This document discusses core material selection for baluns and transformers. It recommends using lower permeability cores like 73 material for receiving applications up to 30 MHz due to their lower turns count. For high power applications, even lower permeability cores like 65 or 61 material are recommended due to their higher Curie temperatures and lower losses. The core's internal length parallel to the conductor has the largest effect on impedance, while diameter and thickness have smaller effects. Minimizing conductor length outside the core window improves performance. Binocular and stacked cores are preferred to minimize external conductor length. Measurements show phase inversion transformers and Beverage matching transformers perform well with the recommended core materials and designs.