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My new PCB design is about 25 mm * 25 mm, this is not enough for a proper 868 MHz antenna ground plane, I am using the 0868AT43A0020 chip antenna where at least a 20 mm * 40 mm GND plane is required. The first prototypes confirm the weak RF performances. what is the standard practice when it comes to dipole antenna when the ground plane of the PCB is extremely small and not sufficient?

A picture of the PCB with the battery behind: enter image description here

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    \$\begingroup\$ why are you asking about dipoles when you deal with a chip antenna? A proper dipole will be much, much larger. \$\endgroup\$ – Marcus Müller Jun 27 '18 at 10:10
  • \$\begingroup\$ The standard practice is to live with the shortfall or make the GP more suitable. BTW a half wave dipole antenna does not need a GP. A quarter wave monopole antenna does. \$\endgroup\$ – Andy aka Jun 27 '18 at 10:39
  • \$\begingroup\$ What range to do you get and what range do you want? \$\endgroup\$ – Steve G Jun 27 '18 at 10:39
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    \$\begingroup\$ are you sure that that ground plane is needed? \$\endgroup\$ – Jasen Jun 27 '18 at 10:40
  • \$\begingroup\$ With those small dimensions its possible for you to tune the antenna to your frequency of interest. However, it will be really narrow band, and detune easily. \$\endgroup\$ – user94729 Jun 27 '18 at 10:43
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the only place in the document you link to that contains 20×40mm² dimensions is the eval board. That's just "randomly" in that size.

What's important is that the chip is offset some 1mm from the edge of a ground plane, and that you have the folded whip as specified, and that you properly match your antenna, as specified in multiple places of the data sheet. My guess is you forgot the latter.

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  • \$\begingroup\$ are these the matching instructions? johansontechnology.com/tuning.html \$\endgroup\$ – Jasen Jun 27 '18 at 10:46
  • \$\begingroup\$ all design instructions are followed and the antenna was matched. However I still get poor RF performance compared to bigger PCBs \$\endgroup\$ – Tarik Mokafih Jun 27 '18 at 11:32
  • \$\begingroup\$ @Tarik, you'll probably need to share your schematic and layout to get a useful answer then. Also are you enclosing the pcb in a housing of some kind? \$\endgroup\$ – The Photon Jun 27 '18 at 13:52

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