Can you provide me with some simple instructions on how to build a groundplane antenna tuned for 868mhz?
Building a groundplane antenna for 868MHz can be performed following a few steps. Here is a basic general guide:

1. Gather materials: You will need a piece of copper wire (around 20-22 gauge), a soldering iron, and some form of connector (such as an SMA connector) to connect the antenna to your device.
2. Cut the wire to length: Using the formula 468/frequency (in MHz) = total length (in inches), calculate the length of wire needed for 868MHz. This should be around 16 inches.
3. Bend the wire: Bend the wire into a U shape, making sure that the total length of the wire is still 16 inches.
4. Solder the connector: Attach the connector to one end of the wire using the soldering iron. Make sure that the connector is securely attached and that there are no stray pieces of wire that could short out.
5. Test the antenna: Connect the antenna to your device and test it to make sure that it is properly tuned to 868MHz.

Note: Groundplane antenna will have a considerable size so it's better if you can use an image of one and follow the measurements.

Please also note that this is a very basic guide and there are many different designs for groundplane antennas. You may need to experiment with different lengths and shapes of wire to find the design that works best for your specific application.
Thanks for your detailed explaination. I will try it out and let you know how it went ^^
No problem. I can't wait to hear the results!