Assuming an antenna system where a transmitter feeds an ideal coaxial cable which is then connected to an ideal ladder line without a balun:
It is often said that common-mode currents appear in the transition between a coaxial cable and a ladder line simply because the coaxial cable is unbalanced while the ladder line is balanced. My question is related to whether this mechanism is related to how the load/antenna looks like. Let us say that the two wires of the ladder line end up in a load without reference to ground (probably impossible to achieve with a real antenna). This can for example be achieved with a simple ideal resistor (dummy load) in the end of the line (and hanging the ladder line in completely free air). Let us also for the sake of the example assume that the characteristic impedance of the ladder line is 50 ohm such as in the coaxial cable - in order to avoid standing differential waves. Could there in this case be common mode currents appearing on the coaxial shield?
Then what if the characteristic impedance of the ladder line is not the same as the coaxial cable and we therefore have (differential) reflections at the transition - does it make a difference for the common mode?
And thirdly, what if the characteristic impedances of the ladder line and the coax are the same, but the resistive load at the end of the cable does not match the characteristic impedance of the cables so that we get (differential) reflections at the load - does it make a difference for the common mode?