Antenna for qrp pixie 2
You do not say where you are.

The radio you reference requires you to hold an Amateur Radio operator's license.  If you are in the United States, you need an Extra Class license (the highest), because the operating frequency is in the Extra Class sub-band of the 40 meter ham band.  If you are somewhere else, check the local laws.  (If you are in Thailand, check with the Radio Amateur Society of Thailand.  I've been to one of their meetings. 
 They're good people and they can and will help you.)

You'll need an antenna designed for the 7 MHz (40 meter) band.  There are LOTS of books out there that will tell you how to cut a simple dipole or inverted-V antenna.