In most cases, transmission line transformers use magnetic cores to improve their low frequency capabilities. Generally ferrite cores are the popular choices.
Ferrites can achieve much higher permeabilities than their counterparts, powdered-irons.
Also toroids are more popular than rod cores because of their closed magnetic path and hence more inductance for the same number of turns. Another consideration is the efficiency of the transformer as a function of the permeability of the ferrite core. In other words, the loss is mainly a dielectric loss. As noted in the basic building block, the voltage drop can be zero as in the delay line or the full input voltage as in the bootstrap and phase inverter connections. Also, the higher the impedance levels, the higher the voltage drops.