I do not believe in magic.
Little cores with a winding with one end connected to a wire and the other end to ??? can’t provide a low loss match to the wire. Your coax braid might help out but that would be magic if completed a low loss match.
This is basic stuff fellas. NO FREE LUNCH.
The original end fed antennas were used on Zeppelins and were fed with open wire line. The mismatch loss was acceptable for the application and it was less that the modern offerings…
I’ve used end fed wires on HF for 50 years, when I could not erect something else. A fair average loss figure for the modern end fed is 3 dB. Yes I have a commercial one, just for evaluation. Not used on QRP, I can’t afford the losses.
You can get back half of the losses by feeding it with 300 ohm ribbon 2 m from the end. And no, a 4:1 balun and coax is not the same.
Just because someone sells stuff with a label on it does not remove the need for some magic to make some offerings work other than as a sub-optimal compromise.
I realize that many EFD users have a religious fervor in relation to their efficacy. Just because you can work people on an EFD does not mean you could not have done better with a better antenna.
Sorry if you feel offended but I was asked to explain.
Any further explanations from me will attract a $200 deposit.