That's easy. Earth is basically very large capacitor (could think of about 1 Farad). If you will feed AC current via smaller capacitor, it will just have large

That's easy. Earth is basically very large capacitor (could think of about 1 Farad). If you will feed AC current via smaller capacitor, it will just have larger resistance, but will also conduct. And with higher frequency need smaller capacitor to achieve same resistance. Practically, antenna size for 50Hz is about 10th of kilometers, but for Kilo-Hertz, need much smaller antennas and smaller capacitors, and in hundreds Megagertz band, current electronics could easy make radio of match size or even smaller.