it have lower losses? How would say a planar core compare to air? Has anyone tried this?
Ferrite has limitations such as:
- hysteresis losses
- eddy current losses
- saturation, limiting maximum current
- limited heat radiation ability, limiting maximum power
- nonlinearities, causing harmonic distortion
On the plus side for ferrite:
- higher permeability, reducing size
- also due to permeability, magnetic field is mostly contained within the ferrite, reducing problems like mutual inductance and other undesirable interactions
Although ferrites have indeed improved, they still have all the losses and limitations above. Air has losses so low they are insignificant for practical radio applications. It doesn't saturate, it's perfectly linear, you don't have to cool it because it has no losses to make it hot. There are resistive losses in the wire, and since the coil is bigger you need more wire. But that's usually easy to solve by using bigger wire.
Because air has no core losses, it will always have a higher Q than ferrite, all else equal.
Air will always have higher Q than ferrite. Air hasn't gotten better, because it can't really get any better. It's loss tangent is so close to 0 that it is typically just set to zero when calculating Q in most small(ish) coils.
That said, ferrite has definitely gotten better. With very careful core selection and design, Q might exceed 600 for values useful in RF design(1).
Calculations will yield insane Q numbers for air core coils, >10,000 in many cases, but practically the ceiling will be somewhere around 1,000 due to shunt capacitance, interaction with other circuit elements, etc. Edge wound coils might do slightly better than that, but you'll be stuck solidly in the low thousands. Regardless, physically large air core coils will always best a ferrite toroid when it comes to Q. The best they can ever hope for is to draw even.