Antenna working on metal surface

[blueskull]

My first post in this section here. Is there a type of antenna that can operate "ON" a metal ground plane?
Looking for 2.4GHz~2.5GHz antenna for WiFi+BT.
Can be adhesive flex PCB antenna or ceramic chip antenna.
I know ceramic patch antenna will work, but the smallest I can find is 12mm*12mm, which is too big for me.
I also came across the 3M "on-ground" antenna, but from its app note, it seems a ground clearance is recommended, so it's not truly "on-ground". Correct me if I'm wrong.
Target size: less than 9mm wide, 20mm long and 5mm thick.

Any suggestions?

[CJay]

I was going to suggest grabbing a copy of the Pi 3 schematics as it's got a ceramic antenna that's roughly 2x5mm and there may be a hint of a part number but the 'schematics' seem to be pinouts of the connectors which is somewhat useless.

[Ice-Tea]

You mean an existing GND plane? Or a PCB you're designing that should have an as-solid-as-possible GND plane?

Something like this: https://www.digikey.com/product-detail/en/proant-ab/PRO-OB-440/1532-1001-2-ND/4895679 maybe?

[Ice-Tea]

Too bad they are a bit big, or they'd fit the bill rather well 

https://www.lsr.com/embedded-wireless-modules/antennas#flexpifa

[cdev]

A flex PCB antenna would be better. If you have a bit more room inside your case, use it.
Even just a wire enclosing a small loop in your flex PCB would likely be better.

Taoglas is a reliable company and their circular ceramic patch GPS antennas are quite decent. I would trust them as far as quality, however that said unless you need it to be in a very small space don't.

Before you commit, if I were you I would get at least one, preferably a few of each antenna you might use so you can try them out in a real world test before buying a bunch of any of them.

Its hard to find a really small 2.4 GHz antenna that performs well at a long distance.

The one that is in the Raspberry Pi 3 doesn't perform super well.

[hagster]

Could you make a slot antenna?. It would mean cutting a thin slit in the metal (or finding an area where there is a natural gap between sections), but slot antennas are really efficient.