You may also use a very easy to build antenna, that provides ~2dB gain, while being electrically short cutted, which increases parasitic rejection: The J-Pole antenna. You may ust this calculator to determine the dimensions:
http://hamuniverse.com/jpole.html
At 867 MHz your antenna will be ~25cm long and will fit into a 16mm electrical plastic conduit.
At the base of the antenna I have winded 3 loops of the coax cable to symmetrize the signal (balun).
The antenna is built in minutes…In the last picture the black wire loop was just used transiently to pull the antenna into the plastic conduit without staining the balun loops.