At Radio-Stuff.com we spend a lot of time reading publication, discussing on air and also watching YouTube to see how other radio amateurs are contributing to our hobby.
Being an owner of a uSDX DL2MAN/PE1NNZ transceiver I decided to Subscribe to DL2MAN channel. Being a manufacturer of EFHW products, I found the video below giving insight into the efficiency and use of different toroids when making EFHW antennas.
If you follow the link below you will see some great content on the efficiency (or not) of some of the cores with different EFHW transformers designs. Below is also also video by MM0OPX again really informative.
I hope you enjoy these video's as much as I did. What I especially liked was the conclusions of core size, windings and frequency. Also the presentation of the results is excellent.