I’d argue that your capacitor and loop are OK (otherwise there would not be a dip at 27 MHz).

But my guess is that the match to 50 Ohm is bad. Try playing around with your feeding loop.

You can ignore the cable if 1) it is properly matched to 50 Ohms on both sides and b) there is no field outside of the cable, meaning the cable does not act as an antenna itself.

You are clearly working on the 1) part, and for the 2) part, it depends on how you feed the loop, unbalanced vs balanced. For coax you either need an unbalanced feed loop, or a balun (“balanced-unbalanced” converter) or a ferrite or similar “Mantelwellensperre” (don’t know the English word, sorry).