Here's a page with a diagram of antenna impedance vs. length that assumes a 0.1 wavelength wire antenna that gives an equivalent of 300 pF resonating with the coil.
https://electronics.stackexchange.co...ystal-radio-be
It seems in your case, you have a 15 foot optimum antenna length (about 0.01 wavelength), which would be about 30 pF, if I'm thinking about this right.
Suggested experiment: put a small capacitor (maybe 30 to 60 pf) in series with your antenna wire, and then lengthen the antenna and see if it works better.