What is the external antenna “looking at” and how distant is it?
The internal antenna doesn’t have to be a pad (the actual term is “patch”)
it could be a “rubber duck” monopole and groundplane like this:
or a simple wire monopole/groundplane like this:
Easy to build from off-the-shelf components.
Ref:
https://openenergymonitor.github.io/forum-archive/node/6238.html