Antenna for receiving shortwave frequencies

I'm pretty new to RTL-SDR and radio in general, just picked one up about a month ago to explore and I find that its quite difficult to get good signal for receiving low frequencies below 20/30Mhz with the antenna that came with the RTLSDR.

Bit of a newbie question, I'm wondering what kind of antenna I could buy that is better suited for those frequencies, bearing in mind that I don't really have much space and would prefer something that can be indoors (if possible).