MATERTAL 61 Moderate temperature stability and high Q for frequencies 0.2 MHz to 15 MHz. Also commonly used for wide band transformers up to 200 MHz. All 4:1 Guanella baluns I build are 2 cores. Individually wound.
Some use PVC tubing for baluns but the problem is if you need to service the balun you cannot. To circumvent this, we use serviceable PVC weatherproof boxes on all baluns.
Major General and Ohio Adjutant General Ohio National Guard. Held callsigns 8ZO and W8GZ. Famous for promoting and developing the Windom antenna that his good friend John Kraus named Windom after him.
All antennas and baluns are tested with an analyzer. No antenna / balun is sold without being up on my 32- foot test pole and run thought the bands. Some require fine tuning, and all are tested with extend transmit times at legal limit.