Superconducting resonators have Q up in the 10^7 range, open a pinhole on that and you'll have an antenna with mediocre efficiency and a more than uselessly narrow bandwidth.
Presumably such a structure could be miniaturized, as a small helical resonator open to space (a Tesla coil as it were), rather than a nearly-closed Helmholtz resonator; at significant cooling cost, of course, given present technology.
Tim