The board has a PCB antenna for the 2.5 GHz ISM Band, for stand alone operation on the Camp Area. For other frequencies the use of an external antenna is highly recommended.
Have a look at Internal Antenna for more details on the internal antenna.
To improve radio reception a little bit you can just press a thumb against the upper right corner and be a human antenna. The most simple real external antenna is a single-wire copper cable (others may also work). This can be pressed against an edge of the little black resistor on the top right corner:
(then the 0 Ohm resistor will be sitting on the R314 pads; R314 and R315 share the upper left pad). Use 2 soldering irons - so you can easily move the little resistor.
The SMA connector should be an edge-launch SMA connector for a PCB thickness of 1.6mm (see RF Kit)
Alternative: Add bridge, remove resistor:
Using a soldered copper cable as antenna:
Adding an SMA connector so that you can connect an external antenna for your specific needs:
WARNING: When soldering, don't connect the unpopulated large pads right below the antenna pads to the pads, this may result in blue smoke! Best to check right after soldering, we did find a few cases of barely visible connection.
Build your own dipole antenna with d1pole
German compendium about HAM-Radio-Antennas, very complete, with a lot of tipps and instructions for DIY-Antennas, Edition from 1968 (7th Edition). Rothammel - Das Antennenbuch Current Edition is 13th, from 2012 ISBN 978-3-88692-065-5
HackRF recommends the ANT500 antenna for beginners. Cheapest way to order in germany/fr/uk seems to be from passion radio for 29€ (VAT not included!), shipping being about 2€. This shop also recommends the apparantly similar antenna Diamond SRH789.