While experimenting with end-fed antennas I've made two 1:49 transformers.
The first one has an air core, as shown in the article by John, M0UKD. This transformer doesn't even work as a transformer until you add a capacitor in parallel to the load. The secondary winding of the transformer and the capacitor should form an LC-circuit resonant on the desired frequency. The transformer works very well, but the bandwidth is narrow.
For the second one, I used FT240-43 ferrite core, as shown in the article by Rudy, N6DOZ. This transformer is wideband and works on 3-30 MHz. Although when loaded to 2450 Ohm non-inductive load (metal-oxide or metal-film resistor) instead of 50 Ohm we see a slightly inductive load:
From what I know this is because the large secondary winding works as an inductance in series with the load. This inductance can be compensated by placing a capacitor in parallel to the primary winding of the transformer. Here is the effect of placing a 57 pF capacitor (47 pF || 10 pF):
The part that I don't quite understand is why the transformers work so differently depending on the core material? Why the one that uses a ferrite core is so wideband while the first one doesn't even work as a transformer without an extra capacitor?