If you are really interested in optimizing your self-made 868 MHz antennas, or all those, you received from your China seller, whom you trust, you should inhale the content of this extraordinary video from Andreas Spiess (the famous "guy with the swiss accent") very thoroughfull.
This video is really very helpful especially for non-antenna-specialists and you may check and trim your self-made 3 $ groundplane (if necessary) within seconds. Each IoT (FAB) - Lab and each TTN Community should have at least one of this tool (N1201SA) available. Remember all those, still ongoing, antenna discussions about HelTec, LiLyGo TTGO V1.0, v2.0 and others in TTN forum.
You may order this famous N1201SA there. Price of today (germany) is : 124.19 €. Pls, add german custom fees too.
You should give Andreas all credits and also a Patreon contribution here.