A Discone is a type of antenna that is designed to be resonant over a wide range of frequencies. Most antenna designs only really receive well on a few resonant frequencies, but a Discone is resonant over a much wider frequency range. This makes it a good partner for RTL-SDR and other SDR units as many SDRs tend to have wide tunable frequency ranges. With a wideband antenna like a Discone connected to an RTL-SDR one can scan over the almost entire tunable frequency range without needing to change antennas for each band. The drawbacks to a Discone however is that the antenna gain is not very high, and that it makes the SDR more susceptible to out of band interference. They also tend to be fairly expensive and difficult to build.

However now over on Thingiverse, mkarliner (aka Mike) has a remedy for the difficulty in building a Discone with his 3D printable Discone design. To construct it you simply need the 3D printed parts, some .3mm and 2mm plastic sheets, a 25mm plastic conduit and some aluminium tape. Mike's design works from 400 MHz and up, but the design could be easily enlarged for better performance on the lower frequencies. He writes:

The Discone antenna is remarkable in that it is capable of receiving and transmitting over a wide range of frequencies with good matching. Because of this, it is a good match for SDR receivers such as the popular RTL-SDR sticks.

The only really tricking thing about making a discone is that the disc has to be balanced at the very top of the cone, which is mechanically awkward.

The two parts here allow the cone to be solidly clamped and provide an adequate base for the disk. There also two holes for bring the coax centre and braid out to the disc and cone.
The base part has a socket at the bottom for 25mm (1 inch) plastic conduit for mounting

This antenna illustrated is designed for 400MHz and up, and as such transmits well on the 70cms amateur band, US and UK PMR channels and 23cms. It also receives aircraft ADS-B signals very well. I used .3mm plastic sheet for the cone and 2mm plastic for the disc, and then covered them with aluminium weatherproof tape. Be sure to check for continuity across the tape stripes.

The screenshot is of a calculator by VE3SQB which can be downloaded from http://www.ve3sqb.com/ if you want to make attenna's for other ranges.

If you're interested in building wideband antenna there is also the planar disk antenna (pdf) which can be built out of pizza pans.