I had a high permeability ferrite rod and an old B&W dipole center insulator in my junk box so I decided to construct a choke type balun for QRP use. I wrapped as many turns of RG-174 coax as I could around the ferrite rod and coated the whole mess with hot glue. The idea was to created as much inductance as possible at HF. This would hopefully cause a high impedance to common mode currents that might otherwise want to flow onto the coax shield in the event of an antenna unbalance. I'll admit that not much critical design effort was expending on this effort. I just used what I had and stuck it all together. In the end it seems to work pretty well, at least in my mind ;-) The only design flaw may be that the high dielectric properties of the hot glue may increase the turn to turn capacitance of the windings which may in turn decrease the usefulness of this balun on the higher end of the HF spectrum. I have not tested this however.
I have posted my QRP Balun just to give ideas and not necessarily to have anyone try to reproduce it exactly. Check out the rest of the photos below to see how it evolved:
Balun itself
Original center insulator assembly cut open, inside bottom
Original center insulator assembly cut open, inside top
Balun installed inside
Balun installed inside, again
Assembly put back together, bottom view
Assembly put back together, top view
Final assembly after cammo paint, top view
Final assembly after cammo paint, bottom view