Re: ferrite core
Yury Kuchura EU1KY
The 28B0562-000 core performs not much better than Micrometals material 61 cores on UHF, but much worse on low frequencies. The plot in the spec is for one turn Z modulo, so it is unclear where is the border between prevailing inductance and prevailing resistance in the impedance. Material 61 spec shows the graph of the complex permeability which is more convenient. If you recalculate it to modulo, you will see a peak at ~30 MHz, though it is still very effective on both 500 kHz and 1 GHz.
After many experiments I've found the RF choke design to be the optimal in the entire frequency range. It utilizes a tiny Amidon BN61-2402 two-hole core, and the line wound on it is just a twisted pair made of dia. 0.2mm enameled wire, Z0 is about ~60 Ohm. The length of the line is short enough to avoid parasitic resonances, though the impedance in the entire frequency range is very satisfying. The line is wound three turns through one hole, and then it continues three turns in another one, see the picture attached. This core and winding layout has once won a Wideband Balun Design contest in some US university years ago (sorry, I've lost the link to the PDF file describing it), and it is really worth this high mark. Don't worry much about the line impedance, it will be fully compensated by later OSL calibration.