wmbusmeters-wiki
Simple 1/4 wave dipole antenna, made from a short length of co-axial cable.
TV Arial or satellite dish cable is suitable and the impedance 75ohm is probably correct for most SDR dongles.
Start with a length of say 15cm, make one end to the connector required by your SDR radio. (F type in my case).
Trim back the insulation, to just short of the connector
Remove the copper foil if present underneath.
Then separate the copper braid, from the inner conductor & its dielectric insulator.
If there is another foil layer underneath remove this.
Twist the braid together so it forms a stiff wire, bend thru 90deg.
Bend the inner conductor & its insulator also thru 90deg in the other direction.
Now trim the length of each wire to 82mm, for 868MHz
There are a few websites with calculators for 1/4 wave antennas if other frquencies are required.