I here describe a wideband HF RF choke design in pictues. The choke covers most of HF from 160m to 10m and will give a resistive part of the impedance R ( R+jX) that should be high enough to not cause overheating of the ferrites. It will also give a significant reacive load, but the resistive load cannot be cancelled by capacitive effects easily. The 9 + 1 core design above will give the resistance shown on the blue trace in the image to the right above. The other traces (red, green, pink) shows 5, 4, 3 turn designs that was first tried. Note that the fewer # of cores in the choke, the higher up in frequency the resistance peak occurs. Note the single external series core with two turns. That single core takes care of extending the resistance up in frequency above the lower peak (the peak that goes out of the graticule to the upper left). The material is the Fair-Rite 2643167851 material.
Today I did some further measurements on the same Fair-Rite material as the other post (Fair-Rite 2643167851). I did the measurements with a calibrated open, short, 50 ohm load S11 measurement. The green trace is the resistive part of the impedance, the red trace is the reactive part of the impedance and the blue trace is the unloaded Q (ratio stored energy in the magnetic field in the core and windings to lost energy in the effective series resitance). As expected, the Q is one when the green and red traces cross each other. This material is not useable as a regular coil for energy storage (high Q, filters and such) over approx 15 Mhz. Below 15 MHz the Q can be quite high, however. This core is probably very good as a dampening material for RFI applications as GM3SEK has indicated. For High power operation when coax common mode current causes problems I think this core may be good. The reason is that the resistive component (real component) of the impedance is dominant. This means that even if a capacitive reactance cancels the inductive reactance, the resistive part of the impedance is still always present. This can be seen from the green trace above. Center of the plot is approx 300Mc/sek. It also rises with QRG up around 450 Mc/sec. At 2 meters and 70 centimeters wavelength it looks like even one turn on a coax (in a low impedance point of the coax!) this choke will do some good. At HF, with more turns it is possible to achieve 1000 ohms over a fairly large range.